It is very likely that no checking whatever at the drums will be found necessary, but that, on the contrary, it will be found desirable to get rid of any resistance by overhauling the heavy chains through the sheaves.
The best day on which to begin the launch was Tuesday, November 3, as it left two or three days of the high full moon spring-tides for the operations, should they be prolonged.
On Monday, November 2, the chief work remaining to be done was stowing kentledge or iron ballast on the cradles, to prevent the timbers floating when the ship should be moved off them. All the appliances were ready, and, except the river tackle, had been carefully examined. This, as has been already said, had not been tested.
It was now for Mr. Brunel to consider whether, in consequence of the river tackle not having been properly tested, he should postpone the launch till the following month.
It was most important for the Company that the ship should be afloat as soon as possible; and, as any defects which might exist in the river tackle would almost certainly declare themselves in the earlier part of the operation, when nothing worse than delay could be apprehended, Mr. Brunel, after a careful review of all the circumstances, determined to attempt the launch.
On the morning of November 3, the work of putting kentledge on to the cradles was completed by firelight, and the rails were rubbed over with a mixture of oil and black-lead. All the shores and props which supported the weight of the ship had been removed, and she was now resting entirely on the cradles.
Later on in the morning the brakes of the drums were tightened down, and the dogshores were removed from the ways in front of the cradles. Mr. Brunel, who had been engaged from an early hour in examining all the preparations, superintended this operation, and, having satisfied himself that all was clear and ready, returned to the upper part of the yard.
By this time it was crowded with people. The Directors, contrary to Mr. Brunel’s expressed wish, and without informing him of their intention, had issued a large number of tickets of admission. A few days before, Mr. Brunel had suggested that four policemen should be obtained, thinking that all they would have to do would be to contend with trespassers. The police force actually present were ignorant of the portions of the yard to be kept clear, and Mr. Brunel had himself to go and assist in ordering visitors away from the neighbourhood of the path prepared for the tackle of the stern hauling gear. The crowd soon became so great that it was almost impossible for the men in charge of the hauling-engine at the stern to see the signals given from the middle of the yard, or for those in the middle of the yard to see what was happening at the stern.
At about half-past 12 o’clock the fastenings of the ship at the bow and stern were let go, and Mr. Brunel ordered a small amount of slack to be given off from each drum. This was done by men turning the handles of the gearing which had been provided for winding the chain on to the drums. The order was then given to haul on the bow and stern tackle, and to pump at the hydraulic presses. It is doubtful what amount of strain was put on by the tackle and the presses, but it was probably not very great.
Presently a shout from the forward cradle announced that it was moving, and almost immediately the stern cradle also started with what appeared to be a considerable speed. The men who had been engaged in turning the handles of the gearing had remained leaning against them. As soon as the ship had moved a few inches, she took up all the slack chain. This made the drum revolve, and the handles of the gearing spun round very rapidly, striking the men, and throwing them into the air. The men who were at the brake-handle next to the gearing ran away. Mr. Brunel, who was standing near the drum when the accident happened, shouted to the men to hold on to the brakes, and ran to the spot. The men who had remained at the other brake-handle hauled it down with the tackle. A great restraining force was thereby brought upon the ship, and her progress ceased; the forward cradle having moved 3 feet, and the after cradle 4 feet 3 inches.
Five men were injured. On the death of one of them it was stated at the inquest, by the foreman of the drum, that, after the slack had been paid out, he had ordered the men to stand clear. Be this as it may, it cannot be denied that the handles should not have been used after the securing chains had been let go; and indeed Mr. Brunel said at the inquest, ‘I may blame myself, for I did not anticipate that the handles would have revolved so rapidly.’
After this accident, Mr. Brunel determined to wait till high water before recommencing the operations. In the meantime the gearing was removed from both drums.
A more important change was also made in the arrangements. When the ship moved, the men on the four middle barges became frightened, thinking she was about to overwhelm them; a rush was made, and one man, jumping into a small boat, shoved off, leaving the rest to their fate. A report was at once sent to Mr. Brunel that the men were untrustworthy, and that they would not remain; and that, as the barges would be of no use without the men, the chains had better be dropped and the barges removed. To this Mr. Brunel consented.
It would, however, have been sufficient to take the men off, leaving a tug-boat and a few steady men to keep the barges out of the way of the ship; they would then have been available if required. Mr. Brunel, a short time after he had given the order, ran round the bow with Captain Harrison to countermand it; but it was too late, as it had been already acted upon. As events turned out no harm was done, as the centre barges alone would not have been sufficient to go on with, after the rest of the tackle failed.
The result of these changes was that when the operations were recommenced, the only hauling gear was the bow and stern purchases; the hydraulic presses were also available to start the ship.
At a little after 2 o’clock the signal was given to haul on the bow and stern tackle, the presses being at the same time pumped up. The brakes of the drums were slackened, but kept all ready for tightening.
Not long after the strain had been brought on the tackle, several of the teeth of one of the wheels of the bow steam crab gave way, and the chief anchor at the stern began to drag, so that no efficient strain could be obtained. On this being reported, the operations were discontinued; and, as there was no possibility of getting things ready by the next day, the launching operations were postponed till December 2, the next full moon spring-tides.
