Fig. 144. An arrangement of stops for a spur-gear.

Fig. 145. A reciprocating circular motion of the top arm makes its attached pawl produce an intermittent circular motion of the crown-ratchet, or ray-wheel.

Fig. 146 represents varieties of stops for ratchet-wheel.

Fig. 147. Intermittent circular motion is imparted to the wheel A by the continuous circular motion of the smaller wheel with one tooth.

Fig. 148. A dynamometer, or instrument used for ascertaining the amount of useful effect given out by any motive power. It is used as follows: A is a smoothly turned pulley, secured on a shaft as near as possible to the motive power. Two blocks of wood are fitted to this pulley, or one block of wood and a series of straps fastened to a band or chain, as in the drawing, instead of a common block. The blocks, or block and straps, are so arranged that they may be made to bite or press upon the pulley by means of the screws and nuts on the top of the lever D. To estimate the amount of power transmitted through the shaft, it is only necessary to ascertain the amount of friction of the drum A when it is in motion, and the number of revolutions made. At the end of the lever D is hung a scale B, in which weights are placed. The two stops C C are to maintain the lever as nearly as possible in a horizontal position. Now, suppose the shaft to be in motion, the screws are to be tightened and weights added in B, until the lever takes the position shown in the drawing, at the required number of revolutions. Therefore the useful effect would be equal to the product of the weights, multiplied by the velocity at which the point or suspension of the weights would revolve if the lever were attached to the shaft.

Fig. 149 represents a pantagraph for copying, enlarging and reducing plans. One arm is attached to and turns on the fixed point C. B is an ivory tracing point, and A the pencil. Arranged as shown, if we trace the lines of a plan with the point B, the pencil will produce it double the size. By shifting the slide attached to the fixed point C and the slide carrying the pencil along their respective arms, the proportions to which the plan is traced will be varied.

Fig. 150. Anti-friction bearing. Instead of a shaft revolving in an ordinary bearing, it is sometimes supported on the circumference of wheels. The friction is thus reduced to the least amount.

Fig. 151. Releasing hook used in pile-driving machines. When the weight W is sufficiently raised, the upper ends of the hooks A, by which it is suspended, are pressed inward by the side of the slot B, in the top of the frame; the weight is thus suddenly released, and falls with accumulating force on to the pile-head.

Fig. 152. A and B are two rollers which require to be equally moved to and fro in the slot C. This is accomplished by moving the piece D, with oblique slotted arms, up and down.

Fig. 153. Centrifugal check-hooks, for preventing accidents in case of the breakage of machinery which raises and lowers workmen, or ores, in mines. A is a framework fixed to the side of the shaft of the mine, and having fixed studs D, attached. The drum on which the rope is wound is provided with a flange B, to which the check-hooks are attached. If the drum acquires a dangerously rapid motion, the hooks fly out by centrifugal force, and one or other, or all of them, catch hold of the studs D, arrest the drum, and stop the descent of whatever is attached to the rope. The drum ought besides this, to have a spring applied to it, otherwise the jerk arising from the sudden stoppage of the rope might produce a worse effect than its rapid motion.

Fig. 154. A sprocket-wheel to drive or to be driven by a chain.

Fig. 155. A combination movement, in which the weight W moves with a reciprocating movement, the down-stroke being shorter than the up-stroke. B is a revolving disc, carrying a drum which winds around itself the cord D. An arm C is jointed to the disc and to the upper arm A, so that when the disc revolves, the arm A moves up and down, vibrating on the point G. This arm carries with it the pulley E. Suppose we detach the cord from the drum, tie it to the fixed point, and then move the arm A up and down. The weight W will move the same distance, and in addition the movement given it by the cord, that is to say, the movement will be doubled. Now, let us attach the cord to the drum, and revolve the disc B, and the weight will move vertically with the reciprocating motion, in which the down-stroke will be shorter than the up-stroke, because the drum is continually taking up the cord.

Figs. 156, 157. The first of these figures is an end view, and the second is a side view of an arrangement or mechanism for obtaining a series of changes in velocity and direction. D is a screw on which is placed eccentrically the cone B, and C is a friction roller, which is pressed against the cone by a spring or weight. Continuous rotary motion, at a uniform velocity of the screw D carrying the eccentric cone, gives a series of changes of velocity and direction to the roller C. It will be understood that during every revolution of the cone the roller would press against a different part of the cone, and that it would describe thereon a spiral motion, the movement in one direction being shorter than that in the other.

