298.—It is considered a precaution with an ordinary level to mark one leg of the tripod and always place this in the same position to the staff. Thus, if the marked leg is placed to the forward staff at first, it is put at the next station backward to the back staff. This corrects any general error from defective work in the instrument and want of adjustment; and if the staves are placed at equal stations any instrumental defect whatever, to act cumulatively upon a distant station, is then prevented, as this principle produces an alternate plus and minus error.

299.—Differences of true level have been found between working southward towards the sun from working northward from it, which are caused by the expansion of the instrument and bubble tube upon the side heated by his rays. These matters of higher refinement may be followed in some of our best works on levelling. Most excellent instructions in this matter will be found in the appendix of A Manual of Surveying for India,[13] in a paper by Colonel J. T. Walker, R.E., F.R.S., etc., of the great Trigonometrical Survey of India, wherein levels have been carried across from ocean to ocean for over 1500 miles of land surface.

300.—Levelling Books which record the levels as they are taken are considerably varied in form, much influenced, no doubt, by the method pursued by the civil engineer for the execution of his work. The illustration, Fig. 115, shows the most general forms, but there are many others.

301.—Entries are very generally made in levelling books in black lead. Faber's artists' pencils, which require no cutting, are very generally used, No. 2 being black and moderately hard. It is very convenient to carry a small file for sharpening the lead frequently. In the author's surveyor's knife, described further on, a file forms one of the blades.

302.—Where it is desirable to make the original levelling book readings permanent for reference or otherwise, they are very commonly written in ink, Morrell's registration ink being very generally used, or the author's drawing ink answers; this being permanent is not liable to corrode the pen, nor permit the writing to be effaced in any degree by moisture.

Fig. 115.—Specimens of levelling books, 1/3 scale.

303.—The Ink Bottle mostly used is that known as the excise bottle. This is of a smooth, oval form, covered with black leather, with a tab and buttonhole to hang upon a button of the coat, Fig. 116. One of the numerous fountain pens is now generally used instead of the bottle described.