As soon as this was known the visitors rushed in on the works, crowding about the cradles and ways; and Mr. Brunel had to postpone those investigations which he wished to make at once.[163] The whole yard was thrown into confusion by a struggling mob, and there was nothing to be done but to see that the ship was properly secured, and to wait till the following morning.[164]
The next day was devoted to an examination of what had gone wrong, and to the consideration of what should be altered before another attempt was made.
At the stern mooring the anchor was bedded into the ground on the further side of the river.
The difficulty with regard to the four centre barges was got over by placing the four crabs with their tackles in the yard, on the landward side of the ship. The four chains attached to the ship, which had before been hauled on directly from the barges, were now passed round sheaves on the barges, and brought back under the ship’s bottom to the tackles in the yard.
The chief alteration, however, was in the arrangement of the hydraulic presses. On November 3, there were, as has been said, two presses. Two additional presses were now provided; each of these consisted of two 7-inch cylinders, and was equivalent to a 10-inch press.
With the object of being able to employ the presses continuously during the descent of the ship, they were arranged to point down the ways at an inclination of 1 in 12. The four presses were placed one on either side of the check tackle at the two ways, and were supported by abutments of timber-work. These abutments each consisted of four rows of piles, one behind the other at intervals of about 8 feet. The press abutted against the row of piles nearest to the ship, which were connected by wooden struts to the piles behind them. Long balks of timber of various lengths were prepared to transmit the pressure to the cradles.[165]
The four presses might be considered equivalent, at their full power, to a force of 800 tons; this was so much in excess of the small force that had moved the ship on November 3, that, even making every allowance for the advantage of the fresh lubrication in the first instance, it seemed reasonable to suppose that with this force the ship could be moved down easily.
As the process of moving the ship with the presses would naturally be a slow one, Mr. Brunel determined to proceed with the operations as soon as everything was ready. On November 19 the work was commenced.[166] The bow tackle was hauled upon first, as the forward cradle was more than a foot behind the after one, and the men at the forward presses were set to work. After a short time the timber backing of the presses began to crack and ‘cry out’; and, without much stress on them, the abutments were forced back some 3 or 4 inches. The mooring chain of the bow tackle also gave way, although there was not any excessive strain on it. On examining the abutments, Mr. Brunel saw the cause of their failure, and ordered the strain to be taken off. The number of piles was sufficient, but the way in which the strain was communicated to them did not enable them to exert the proper amount of resisting power.
This defect was cured by tying the heads of the rearward piles with bolts to the foremost piles. The ship being secured, each press was tested to a full strain, and the adequacy of its abutments ascertained.
It was different, however, with the river tackle. The chain which had parted was an old river mooring-chain of great size. Much delay in replacing it was caused by dense fogs, which made it almost impossible to work on the river. Moreover, there seemed a fatality about every attempt to get a regular trial of any part of the tackle. When, at last, a trial took place, and a strain was put on, a mooring-chain gave way; then this had to be fished up from the bottom of the river, and pieced together, the accident being ascribed to a defective link in the chain. The trials were, therefore, so few that it was only proved by degrees that all the regular moorings were worthless; although they had large chains which ought to have been good for three times the strain put on them.
The stubbornness of the ship on November 19 gave Mr. Brunel great anxiety; not from any fear of being unable to apply sufficient power to move her, but because, on continued consideration of the subject, he apprehended that a serious difficulty might arise, if there should be a prolonged delay at a particular part of the ship’s progress.
It has been explained that Mr. Brunel, with a view of obtaining uniformity of bearing over the surface of the ways, had not attempted to support them rigidly on piles, but had rested them on the river-bank. As, however, the foundation of the building slip was comparatively rigid, he feared lest an unequal subsidence might cause injury to the ship, if she were stopped for any length of time before she had completely left the ground on which she was built. He thought that if the ways sank at this point they would assume a slightly convex form, and tend to force upwards the flat bottom of the ship. The main part of the ship’s bottom, between the longitudinal bulkheads, could bend in slightly under a heavy upward pressure; but this action could not take place at the transverse bulkheads, as they would not yield without injury. Mr. Brunel shrank from proceeding with the launch without having in reserve such an amplitude of power as would prevent the ship’s being stopped at this critical point. This consideration, together with the continued failure of the river tackle under such tests as were applied to it, led him to address the following communication to the Directors:—
November 26, 1857.