Fig. 158. An engine governor. The rise and fall of the balls K are guided by the parabolic curved arms B, on which the anti-friction wheels L run. The rods F, connecting the wheel L with the sleeve, move it up and down the spindle C D.

Fig. 159. Toe and lifter for working poppet-valves in steam engines. The curved toe on the rock-shaft operates on the lifter attached to the lifting rod to raise the valve.

Fig. 160. Mercurial compensation pendulum. A glass jar of mercury is used for the bob or weight. As the pendulum-rod is expanded lengthwise by increased temperature, the expansion of mercury in the jar carries it to a greater height therein, and so raises its centre of gravity relatively to the rod sufficiently to compensate for downward expansion of the rod. As rod is contracted by a reduction of temperature, contraction of mercury lowers it relatively to rod. In this way the centre of oscillation is always kept in the same place, and the effective length of pendulum always the same.

Fig. 161. Compound bar compensation pendulum. C is a compound bar of brass and iron, or steel brazed together with brass downward. As brass expands more than iron, the bar will bend upward as it gets warmer, and will carry the weights W, W, up with it, raising the centre of the aggregate weight M, to raise the centre of oscillation as much as elongation of the pendulum-rod would let it down.

Fig. 162. Watch regulator. The balance-spring is attached at its outer end to a fixed stud R, and at its inner end to staff of balance. A neutral point is formed in the spring at P, by inserting it between two curb-pins in the lever, which is fitted to turn on a fixed ring concentric with staff of balance, and the spring only vibrates between this neutral point and staff of balance. By moving lever to the right, the curb-pins are made faster, and by moving it to the left, an opposite effect is produced.

Fig. 163. Compensation balance. t, a, t´ is the main bar of balance, with timing screws for regulation at the ends. t and t´ are two compound bars, of which the outside is brass and the inside steel, carrying weights b, b´. As heat increases, these bars are bent inward, diminishing the inertia of the balance. As the heat diminishes, an opposite effect is produced. This balance compensates both for its own expansion and contraction, and that of the balance-spring.

Fig. 164. Parallel ruler, consisting of a simple straight ruler B, with an attached axle C, and a pair of wheels A A. The wheels, which protrude but slightly through the under side of the ruler, have their edges nicked to take hold of the paper and keep the ruler always parallel with any lines drawn upon it.

Fig. 165. Compound parallel ruler, composed of two simple rulers A, A, connected by two crossed arms pivoted together at the middle of their length, each pivoted at one end to one of the rulers, and connected with the other one by a slot and sliding pin, as shown at B. In this the ends as well as the edges are kept parallel. The principle of construction of the several rulers represented is taken advantage of in the formation of some parts of machinery.

Fig. 166. A simple means of guiding or obtaining a parallel motion of the piston rod of an engine. The slide a moves in and is guided by the vertical slot in the frame, which has been planed to a true surface.

Fig. 167. Parallel motion for direct-action engines. In this, the end of the bar B C is connected with the piston-rod, and the end B slides in a fixed slot D. The radius bar F A is connected at F with a fixed pivot, and at A midway between the ends of B C.

Fig. 168. Oscillating engine. The cylinder has trunnions at the middle of its length, working in fixed bearings, and the piston rod is connected directly with the crank, and no guides are used.

Fig. 169. Inverted oscillating or pendulum engine. The cylinder has trunnions at its upper end, and swings like a pendulum. The crank shaft is below, and the piston rod connected directly with crank.

Fig. 170. Section of disc-engine. Disc-piston, seen edgewise, has a motion substantially like a coin when it first falls after being spun in the air. The cylinder heads are cones. The piston rod is made with a ball to which the disc is attached, said ball working in concentric seats in cylinder-heads, and the left-hand end is attached to the crank arm or fly-wheel on end of shaft at left. Steam is admitted alternately on either side of piston.

Fig. 171. The gyroscope, or rotascope, an instrument illustrating the tendency of rotating bodies to preserve their plane of rotation. The spindle of the metallic disc C is fitted to return easily in bearings in the ring A. If the disc is set in rapid rotary motion on its axis, and the pintle F at one side of the ring A is placed on the bearing in the top of the pillar G, the disc and ring seem indifferent to gravity, and instead of dropping begin to revolve about the vertical axis.

