18 



ordinary instruments, but it must be renewed quite often. In the finer class oi 

 leveling instruments, the centers should be lubricated with oil only, us in transits. 



A great deal <>t annoyance is caused to the engineer if the eye-piece or the object- 

 slide of the telescope move too freely in their tubes, requiring a re-focussing of tin; 

 cross-wires and object at every revolution of the telescope in altitude. If the eye- 

 piece can be retained in its socket, with sufficient friction to keep it focussed to the 

 cross-wires, no matt or how much it may wabble otherwise, this imperfection (in old 

 instrument-) will not lead to any Inaccuracy, but if there is not sufficient friction to 

 keep it focussed to the win's, a little rendered tallow or marrow applied to its bearing 

 surt'ac.-s in most cases will remedy this evil. Wabbling in the object-slide, however, 

 leading to inaccuracy of collrination, or back-lash in its rack or pinion motion, can be 

 remedied only by a maker; but if the object-slide moves too freely in and out of its 

 tube only, this may be remedied by applying a little tallow to the bearing parts of 

 the rack and pinion, or by tightening the screw in the pinion-head. If not entirely 

 successful, a thin disk made of parchment, or a thin leather-washer, both .:_! 

 with tallow, and inserted between the flanges of the pinion-head and its socket. 

 will insure the desired result. These latter remarks apply to transit and level 

 telescopes of the customary design. In telescopes, where the object-glass is MM Mint cd 

 permanently to the telescope-tube, the eye-piece tube, containing the cross-irirrs, 

 becomes the slide with which to focus the object. Its motion must be in a lino 

 parallel to the optical axis. Any wabbling in this eye-piece slide would lead to in- 

 accuracy in sighting through the telescope, hence it requires the most careful 

 treatment on the part of the engineer. 



Care in the Use of Spirit-Levels. 



Spirit-levels are very susceptible to the least change in temperature, as will be 

 readily seen by the difference in the length of its bubble in varying temperatures. 

 Hence, to guard against inaccuracies from this source, it is necessary that the 

 bubble should lengthen symmetrically from the center of its graduated scale (sup- 

 posed to be made by the maker), and that both of its ends should be read. Suffi- 

 cient time must also be allowed for the bubble to settle before a reading is made. 



The fluid ordinarily used for levels is pure alcohol, and requires, according to 

 curvature, diameter and length of tube and length of bubble, from twenty sec- 

 onds to one minute to attain its equilibrium. The composition fluid used in our 

 levels for field instruments requires only from five to fifteen seconds of time ; those 

 filled with pure ether, a few seconds only. 



A great source of error in spirit-levels, however, increasing with their greater 

 sensitiveness, is occasioned by an unequal heating of the level-tube, as the l>ti/>l,lt> 

 will always move towards the warmer spot or end, thereby imparting to the instrument 

 an inaccurate position. This must be attributed to a changed condition in the 

 adhesiveness of the fluid in the level-tube, and not to a change in the form of 

 the tube itself. Therefore, to guard against inaccuracy resulting from sudden 

 changes of temperature, a spirit-level, while in use, should be protected from the 

 sun, and no part of it or its mounting should ever be touched with bare lingers; 

 neither should it be breathed upon, nor the face of the observer come too cl<< to 

 it. For this reason, in the finer instruments the mountings of our spirit-levels 

 are cloth-finished, and if the levels are detachable they arc provided with wooden 

 handles, as the case may require, and glass covers are placed over them whenevei 

 deemed necessary. 



If at anytime during the progress of field-work a spirit-level lias been improperly 

 exposed, it is best to cover it with a cloth for from five to fifteen minutes, before 

 proceeding with further work. 



Mounting Spirit Levels. To prevent any undue strain and change of 

 curvature in spirit levels used in astronomical instruments, they arc mounted by us 

 in wyes, as .-hm\n in the cuts of these instruments, and are protected tnun injury, or 

 inaccuracy caused by the breath of the observer and ot her air currents, by a cover 

 of glass placed over them. " Such amounting, while most suitable for su-h deli.-ato 

 levels, would, however, require constant attention and expose a spirit level to break- 

 age in field instruments. To guard against this danger and to lessen the expense 

 and weight, the spirit levels for field instruments are mounted in a brass tube; but 