We proved two of the presses yesterday afternoon up to the full pressure. A third, the largest, was proved partially; it required some additions, which are nearly completed, and will be in a few hours. The fourth may, I think, also be relied on to the same extent, nevertheless, after a careful examination of the effects of these strains and other circumstances, I have, after a night’s consideration, come to the conclusion that our means are too imperfect to justify my moving the ship with them in their present form. The presses would start the ship, but it is evident that if required to be used constantly, that is repeatedly, the piles would become loosened so as to draw and rise; this again might be remedied by loading, but clumsily, and with other contingencies, which I will report, combine to render it hazardous to depend upon them. My original intention, the right one, was to fit up properly such an hydraulic apparatus as should be fitted to move the ship the whole length of the ways, and to depend upon the whole river tackle only in the event of her moving very easily and for getting her off the ways at the end. From an unwise attempt to economise I determined to dispense with the immediate costly apparatus for pushing, and by sufficient power merely to move the ship at starting or in the event of sticking, and to depend upon the same river tackle to keep her moving down the ways. The power originally calculated upon for the river tackle has gradually, step by step, failed us; the moorings supposed to be sufficient for certain strains have failed us at one-third of those strains, another has parted since our last attempt, and, instead of full 350 tons of power from this source, we cannot now depend upon 200, and this, added to the inefficiency of the pushing power, would risk the sticking of the ship, which might occur exactly at a point which would involve serious difficulty to remedy. I am assuming a combination of adverse circumstances, perhaps not likely to occur, but quite possible; and the conclusion I am compelled to come to is that our apparatus is too defective, and that the original plan of a proper and sufficient hydraulic apparatus, arranged in a complete well-constructed mechanical manner, to push the ship continuously down the ways, ought to have been followed out, and is now the only mode of doing the work safely, that is, without the risk of being involved in a difficulty much greater and more costly.
I have only to add that bad as this report of our condition is, it is at any rate the worst that can be made of it, that nothing whatever has occurred to show that any new difficulty has arisen or anything whatever to create any new difficulty. We could move the ship now if it were wise to do so, but with great doubts whether our pushing apparatus in its present form, imperfect and unmechanical, would continue effective if repeatedly used, and the certainty that our river tackle is far inferior to what is required, and also of doubtful and more than doubtful permanency for repeated strains, it would not be right to commence....
Mr. Brunel at the same time determined to obtain, on a large scale, a measure of the deflection that might be expected from the weight of the ship coming on the ways. More than 100 tons of kentledge was piled on a portion of the ways 10 feet square, in such a manner as to give a pressure thereon of about double that which would be produced by the weight of the ship.
It was necessary that this test should not be tried on too small a scale, as a weight resting on an isolated patch would receive support from the surrounding ground, which it could not of course do if that ground was equally loaded. The ways sunk so little under the test as completely to reassure Mr. Brunel, and to show that no serious evil need be contemplated in the passage of the ship from off the place where she was built on to the newly made ways, even though she might be again stopped for some time. He therefore determined to go on at once with the launching operations.
The result of the test was very satisfactory to him, and it enabled him to carry on the work with the same confidence as he had at the first felt—‘that the ship may stop halfway or not move at all ... but no injury to the ship can result from any failure in the course of proceeding in this mode of launching.’
Shortly before the second attempt to move the ship, on November 19, the experimental cradle had been again put up with a view of obtaining some additional data as to the hauling strain that might be required. The deductions made from them were the same as those obtained in the commencement of the year, and encouraged the hope that the motive power required would not be excessive.[167]
By Saturday, November 28, the four presses had been got ready; and the river tackle, though still far from being beyond reproach, had been got into place, and partly tested.
The brakes were eased, and a small amount of slack was overhauled on the check-tackle chains by the men stationed on the ways for that purpose. As on the previous occasion, the pressure was to be first put on the presses at the foremost cradle.
Arrangements were made for promptly following up the ship if she moved freely. A black board was placed on each cradle for recording the progress of the ship.
Mr. Brunel stood on a low platform in the centre of the yard, as a convenient position from which to watch and command the operations. A little before ten he gave the order to commence pumping, and the men at the hydraulic presses got to work. When the pressure came on the timber framing which formed the abutments, there was considerable noise of creaking and crushing as the several parts subjected to strain came in to their proper bearing. The men soon changed from the large plunger handle to the small one which put on the full pressure; the timbers of the abutments kept on crying out, but it was evident that they were not yielding as they had done before. Presently, while the noise of the timber-work was still attracting attention, the man in charge of the measuring apparatus recorded on his black board a movement of one inch; the ship was again in motion.
She moved steadily, but slowly, under the force of the presses, at a rate of about one inch a minute, and as soon as the forward cradle had been moved about a foot in this way, the presses at the after-ways were set to work, and the river tackle was put into operation, first the bow and stern tackle, and then the four middle purchases. All went well with the presses, but the strain had not been put on long, when the stern mooring-chain and one of the two chains at the bow broke; an anchor at the bow had also begun to drag.
Later in the day part of the moorings of the centre barges also gave way. Captain Harrison set to work to repair these defects as fast as they occurred; in no way dismayed that, as he was at work remedying one mishap, the news of another was brought to him. Barges had to be got into place, the broken ends of cables fished up or under-run and pieced together, and this often in the dark; for it must be remembered that the work was going on at the end of November, when the sun rose, invisible for fog, at half-past seven, and set at half-past four.
With the exception of the river tackle, all had gone well; the presses and their abutments had acted efficiently, and the ship had been moved easily down the ways about 14 feet before work was suspended at night.
Though the progress had not been great, there still seemed a reasonable hope that, by pushing on, the ship might be got down to the bottom of the ways in time for floating her off at the next spring-tides, namely, on December 2. Mr. Brunel therefore decided to go on with the operation on the Sunday. Early in the morning the presses and crabs were again set to work. The river tackle soon gave way; and, indeed, there was no reason why it should be superior to that used the day before, as almost all that could be done in the night was to piece together the broken chains, and to replace the anchors. The moorings at the bow and stern began at once to drag, and two of the mooring-chains amidships parted. The hydraulic presses were then the only available power; and, although the full pressure was put on, the ship did not move. This was very disheartening; it was, however, thought that the resistance was due to some exceptional adhesion. Every effort was therefore made to get together the means of giving the ship a first start.