Fig. 172. Bohnenberger's machine, illustrating the same tendency of rotating bodies. This consists of 3 rings, A, A´, A2, placed one within the other, and connected by pivots at right angles to each other. The smallest ring, A2, contains the bearings for the axis of a heavy ball B. The ball being set in rapid rotation, its axis will continue in the same direction, no matter how the position of the rings may be altered; and the ring A2, which supports it, will resist a considerable pressure tending to displace it.

Fig. 173. What is called the gyroscope governor, for steam-engines, introduced by Alban Anderson in 1858. A is a heavy wheel, the axle B B´ of which is made in two pieces connected together by a universal joint. The wheel A is on one piece B, and a pinion I on the other piece B´. The piece B is connected at its middle by a hinge-joint with the revolving frame H, so that variations in the inclination of the wheel A will cause the outer end of the piece B to rise and fall. The frame H is driven by bevel gearing from the engine, and by that means the pinion 1 is carried round the stationary toothed circle G, and the wheel A is thus made to receive a rapid rotary motion on its axis. When the frame H and wheel A are in motion, the tendency of the wheel A is to assume a vertical position, but this tendency is opposed by a spring L. The greater velocity of the governor, the stronger the tendency, above mentioned, and the more it overcomes the force of the spring, and the reverse. The piece B is connected with the valve rods by rods C, D, and the spring L is connected with the said rods by levers N and rod P.

Fig. 174. Pair of edge runners or chasers for crushing or grinding. The axles are connected with vertical shaft, and the wheel or chasers run in an annular pan or trough.

Fig. 175. Rotary motion of shaft from treadle by means of an endless band running from a roller on the treadle to an eccentric on the shaft.

Fig. 176. Tread-wheel horse-power turned by the weight of an animal attempting to walk up one side of its interior; has been used for driving the paddle-wheels of ferry-boats and many other purposes. The turn-spit dog used also to be employed in such a wheel in ancient times for turning meat while roasting on a spit.

Fig. 177. The treadmill, employed in jails in some countries for exercising criminals condemned to labour, and employed in grinding grain; turns by weight of person stepping on tread-boards on periphery. This is supposed to be a Chinese invention, and it is still used in China for raising water for irrigation.

Fig. 178. A. B. Wilson's four-motion feed, used in Wheeler and Wilson's, Sloat's, and other sewing machines. The bar A is forked, and has a second bar B, carrying the spur or feeder, pivoted in the said fork. The bar B is lifted by a radial projection on the cam C, at the same time the two bars are carried forward. A spring produces the return stroke, and the bar B drops of its own gravity.

Fig. 179. Mechanical means of describing parabolas, the base, altitude, focus, and directrix being given. Lay straight edge with near side coinciding with directrix, and square with stock against the same, so that the blade is parallel with the axis, and proceed with pencil in bight of thread, as in the preceding.

Fig. 180. Mechanical means of describing hyperbolas, their foci and vertices being given. Suppose the curves two opposite hyperbolas, the points in vertical dotted centre line their foci. One end of thread being looped on pin inserted at the other focus, and other end held to other end of rule, with just enough slack between to permit height to reach vertex when rule coincides with centre line. A pencil held in bight, and kept close to the rule, while latter is moved from centre line, describes one-half of parabola; the rule is then reversed for the other half.

Fig. 181. Cyclograph for describing circular arcs in drawings where the centre is inaccessible. This is composed of three straight rules. The cord and versed sine being laid down, draw straight, sloping line from ends of former to top of latter; and to these lines lay two of the rules crossing at the apex. Fasten these rules together, and another rule across them to serve as a brace, and insert a pin or point at each end of chord to guide the apparatus, which, on being moved against these points, will describe the arc by means of pencil in the angle of the crossing edges of the sloping rules.

Fig. 182. Proportional compasses used in copying drawings on a given larger or smaller scale. The pivot of compasses is secured in a slide which is adjustable in the longitudinal slots of legs, and capable of being secured by a set screw; the dimensions are taken between one pair of points and transferred with the other pair, and thus enlarged or diminished in proportion to the relative distances of the points from the pivot. A scale is provided on one or both legs to indicate the proportions.

Fig. 183. One of the many forms of rotary engine. A is a cylinder having the shaft B pass centrally through it. The piston C is simply an eccentric fast on the shaft, and working in contact with the cylinder at one point. The induction and eduction of steam take place as indicated by arrows, and the pressure of the steam on one side of the piston produces its rotation and that of the shaft. The sliding abutment D, between the induction and eduction ports, moves out of the way of the piston to let it pass.

