Surveying and Levelling Instruments, Theoretically and Practically Described. / For construction, qualities, selection, preservation, adjustments, and uses; with other apparatus and appliances used by civil engineers and surveyors in the field.

by William Ford Stanley

Larger image

181.—The Level Tube may form a Complete Instrument in itself.—In this case the lower surface is ground to a flat plane to rest on any plane surface. This level is generally contained in a small pocket case, and is most convenient for adjusting instruments to level. It is commonly used with the black glass artificial horizon, to be described.

182.—Mounting Tubes.—Level tubes when applied to instruments are generally mounted in brass covering tubes. Small level tubes under 2½ inches are conveniently mounted in such tubes with a fixing of slaked plaster of Paris inserted at each end of the brass tube. Larger level tubes should be bound round with thin paper pasted round the ends, which is allowed to get quite dry, to be afterwards fitted to the brass tube with a file. Fitted in this manner the tubes admit of adjustment to the difference of expansion of the metal and glass by change of temperature without distortion. There is no objection, however, to thoroughly fixing one end of the tube with plaster if the other be left free, and this is perhaps advisable for portable instruments.

It is convenient in mounting level tubes to place white glazed paper under the bubble to reflect the light that passes through it to ensure better observation.

183.—In Fixing Undivided Level Tubes, or replacing them in instruments, it is important to observe that the side with the test mark, which is a small ground facet, should be placed on the top.

184.—In the Use of Level Tubes generally, it is not well to have them of greater sensitiveness than the general construction of the instrument upon which they are placed permits. Thus the centre of a surveyor's level that may be under constant strain from the unavoidable inequality of the pressure of parallel plate screws, will appear never to reverse properly if it has a very sensitive bubble, the cause of the irregularity being entirely due to the distortion from the strain on the vertical axis of the instrument. The same irregularity occurs in a lesser degree with the Y's of a theodolite, where these and the collars become corroded by exposure. The optician often gets an undue amount of credit for perfecting such instruments when he has merely replaced the sensitive bubble by a dull one—that is by doing what is really in this case the best for the instrument.

185.—When an instrument that depends entirely upon the level for its possible working is to be used abroad, an extra tube should be taken, as the level tube is very generally more exposed and is more delicate than any other part of the instrument. The tube may not only be accidentally fractured with a slight bow, but even the heat of the sun's rays will sometimes burst it.

186.—Reading the Bubble.—The exact position of the capillary concave surface of the spirit in the tube is liable to deceive the observer by the difference of refraction and reflection it gives, whether the light is towards the right or left hand. To avoid this cause of error it is better, in sunlight, to hold a piece of white paper at a short distance over the end of the bubble during the observation taken of its terminal reading into the divided scale on the tube. It is also important to note that the observer should stand at right angles to the tube to see the position exactly where the upper capillary line of the spirit cuts, as the tube itself refracts the light unequally. It is not at all difficult to read the bubble if the observer stand over it; but generally, as it is mounted upon an instrument, it is at the height of the eye. In this position the hollow surface round the bubble, caused by the adhesion of the liquid to the sides of the glass tube, reflects the light in a manner that the hollow may be taken for the end of the bubble, and a false reading made. It is better if possible to take the convenient side reading first, and afterwards get a glance at the upper surface reading for certainty. In some cases this may be much assisted by the employment of a small mirror of about the size of a spectacle eye, which is carried open in the pocket, or, as the author has made it, it may close in a horn case with a pocket lens, as in the Fig. 52 shown below. C sheath, M mirror, L lens.

Fig. 52.—Pocket lens and mirror.