It was not till the afternoon that a large number of screw-jacks and hydraulic jacks which had been procured were got into place; they were then screwed up hard, and the hydraulic presses being set to work, the ship began to move in a manner very similar to that of the day before. There was not, however, much daylight left; and, when night came on, the distance traversed was only about 8 feet. The comparative facility with which the ship moved when once started gave hopes that good progress might be made the next day.
On Monday morning the ship moved without more difficulty than when she had stopped the evening before, and the work went on quite satisfactorily. She continued to move slowly, and by dinner-time had gone about 8½ feet. Three feet an hour was not much, but still if it could be kept up it would suffice to get the ship down by the next spring-tides. Arrangements were therefore commenced for lighting up the ways and pumping machinery, so that the work might be continued through the night. The repairs of the river tackle were pushed on, the ship’s anchors, which had just been finished and tested, were laid down for part of the moorings, and some of the chains were replaced with chains lent by the Government and by Messrs. Brown and Lenox.
When work was recommenced after dinner the ship made a short slip of about 7 inches. On pressure being again applied the 10-inch press at the forward cradle burst. This put an end to all work for the day, and it was then determined to replace the broken press and to add two more presses to each cradle, before proceeding with the launch.
The preparations for the new presses were pushed on vigorously, but it was not till the afternoon of Thursday, December 3, that things were again ready for a start.
The pumps were set to work and the tackle hauled upon. The ship made several short slips of a foot or so, and then moved more than 5 feet at one slide. When darkness set in she had moved about 14 feet, in slips of greater or less length.[168]
On Friday, December 4, all was ready early, and during the morning everything went as well as on the day before; but in the afternoon increased difficulty was found in getting the ship to move, and the 14-inch press at the after cradle burst, as did also a 7-inch cylinder of one of the coupled presses.
Notwithstanding the delay due to the bursting of the presses, the ship was moved some 30 feet; but there was no longer any chance of getting her afloat at the spring-tides, and the increased adhesion gave cause for the fear that still more power would have to be applied.
On the next day, Saturday, December 5, the ship made a short slip; but, although the pressure was kept constantly on, no further advance was made until late in the afternoon. Mr. Brunel then tried suddenly letting go the strain on the stern tackle. The sudden relief of the side-way strain on the end of the ship sent a tremor through the hull, which served to destroy the adhesion, and she slid several inches. This operation was several times repeated, and although there were a number of vexatious delays from pushing-pieces giving way and other mishaps, she was moved by the evening a distance of about 7 feet, the resistance due to adhesion being very great.
On Sunday some of the presses farthest from the cradles were moved down the ways nearer to the ship, so as to avoid the necessity of using long pushing-pieces, which required much attention to prevent their bulging sideways. The river tackle now consisted of the bow and stern steam-engine purchases, and two crabs and tackle, one at each end of the ship. The moorings opposite the centre of the ship having proved worthless, it was necessary to lay down new moorings, and it was found more convenient to lay them opposite the ends of the ship.
The next day, Monday the 7th, after the commencement of operations, considerable delay was caused by the failure of some of the feed pipes of the presses. These defects were not cured till after dinner-time, when the operations were resumed, and before dark the ship was moved about 6 feet.
On the following morning several short slips were made, and the ship had been moved about 4½ feet; but, at 10 o’clock, a dense fog came on, and rendered it impossible to proceed. The next day was occupied in re-arranging the tackle and presses.
These were not ready till the morning of Thursday, December 10; when, on the presses being again set to work, and one of the chains being as before suddenly slackened, the ship made one slip of a little over a foot; but, on the strain being again applied, two of the anchors began to drag. As it was now essential to have the river tackle, in order by shaking the ship to destroy the adhesion, and by the drag of the catenaries to increase the length of the slides, Mr. Brunel determined to dispense with the anchors, and to attach the chain cables to piles connected by framework. These abutments or pulling points for the chains were now constructed on the other side of the river, opposite the bow and stern of the ship.
The launching operations last described, namely, from December 3 to December 10, were full of incident. Nor was the scene wanting in that animation which agreeably interests a bystander, the more so if he is not thoroughly conversant with the meaning of all he sees and hears, so that he mistakes a loudly spoken word, loudly spoken merely that it may be plainly heard, for a prompt and urgent command.
The labourers at the pumps relieved the monotony of their work, and shook off the cold, by taking a lively and talkative interest in the progress of the launch, and echoed the orders given them to pump with the ‘big plunger,’ or ‘little plunger’ of the pumps, or to ‘fleet’ the press. This and the singing of the gangs, which were constantly at work moving chains for the repair of the river tackle, or rolling logs of timber on to the ways to serve as pushing-pieces for the presses, gave plenty of life to the operations; and then when the pressure had been got on the presses, and shouts from the bow and stern of the ship passed the word that the river tackle was hauled taut, the order would be given to ‘let go’ the chain at one end of the ship. Immediately the rattling noise announced that this had been done, and, after a second or two of anxious watching, the ship slid off, the timbers, abutments, and pushing-pieces creaking and groaning as the strain was suddenly relieved. While the ship was in motion, the whole of the ground forming the yard would perceptibly shake, or rather sway, on the discharge of the power stored up in the presses and their abutments. The appearance of the ship moving sideways in these short slips, when seen from the ways, was very imposing.