Fig. 184. Another form of rotary engine, in which there are two stationary abutments D, D, within the cylinder; and the two pistons A, A, in order to enable them to pass the abutments, are made to slide radially in grooves in the hub C of the main shaft B. The steam acts on both pistons at once, to produce the rotation of the hub and shaft. The induction and eduction are indicated by arrows.

Fig. 185. Jonval turbine. The shutes are arranged on the outside of a drum, radial to a common centre, and stationary within the trunk or casing b. The wheel c is made in nearly the same way; the buckets exceed in number those of the shutes, and are set at a slight tangent instead of radially, and the curve generally used is that of the cycloid or parabola.

Fig. 186. A method of obtaining a reciprocating motion from a continuous fall of water, by means of a valve in the bottom of the bucket which opens by striking the ground, and thereby emptying the bucket, which is caused to rise again by the action of a counterweight on the other side of the pulley over which it is suspended.

Fig. 187. Overshot water-wheel.

Fig. 188. Undershot water-wheel.

Fig. 189. Breast-wheel. This holds intermediate place between overshot and undershot wheels; has float-boards like the former, but the cavities between are converted into buckets by moving in a channel adapted to circumference and width, into which water enters nearly at the level of axle.

Fig. 190. Horizontal overshot water-wheel.

Fig. 191. A plan view of the Fourneyron turbine water-wheel. In the centre are a number of fixed curved chutes, or guides, A, which direct the water against the buckets of the outer wheel B, which revolves, and the water discharges at the circumference.

Fig. 192. Warren's central discharge turbine, plan view. The guides A are outside, and the wheel B revolves within them, discharging the water at the centre.

Fig. 193. Volate wheel, having radial vanes A, against which the water impinges and carries the wheel around. The scroll or volute casing B confines the water in such a manner that it acts against the vanes all around the wheel. By the addition of the inclined buckets c, c, at the bottom, the water is made to act with additional force as it escapes through the openings of said buckets.

Fig. 194. Barker, or reaction mill. Rotary motion of central hollow shaft is obtained by the reaction of the water escaping at the ends of its arms, the rotation being in a direction the reverse of the escape.

Fig. 195 represents a trough divided transversely into equal parts, and supported on an axis by a frame beneath. The fall of water filling one side of the division, the trough is vibrated on its axis, and at the same time that it delivers the water the opposite side is brought under the stream and filled, which in like manner produces the vibration of the trough back again. This has been used as a water-meter.

Fig. 196. Persian wheel, used in Eastern countries for irrigation. It has a hollow shaft and curved floats, at the extremities of which are suspended buckets or tubs. The wheel is partly immersed in a stream acting on the convex surface of its floats; and as it is thus caused to revolve, a quantity of water will be elevated by each float at each revolution, and conducted to the hollow shaft at the same time that one of the buckets carries it full of water to a higher level, where it is emptied by coming in contact with a stationary pin placed in a convenient position for tilting it.

Fig. 197. Machine of ancient origin, still employed on the river Eisach, in the Tyrol, for raising water. A current keeping the wheel in motion, the pots on its periphery are successively immersed, filled, and emptied into a trough above the stream.

Fig. 198. Application of Archimedes screw for raising water, the supply stream being the motive power. The oblique shaft of the wheel has extending through it a spiral passage, the lower end of which is immersed in water, and the stream acting upon the wheel at its lower end produces its revolution by which the water is conveyed upward continuously through the spiral passage and discharged at the top.

Fig. 199. Common lift pump. In the upper-stroke of piston or bucket the lower valve opens and the valve in piston shuts; air is exhausted out of suction pipe, and water rushes up to fill the vacuum. In down stroke lower valve is shut and valve in piston opens, and the water simply passes through the piston. The water above piston is lifted up, and runs over out of spout at each up stroke. This pump cannot raise water over thirty feet high.

Fig. 200. Ordinary force pump, with two valves. The cylinder is above water, and is fitted with solid piston; one valve closes outlet pipe, and other closes suction pipe. When piston is rising suction-valve is open, and water rushes into cylinder, outlet valve being closed. On descent of piston suction valve closes, and water is forced up through outlet valve to any distance or elevation.