All these somewhat striking surroundings of the operations were naturally heightened in effect, when the work was being carried on in the early morning or late in the afternoon; and when the timber-framing and the groups of men at work were illuminated by the glare from the open fires which were kept burning near the pumps and presses.
The preparations already described were not completed till December 15. In the meantime Mr. Brunel had been joined by his friend Mr. Robert Stephenson. Mr. Stephenson had not been aware of many vexatious circumstances which had even prevented Mr. Brunel from making full use of his own staff of assistants. Mr. Stephenson expressed to a common friend his regret that Mr. Brunel had not invited him down to the ship, and said that he should have gone down uninvited, but that he thought Mr. Brunel had reasons for not wishing it. On the state of affairs being explained to him, Mr. Stephenson said, ‘I’ll go down to him at once;’ he did so, and his arrival at Millwall was very welcome to Mr. Brunel.[169]
Mr. Stephenson agreed with Mr. Brunel as to the expediency of suspending operations until an ample excess of power was applied. Fortified by the support of Mr. Stephenson, Mr. Brunel was prepared to advise the Directors to adopt this course; but, as the preparations for recommencing the work were just completed, it was determined to make a trial on the afternoon of December 15. The presses were all pumped up, and the river tackle hauled on; but, although the force applied was at least 300 or 400 tons greater than that which had last moved her, the ship did not yield, and the attempt was abandoned. After a careful consultation on the depressing result of this day’s work, it was determined to make another attempt the next morning, in order to see if any new form of difficulty had arisen; and that after this operations should be suspended, and an ample number of additional presses provided.
The following day, December 16, as soon as Mr. Brunel and Mr. Stephenson had arrived, the pressure was again put on the presses, and the river tackle having been hauled taut, the chains at the bow were let go, and, to the great satisfaction of all present, the ship made a short slide. The record of her movement showed that, although the adhesion was much greater, the retarding force of friction was about the same as before, and that therefore there was no reason to assume the existence of any special obstacle. Another short slip was made; but, in getting up the pressure again, a press was burst, and the work was then stopped.[170]
Mr. Brunel’s decision to suspend the launching operations at this point was approved at a meeting of the principal shareholders held the next day. His report to the Directors, and a memorandum of a verbal statement which he made to the meeting, are as follows:—
December 17, 1857.
In my letter of October 23, which was published at the time in the daily papers, I referred to the possible contingency of the power provided to move the vessel down the ways proving insufficient, and the operation then about to be attempted being so far unsuccessful; and, referring to what I considered a countervailing advantage in the absence of risk, I stated, ‘the ship may stop half-way, or may not move at all, more power may have to be applied, but no injury to the ship can result from any failure in the course of proceeding in this mode of launching.’
The result has been that after moving the vessel nearly half the distance to low water, it has become necessary to increase very considerably the power which has effected this much, although it had already been much added to during the operation.
This will unavoidably be attended with some expense and delay, but not considerable, as the requisite hydraulic presses can be obtained ready made, and their application is simple, and the result cannot, I apprehend, be doubtful.
I do not mean to imply that I contemplated any such great increase of resistance as probable, such experiments as could be made before moving the ship having given me good reason to hope for a different result; but the possibility of it was contemplated, and I refer to this merely as explaining the statement I now make, that the difficulty is simply one of degree, of more or less power being required, and that nothing whatever has occurred to create any new class of difficulty. The launching ways, about which anxiety had been expressed, and not unnaturally, have stood perfectly and without any settlement or any derangement by being passed over. There is no change of gradient or inclination in the ways capable of producing any effect, as has been supposed; the upper part of the ways having an inclination of 1·025 inches per foot, and the lower part, where the ship now is, one of 1·000 per foot, a difference too small to be appreciable, but which possibly by some mistake of figures may have led to the erroneous impression referred to.
The amount of resistance upon the ways in their present condition and inclination has now been positively ascertained, and an ample excess of power being applied, there can be no reason to doubt the result. I propose to apply that excess by going considerably beyond the amount which the calculation founded upon the results actually obtained would give as the maximum, and to double the power which has last moved the vessel.
Memorandum of a verbal Report made to the Directors, and a small Meeting of the Principal Proprietors.
December 17, 1857.
That after full consideration of all the circumstances, and assisted by the best advice I could call in to my aid, namely, that of my friend Mr. Robert Stephenson, I considered that the only mode of proceeding, and one which there appeared no reason to doubt would succeed, was to apply considerably more press power; that I proposed to double what we had; that I believed I was able to put my hands upon the requisite presses; that the river tackle so far as it went might now be considered good, but that unfortunately we were obliged to take up the principal part of the chains, which with great kindness and liberality Messrs. Brown and Lenox had lent us, and were now peremptorily called upon to deliver up; but that with their assistance I could replace them....
A large number of presses were obtained, the owners for the most part lending them free of charge. Among these presses was the large one, with a 20-inch cylinder, which had been used for lifting the tubes of the Britannia Bridge.
On each of the ways were placed nine presses. The total sectional area of the cylinders at the forward cradle was 1,066 circular inches, and that of the cylinders at the after cradle was 1,358 circular inches; but the Britannia press was not to be worked to its full power, so the total area of the cylinders may be taken as 2,300 circular inches, or 1,800 square inches. The presses might be considered as good for at least 2½ tons on the square inch; this gave a power of 4,500 tons, which, with the 1,000 tons due to gravity, gave 5,500 tons, or equal to nearly half the weight of the ship. The presses were now coupled together in groups, in order to ensure that an equal pressure should be brought on them; and to each of these groups an accurate pressure gauge was attached.
All the presses having been tested, it was determined to recommence the actual operation of launching on Tuesday, January 5.
So much of the water in the pipes had been frozen that it was eleven o’clock before the order was given to the men to pump. When at one group after another the pressure was shown to be one ton on the circular inch, the pumps were stopped. As the backing of the presses continued to yield slightly, a stroke or two of the handles had to be made from time to time, to keep up the required strain. For six minutes there was perfect silence, and then the ship moved, sliding down about 3 inches.
The same process was repeated at the stern cradle once or twice, and then at both cradles. After this the order was given that the pumps should be kept going till she moved. This was accordingly done, and when the pressure amounted to 1¼ ton on the circular inch the ship made a slide of about 4 inches. In this manner she was moved about 5 feet before work was stopped in the evening.[171]
On January 6, there was a singular change in the behaviour of the ship. During the whole of the forenoon she moved gradually, yielding to the pressure at a rate of about an inch in four minutes. In the afternoon, however, she moved in short slides.
During this and the three following days her progress was about 10 feet each day. After this the ship, being to a considerable extent waterborne, was moved with greater ease, and on Tuesday, January 12, 20 feet was accomplished in less than four hours.
By Thursday, the 14th, the ship had traversed a distance of 197 feet at the forward cradle, and 207 feet at the after cradle. It was thought unwise to advance further till the coming spring-tides on the 19th of the month were past, lest an exceptionally high tide might come unexpectedly, and partially float her. As soon as the spring-tides had passed, she was moved on cautiously, a short distance at a time, and the depression of the ways was carefully observed. This was found to be inconsiderable, and the cradles were gradually pushed 25 feet off the ways. As the spring-tides came on, water was run into the ship, to prevent her from floating prematurely.
The upright struts of the cradle on the side next to the river were all removed, and the wedge-pieces had chains fastened to them, with the ends brought on deck; so that, if any of the wedge-pieces got jammed and did not come out when the ship floated, they might be hauled out by the chains.
The river tackle now consisted of two purchases at the bow and two at the stern. To keep the ship, when she floated, from being drifted by the tide or wind, chains were carried from the bow and stern to moorings, by which her movement up and down stream might be regulated. Four tugs were in attendance to tow the ship to her berth, and a floating fire-engine was also ready to pump water into her, should this be necessary owing to any sudden postponement of the launch.
Nothing now remained but to watch carefully for a suitable tide.
It was determined that, if the weather were favourable, the floating should be effected on Saturday, January 30. The tides had been below the average, and on the Friday matters did not look promising; the tides had continued low, and the weather was bad. A careful watch was kept on the tide, observations being taken every half hour, and plotted on a diagram so as to show at a glance the probable height to which it would rise.
The tides showed signs of improvement, and they commenced to pump water out of the ship on Friday night; but, as time went on, the weather did not mend, and the wind was blowing from the south-west against her broadside; therefore in the early morning Mr. Brunel, who was in person attending each turn of affairs, ordered water to be pumped in by the fire-engine. There was hard rain and strong wind; and telegrams which, according to arrangement, were being frequently sent from Liverpool and Plymouth, showed similar weather. This continued throughout the Saturday, and the tide was low; but, when it began to rise in the evening, it gave indications of being a very high one. As soon as the tide reached the Kingston valves, Mr. Brunel had water run into the ship. Although she rested uneasily on the cradles, she remained safely in her position.
In the evening the rain came down in torrents; nevertheless, after midnight the weather mended, and the wind went round to the north-east. As the telegraph gave the same report from Liverpool, Mr. Brunel, encouraged by every sign of fine weather, and having the good promise given by the high tide of the night before, determined early on the morning of Sunday, January 31, to float the ship on that day.
The pumps were immediately started to discharge the water from the ship. The bolts securing the wedge-pieces of the cradles were unfastened at daybreak, and the ship was then ready.
The morning broke with great splendour after the gloom of the previous days, and the tide, as soon as it had turned, began to rise with unusual rapidity. It had been arranged that all the men should be at their posts at the presses and crabs by eleven o’clock; but the tide was not only very high, but exceptionally early; and, although a considerable margin had been allowed, it was not sufficient. Mr. Brunel and his assistants hurried the men to their places, the presses were set to work, and the ship was put in motion down the ways for the last time.
At a little before one o’clock observations taken by levels showed that the ship had ceased to descend, although she was still being pushed forward. Shortly afterwards Captain Harrison, who had gone on board, sent Mr. Brunel word that all the wedge-pieces had floated up on the outer side of the ship; and at twenty minutes past one the stern was seen to be afloat. Mr. Brunel had been loath to haul out the ship by the river tackle, lest the wedge-pieces might get jammed; but, as soon as he was informed that they had floated up, he sent orders for a strain to be put on the bow tackle. This was at once done, and by twenty minutes to two the bow rose from the cradle.