Fig. 201. Double-acting pump. Cylinder closed at each end, and piston-rod passes through stuffing-box on one end, and the cylinder has four openings covered by valves, two for admitting water and like number for discharge. A is suction pipe, and B discharge pipe. When piston moves down, water rushes in at suction valve 1, on upper end of cylinder, and that below piston is forced through valve 3 and discharge pipe B; on the piston ascending again, water is forced through discharge valve 4, on upper end of cylinder, and water enters lower suction valve 2.

Fig. 202. Common windmill, illustrating the production of circular motion by the direct action of the wind upon the oblique sails.

Fig. 203. Ordinary steering apparatus. Plan view. On the shaft of the hand wheel, there is a barrel on which is wound a rope, which passes round the guide-pulleys, and has its opposite ends attached to the tiller, or lever, on top of the rudder; by turning the wheel, one end of the rope is wound on and the other left off, and the tiller is moved in one or the other direction, according to the direction in which the wheel is turned.

Fig. 204. Capstan. The cable or rope wound on the barrel of the capstan is hauled in by turning the capstan on its axis by means of handspikes or bars inserted into holes in the head. The capstan is prevented from turning back by a pawl attached to its lower part and working in a circular ratchet on the base.

Fig. 205. Lewis bolt for lifting stone in building. It is composed of a central taper-pin or wedge, with two wedge-like packing pieces arranged one on each side of it. The three pieces are inserted together in a hole drilled into the stone, and when the central wedge is hoisted upon it, it wedges the packing pieces out so tightly against the sides of the hole as to enable the stone to be lifted.

Fig. 206. Tongs for lifting stones. The pull on the shackle which connects the two links causes the latter so to act on the upper arms of the tongs as to make their points press themselves against or into the stone. The greater the weight, the harder the tongs bite.

III

THE WEATHER AND INDOOR WORK

The measure of rainfall varies considerably within comparatively small areas, and this renders it no easy matter to get correct figures, so that the nearest records are those taken from a number of gauges within a limited district, and generalized. The more this is done, the less will be the inaccuracy in referring to the rainfall of any particular district or country.

As the fall of rain is always measured in inches gauges are made to indicate the equivalent of a cubic inch of rain on the surface of the earth. The simplest form of rain-gauge is a square or circular box or jar with a perfectly flat bottom and perpendicular sides (see Fig. 207). If the depth of water in such a gauge be measured after a fall of rain, one can ascertain in inches, or parts of an inch, the amount of rain that has fallen on the surface of the earth. Care must be taken to have the edge of the gauge thin and free from dents, the sides perpendicular and the bottom of the jar perfectly flat, for though in one measurement these irregularities may not make much difference, they would lead to a very decided error in a large number of measurements. Evaporation is also liable in such a gauge to give rise to errors, and extraneous matters are easily introduced. The better rain-gauges are constructed to avoid these contingencies, as far as possible and to depend only on the area of entry for the accuracy of the measurements. This area may be a square, but is usually circular for convenience. The circle must be accurate, and its area is then easily calculated, so that one can estimate the amount of rainfall, however large the receiving vessel may be. The edge of the circle, which may be made of copper, more durable than iron, must be sharp, with an overlapping rim to prevent raindrops from being whirled out of the receiver, and connected by a shoulder to a funnel, which directs the water into the receiver. This may be a glass bottle fitted with a cork to hold the funnel firmly, and prevent leakage between the outside of the funnel and the neck of the bottle (see Fig. 208). A more convenient receiver, and one less likely to be broken, is a round tin case of convenient size, with a top fitting accurately under the overlapping edge of the funnel-shaped cover. In this large receiver may be placed a small tin mug, with a lip just under the funnel, for conveniently measuring small quantities of rain, and preventing waste by evaporation. Any overflow from the mug will be caught in the large receiver (see Fig. 209). The circle of entry may, of course, be of any size; but one whose diameter is between 4 or 8 inches will be most convenient. Make the circle determine its area by careful measurement, using the following formula: D² × .7854 = area, each square inch will give cubic inches for area. Take this amount of water and pour it into a glass, marked at the top of the water, and then divide the intervening space between this mark and the bottom into 100 equal parts. This graduated glass will give the rainfall in inches and 100ths of an inch. As an inch glass is somewhat cumbersome, a half-inch glass is usually sent out with a rain-gauge. It may, however, be sometimes convenient to use an ordinary ounce measure, as graduated glass measures, when broken, are not always easily replaced; so that it may be necessary to find the corresponding relation between the cubic inches of receiving area and ounces and drachms. To do this, we will suppose the diameter of the circular top of gauge to be 4.7 inch; this squared = 22.09, multiplied by .7854 = 17.349486, divided by 1.733 (an ounce avoir. = 1.733 c. in.) = 10.011 oz. avoir.