Mr. Brunel then ordered the checking gear to be secured, that the ship might be sooner hauled clear of the cradles, and he went on board. He had scarcely done so when a serious difficulty arose. It has been already said, that in order to keep the ship from moving up or down stream, cables had been carried out to moorings ahead and astern; and both these chains had been hauled up tolerably taut, at least the slack had been taken out of them, so that if required they might be at once available. Now when Mr. Brunel had given the order for the bow out-haul tackle to be hauled upon, Captain Harrison, in order to supplement it, ordered the tug-boats to haul the bow off. This order was by some one conceived to have been given with the object of hauling the ship ahead; and to facilitate this the stern mooring-chain was let go.
By this time, though the tide was still running up the river, its strength was much diminished; and the drag of the chain at the bow of the ship was sufficient to pull her forward against the tide. The paddlewheel on the shore side then came in contact with the upright timbers of the forward cradle. Mr. Brunel ran down from the ship into a boat and examined the place where the wheel was fouled. He then hurried back on board, where, as through some blunder the stern outhaul had also been let go, he had now little but the tug-boats to depend upon. They were of course of but small value for a dead pull as compared with the chain tackle. He had the bow chain veered out and the tugs all set to work, assisted by the tide, to haul the ship up the river.
In about twenty minutes time, the paddlewheel was got clear of the cradle, and this great difficulty was overcome. Fortunately, the tide was an extraordinarily high one, and the time of available high water was long.
The ship had not been moved far towards her berth when another mishap delayed her progress. The barge of the bow purchase came foul of the starboard paddlewheel, and the only way of freeing the ship was to scuttle the barge. When this was done it sank away clear of the wheel, and the ship proceeded to her berth on the Deptford side of the river.[172]
By about seven o’clock she was safely moored; and the cheers of the men, as Mr. Brunel went down her side, announced that the launch of the ‘Great Eastern’ was at length accomplished.
Experiments and Observations on Friction.
In January 1857, Mr. Brunel took steps to form an estimate of the amount of hauling or of retarding force that would probably be required in the launch.
Two rails were laid at an inclination of 1 in 12, and upon them an experimental cradle was placed, with three cross-bars similar to those which were to form the under surface of the cradles. The three cross-bars therefore made six intersections on the two rails, and the small cradle was loaded with about 8 tons, so that the weight on each intersection was about equal to that which would come on each of the intersections of the actual cradle. This arrangement was not therefore a model, but a correct representation of a part of the cradles, and which might, with an exception to be noted presently, be taken to exactly represent, by its conduct, the conduct of every similar part of the actual cradles. Experiments were made with one or two kinds of unguents, and, what was a more correct representation of what was likely to occur, with the rails and bars clean but not bright, and without lubrication.
The experiments with lubrication were useful rather as comparing the various lubricants one with another than as representing, by a mere process of multiplication, what would be the behaviour of the ship on her cradles, because, for the reason already pointed out in the case of wooden sliding-surfaces, the lubrication would be more and more rubbed away as more of the cradle passed over it; thus the experimental cradle, when tried with lubrication, represented rather the behaviour of the front part of the cradle than that of the whole. Had the ship herself been moved uninterruptedly down the ways, the state of things would have been something between good lubrication and none at all. As the under sides of the bars were lubricated, any motion of one end of the ship before the other would tend to move the bars sideways over the rails, and so to spread the lubrication, and to pick it up and re-deposit it. Mr. Brunel thought but little of the black-leading of the ways, considering that it would be rubbed off by the leading bars of the cradles; but a very little lubrication on metal surfaces is sufficient; and doubtless, had the ship been moved continuously down the ways, considerable assistance would have been derived from the lubrication which was applied.
The results of the experiment were curious. The generally received notion is, that friction between rubbing surfaces is independent of the velocity; that is to say, that whether a body be moving fast or slow within reasonable limits, the retardation due to friction is the same; that if a body be sliding at a given velocity, whether that velocity be great or small, a drag of a certain number of pounds will keep it moving at that velocity. It was, however, always understood that a greater force was necessary to start a body from rest, to overcome adhesion. The experiments made with the experimental cradle distinctly showed that any rule as to friction being constant at different velocities was untrue. It was evident that, as the speed increased, the power required to overcome the friction became less. No exact records are extant of the experiments made with this experimental cradle before the launch; they were, however, repeated during the launch with great care, and the results very carefully analysed. The experiments showed generally that the tractive force, including the action of gravity, was never more than ⅛, or less than 1/15, of the weight.
Although the experiments showed that the amount of friction in the case of the actual launch would lie between the limits above mentioned, they at the same time indicated that it would not probably approach either of those limits.