Now if the rainfall is collected daily at a certain time in an ounce measure, the amount may easily be recorded in inches by reference to the accompanying table:

A similar calculation can be made and table prepared for any larger circle of entry by the same method.

The amount of rainfall in any country is a matter of great importance to that country, and, like the rise of the Nile in Egypt, it indicates the coming state of the crops. If we have too small a rainfall, drought and withered crops follow, and if we get too great a fall of rain, drowned out crops, and disastrous floods occur, so you see how necessary it is that those people who are elected to look after the welfare of a nation, should keep posted on matters of rainfall in all its phases. In India, China and some other parts of the world the question of rainfall is one of life and death to the people, and most of the great famines of the past have been due to the small rainfall. Hundreds of thousands of people used to perish by famine and disease year after year. Much of this danger from shortage of rain has happily been avoided in India by the efforts of the British government, which has inaugurated and carried out great schemes of irrigation and artificial waterways to prevent the recurrence of famine from drought. Our own government also is expending large sums of money on irrigation plans now being executed in Arizona, Texas, Colorado and other states, which will render immense territories fit for cultivation, which would otherwise have remained barren and of no use. The matter of rainfall is of the highest importance to a nation and to the men and beasts inhabiting it.

"Will it rain to-day?" is a question frequently asked, as regards the weather, showing how important the subject is, and while I am talking on it, it may not be amiss to make a few remarks regarding the formation and distribution of rain, as formulated by learned meteorologists. We are told that the two great causes of rain are the sun and the ocean—the latter, of course, includes the great lakes and rivers—and since these two factors may be taken as constant, it follows that the rainfall over the earth as a whole will always be constant, while the local variations will be due to local conditions. The rain which falls on this continent is drawn up by the sun from the various sources, but the conditions which cause its precipitation may be said to be local. To your imagination may be left the tracing of the journey of the rain drops back to the ocean again. The starting points in considering the causes of rain are, therefore, heat and moisture. From the surface of land and water moisture is continually evaporating into the atmosphere, and the higher the temperature of the air the more watery particles it can hold. If any reduction in the temperature of this saturated air should take place, the vapour becomes visible as fog, mist, or cloud, and it is from this vapour that the rain drops are formed. Recent research says that these watery particles require minute dust atoms as nuclei before they can form, and it has been estimated, by experiment, that there are one thousand millions of them in a cubic foot of saturated air, though their total weight amounts to only 3 grains. Accepting these figures, the mathematically inclined may be told that it would require a cloud three miles thick to produce one inch of rainfall. But before these watery particles can fall to the earth as rain, they must first form into rain drops, and the question arises, how are rain drops formed?

These watery particles pass into the air by evaporation, and there are several ways by which the reduction in temperature necessary to render them visible can be brought about. It may take place through contact with a colder body of air, by expansion, or by a reduction of pressure owing to a rise in altitude. Clouds are said to be formed by this last method, for a volume of hot air rises higher and higher until it presently reaches a point when its contained vapour condenses, and becomes visible as a cloud. Meteorologists repeat one of these processes in the laboratory, by releasing from pressure damp air placed in a convenient glass globe, and are able to see something of the methods of cloud formation. It has been customary to speak of a cloud as being composed of watery particles floating motionless in the upper air; but although it may appear unchanged in form, it is all movement. So soon as ever a cloud is formed, its particles of moisture commence to fall slowly, the rate of fall being in proportion to the diameter of the particles, and this is due to the slight resistance the air makes to such very small atoms. In passing, it may be said that one observer estimates the diameter of these particles as from .00033 inch to .00025 inch. The component parts of a cloud are always in motion and recognizing this fact it becomes possible to take the first step in considering the formation of a raindrop.

An easy way out of the difficulty of explaining the formation of a raindrop, is to say that, since clouds are so often of two opposite electric potentials, there is always a continuous bombardment of watery particles taking place, and some of these must unite and fall as rain. The meteorologist is always tempted to call in electricity as an agency whenever he is anxious to discover a cause for some particular phenomenon. This often explains one mystery by another. The production of rain, snow, and hail has for many years been explained by vaguely ascribing them to the action of electricity, without any information being forthcoming as to the precise way in which this action takes place. Meteorologists are at present attempting to find a more satisfactory explanation. Another theory is that the particles of moisture in a cloud, like all other objects, radiate heat, and, growing cold, condense moisture upon their surfaces, thereby increasing in weight until they assume the proportions of a drop. This seemed a reasonable explanation of the formation of a rain drop until modern research decided that whenever moisture is condensed, latent heat is set free, so that all moisture deposited on a watery particle only serves to raise its temperature, and cause evaporation of the moisture thus acquired. The particles of water could not by this means grow to the full estate of a rain drop, and the theory is being gradually abandoned.

A rain drop is, according to modern meteorologists, explained in a very simple way. It has been seen how the hot, damp air is formed into a cloud, and also how the minute particles of water at once commence to fall slightly earthwards. Now these little particles as they pass into a warm layer of air would soon be evaporated, and would never reach the earth at all. Their downward journey, however, is often through a cloud many miles thick, and the most modern and simple theory is that in this journey they overtake some of their fellows, and the joined particles increase their rate of travel, overtake more and more particles until they presently become heavy enough to take the final plunge to earth. Were it possible to be just beneath a cloud, an observer would see rain drops coming from it of all sizes. The same process goes on in drops, which trickle down a window pane, or in the effervescing globules in a bottle of seltzer water. In the latter instance, the process is reversed, for the globules are seen overtaking one another in an upward direction. There are many points in favour of this theory of the formation of rain drops, and at least it gets rid of those elaborate complications, electricity and condensation. With respect to the formation of rain by the impinging of clouds upon the tops of cold mountains in the northwest, one authority argues that moisture is in these circumstances not condensed solely because of the contact with the cold hills; that rain there is due to a mechanical cause, the watery particles being squeezed together by the grinding effect of the clouds on the sides of the mountains in such a way that they coalesce, and fall as drops.

A rain drop's roundness is due to the action of capillarity. Just as a circle made by dropping a stone into water owes its shape to the fact that the force is able to act equally in all directions, so a rain drop is spherical, owing to similar untrammelled action on the part of capillarity. These are some of the explanations of the formation of a rain drop, but meteorologists still have the subject under consideration.

The periods of rainfall are divided broadly into times of drought and times of flood, and it is in these matters that meteorology is seen in its practical aspect. Some people ask, "Where does all the rain come from?" Others are surprised that rainfall totals up to such large quantities.

A fall of rain to a depth of one inch over a very limited area, represents millions of gallons, but in spite of this vast quantity of falling water, many times multiplied if the annual rainfall be taken into account, there still are water famines. The question has often been debated whether man can modify climate or effectively tamper with the processes which produce rain. Rain making has not, so far, been a success, though the firing off of heavy guns has been tried, along with the legitimate avocations of the meteorologist. The afforesting or deforesting of a district has, however, a marked effect upon rainfall. Three notable instances are Ascension Island, Malta, and the neighbourhood of the Suez Canal, where the planting of trees seems to have had the result of increasing the rainfall. The effect of trees is felt more in the storage of rain water, while leaves and roots serve to retain moisture that would otherwise quickly drain away. A hill may be converted into a sponge by the judicious planting of trees. The question of the storage of rain water becomes more pressing each year, and the longer the settlement is put off, the more difficult will decision become. Engineers called upon to prevent floods and to conserve rain water reply, "Save our forests, cover the land with trees."

The fact that such problems arise, serve to show how great is the amount of water formed by the continual falling of the tiny raindrops. As long as this beneficent downpouring is allowed to drain away unused or uncontrolled, so long will droughts annoy and water famines bring distress.

In recording weather conditions, symbols are sometimes used in order to shorten reports and, while not universal, most nations adopt these: The symbol for rain is ●, a small circle filled in; for lightning [o]; for thunder T, while the two latter combined make T[o], the symbol for a thunder-storm. Nearly every weather component has a distinctive symbol, and since a great part of the meteorologist's work consists in going over records of observations to search for the number of times the different phenomena occur during each week or month, the task is much simplified when observers employ the symbols, as it is easier to pick out a symbol from a printed or written page than it is to recognize a word. These symbols, moreover, have been agreed upon as a sort of international notation, and make it easier for the meteorologists of different countries to understand the records of foreign meteorological services.