Shortly after the commencement of the launch, Mr. Brunel had the experimental cradle and ways re-erected. A very simple arrangement was fitted up, by which the forces at work at each period of the progress of the cradle in each experiment might be deduced. The results of these experiments, which, as may be supposed, were similar to those obtained in the commencement of the year, were most instructive; they showed quantitatively the decided diminution in friction which took place as the velocity increased, and the amount of that diminution. The apparatus was very simple. The experimental cradle, which has already been described, was made to slide down its ways by a chain attached to a suspended weight. The weight employed was generally about 5 cwt. After the cradle had run a certain distance, the weight reached the ground and the cradle proceeded with the momentum it had obtained. The velocity given to the cradle down the ways was measured in the following manner. A long piece of tape was coiled round a reel placed at the top of the inclined rails or experimental ways, so that it could revolve freely and pay out the tape as required. One end of this tape was attached to the cradle, so as to be drawn after it as it ran down the rails. The tape, as it ran off the reel, passed over a guiding board over which swung transversely a pendulum arranged to swing once every quarter of a second. At the lower end of this pendulum was attached a brush which was filled with paint; and as soon as the model cradle moved, the pendulum was set oscillating by a self-acting trigger arrangement. The pendulum in its oscillations made marks on the tape as it ran out at every quarter of a second of time. Thus, by an examination of the tape, could be determined the exact distance which had been passed over by the cradle during each quarter of a second of the time during which it was moving. The rate of progress being thus known, and the actuating force (gravity acting on the cradle and on the suspended weight) being also known, it will be understood that the exact amount of the resisting force, namely, friction, could be calculated exactly, and this for each moment and position of the descent of the experimental cradle.
The following results of these experiments were recorded in terms of the corresponding amounts of tractive force that would be required to produce similar results in the case of the ship and cradles, a weight of 12,000 tons.
| Force in tons required to move or restrain ship on incline of 1 in 12 |
Force in tons required to move a weight equal to the ship on similar ways, but on the level | |||||||||||
| Velocity, feet per second | 0 | 0 to 1 | ·75 | 1 | 1·5 | 2 to 3 | 0 | 0 to 1 | ·75 | 1 | 1·5 | 2 to 3 |
| RAILS AND CRADLE BARS. | Retarding | |||||||||||
| No. 1. Ample lubrication | 60 | 0 | 110 to 200 |
1,060 | 1000 { | 890 to 800 | ||||||
| No. 2. Medium lubrication | 120 | .. | .. | 0 | 60 | 1,220 | .. | .. | 1,000 | 940 | ||
| No. 3. Very little lubrication | 400 | 200 | .. | 0 | .. | 1,400 | .. | 1,200 | .. | 1,000 | .. | |
| No. 4. No lubrication { | 560 to 400 |
.. | .. | 0 | .. | 1,560 to 4,400 |
.. | .. | 1,000 | .. | ||
In every case where a velocity approaching to 3 feet per second was attained, whether the ways were lubricated or were quite dry, the model, though there was no tractive force acting on it other than that of gravity down the incline of 1 in 12, rapidly increased its speed till it reached the end of the ways.
These experiments are worthy of note for the contradiction, already referred to, which they gave to the received rules relative to friction. It will be seen by these experiments, and as will hereinafter appear from the results of the movements of the ship herself during the launch, that with different degrees of velocity very great variation in the friction was apparent, amounting to a difference of about thirty per cent. in the case of unlubricated surfaces, according as the velocity was nearly nil or was 1·5 feet per second, which is a comparatively small velocity. The friction in this case was, on a weight of 12,000 tons, 1,500 tons at a very low velocity, and but 1,000 tons at a velocity of 1·5 feet a second or about one mile an hour, the friction at the very low velocity being fifty per cent. greater than that at one mile an hour.
As soon as the ship began to move by slides, the recording apparatus of tape and pendulum was applied to record the nature of the ship’s movements. This apparatus was similar to that already described. The tape was attached to the bottom of the ship under her centre of gravity, and recorded the rate of retardation of the ship when left to herself after the motive appliances had, with the exception of gravity, ceased to act; and the amount of friction acting to retard the ship was determined with a very considerable amount of accuracy.
The best experiments made were on December 7, 8, and 10; and the results are very interesting. The dirt and rust of the sliding surfaces had increased the adhesion very much, and a considerable force was necessary to start the ship. There being no good pressure gauges to the presses, it was impossible to decide exactly what was the force required to start the ship, for of course the tape record gave no information on this point; but there is no doubt it was considerable, probably 800 or 1,000 tons in addition to gravity, and was thus far greater than the force to start that had been observed with the experimental cradle.
But the remarkable fact was that, notwithstanding the deterioration of the sliding-surfaces as evinced in the increased difficulty in starting, the friction, when once motion was established, was proved not to be very largely in excess of that which had been exhibited in the experiments.
In the various experiments tried, it was shown that when the ship had a velocity of between 6 and 8 inches per second, the amount of friction was only about 100 tons in excess of the action of gravity down the incline; while as the velocity became less, the friction became greater, till, as the velocity became smaller, the friction increased from 200 to 300 tons in excess of the action of gravity.
The results obtained by the observations made on the motion of the ship having shown that its behaviour when in motion accorded with that of the experimental cradle, there is every reason to believe that if the ship had ever attained a velocity of 1½ to 2 feet per second, which might have happened had the river tackle acted well, the friction would, as in the experimental cradle, have become less than the action of gravity down the slope, and the brakes would have had to be employed to check the motion.
Letter from Mr. Brunel to W. Froude, Esq.