Everybody does not know the Russian word for snow, or the Dutch for hail, or the Bosnian for rain, but all who run, may read when "snow" is universally written, and hail represented by a wedge-shaped figure with lines drawn across. Time and space being limited, nearly all published records of weather merely set forth the number of days throughout the year on which the different phenomena occurred, and should snow, hail or thunder happen two or three times in one day, it would still be counted only as one day. The yearly totals, therefore, show the number of days on which these conditions have been observed. It is now an almost universal custom to count .01 inches or more during the twenty-four hours as a day of rain. Accordingly, where observers read their rain-gauge to three places of decimals, that on which less than .005 inch fell would not be counted as a rainy day. Smaller amounts would, however, be included in the total. Dew may sometimes fall to the amount of .01 in. or more; and that is counted as a rainy day, the rule being to consider the amount of precipitation, irrespective of the manner in which it has fallen. If you wish to make these observations comparable with published records you would do well to conform to these rules.

HAIL

Hail, the next weather component to be considered, presents many difficulties when the attempt is made to explain its origin and formation. Those who have anything to do with scientific matters are well acquainted with the hypothesis, which explains a given fact, and in considering the subject of hail, the meteorologist hears of many hypotheses which are put forward as complete explanations of this phenomenon. Caution is, therefore, to be exercised and every reported statement severely questioned. Remembering the aphorism: "The man or boy who never makes a mistake will never make anything," meteorologists have attacked the question of hail formation, and, although many mistakes have probably been made, the subject has lost a good deal of its mystery. For many years, it was customary to be content with a recognition of the fact that hail and lightning very often occur together, and the conclusion was drawn that the one was in some way responsible for the other. Sufficient corroboration of this hypothesis was to some meteorologists, found in the fact that thunder and lightning are said to be almost unknown in the Arctic regions, and this supposed companion, hail, almost unknown. Roughly speaking, the assumption was that lightning, as it flashed through a cloud laden with watery particles, caused hail to form. Such an explanation only tended to make the subject more mysterious, and the question, How is hail formed? practically remained unanswered. Many simpler explanations of hail have been propounded as the result of modern research, and, like rain and lightning, it has been demonstrated that hail owes its origin to the movement of the minute watery particles found everywhere in the atmosphere.

The clouds from which hail fall are ordinarily of great height above the earth, 40,000 feet or even higher. These are the well-known cirrus. The first condition necessary to the formation of hail is a powerful ascending current of hot, moist air, which may condense its moisture in the shape of the large woolly cloud, known as cumulus. Such a cloud may be 100 cubic miles in volume, and as long as it retains its shape nothing is likely to fall from it to the earth beneath. Before the formation of a thunder-shower, cirriform fibres in some instances break away from the upper portion of this cloud, the electrical tension is lowered, and rain falls. The coalescing of the particles of moisture has a great deal to do with the changes which take place in a cloud. All these changes take place in the higher clouds in a marked degree, and the varying strata through which the watery particles pass in ascending to and descending from this great height bring about the violent change essential to the formation of hail. The necessary conditions for hail are, therefore, a powerful, hot, ascending current of air and great variation in the strata of the atmosphere as regards moisture and temperature. Mountains assist in forcing currents of air upwards, and one mass of air impinging on another is also thrown upwards, so that condensation of moisture rapidly takes place. A hail cloud may be described as a tower of hot air, from the top of which, vapor is ejected into a frosty region. Hot plains are accordingly the most favourable spots for the formation of hail, and in mountainous districts, more hail falls at a distance from the mountains than among them. Snow is observed in all latitudes and at all heights, but hail is confined to middle latitudes, and is rare in high latitudes. The places most affected by hail are those in which, the temperature and humidity of the air are high, while above, at a great height, there is a cold area below the temperature of freezing point; but, as in the case of the rain drop, before anything can be definitely stated, it must be shown how the particles of moisture coalesce to form hail.

SNOW

Snow is frozen water which falls instead of rain when the temperature is below the freezing point. The ultimate constituents of snow are tiny, six-pointed crystals of ice. They assume in combination a thousand different figures (Fig. 210), all exceedingly beautiful. Professor Tyndall has shown, further, that the ultimate particles of ice are also these six-pointed stars. The white colour of snow is caused by the commingling of rays of all the prismatic colours from the minute snow crystals. Separately the crystals exhibit different colours.