29 



Carrying an instrument in cold weather into a warm room, without the pro- 

 tect ion of its box <>i- bag, will cause a sudden exchange of ;iir within the hollow 

 spaees. and ca-rry with it dust and other substances through the minutest openings. 

 The vapor, also, that will thus condense on the metal surfaces, if it were not pro- 

 i, will have a tendency to settle a film on exposed graduations, making them 

 indistinct and difficult to read. 



Failure to protect the lenses of the eye-piece and object-glass of a telescope, 

 when not in actual use,* from the effects of moisture, dust, etc., by the covers pro- 

 vided for them (eyepiece-lid and cap) will result in a more frequent settling of a 

 thin film, which, like the fatty substance left by the touch of the fingers, greatly 

 impairs the clearness of vision. That the too frequent cleaning of the lenses must 

 in tlu 1 course of time bo detrimental to their brilliant polish, and lead to a corres- 

 ponding loss of transparency so essential to the proper working of a good telescope, 

 is apparent. Too much care cannot be taken to guard the lenses, and particularly 

 the Inner 'surfaces of the lenses comprising the objective, against any film that may 

 set t le on them. The ill effects of such a film are especially noticeable in high-pow- 

 ered telescopes of first-class geodetic and astronomical instruments. In short, it 

 should be remembered that the slightest film, scratch, or dirt will, according to their 

 nature and location, impair the sight through a telescope, and often render it unfit 

 for accurate work. 



The glass covers protecting the compass, arc, and verniers from exposure need 

 very car- >ful brushing and cleaning, the same as the lenses, as any scratch or film 

 will impair their transparency. If at any time the ground-glass shades should lose 

 their pure whiteness, by either dirt or film, and will not act as illuminators of the 

 verniers and graduation, take them out of their frames and simply wash them with 

 soap and water. 



To prevent loss of magnetism in the needle of instruments provided with a com- 

 pass : when storing away, allow the needle to assume magnetic North and South ; 

 then, by means of the lifter, raise it from the center-point against the glass cover. 



If an instrument has met with a fall, bending centers and plates, etc., it should 

 not be revolved any more, in order to preserve the graduations from still further 

 injury, but recourse should be had at once to the nearest competent maker. 



If the box or tripod should have become wet, they should be rubbed dry, and 

 the varnish should be renewed whenever found wanting. 



Loose or detached resting-blocks in the instrument-box, or any looseness of the 

 instrument in them, are very detrimental to the instrument and its adjustments. 

 Cracks in the instrument-box, the absence of rubber cushions under it, worn-out 

 straps and defective buckles, hinges, locks, and hooks, should never be tolerated, 

 as the remedy is so easily applied by any mechanic. Such defects and imperfections 

 are known to lead to injury of the instrument. 



The place where instruments are kept or stored away should be thoroughly dry 

 and free from gases. The placing of fused chloride of calcium, or caustic lime, in 

 an open vessel in the instrument-box is to be recommended where there is damp- 

 ness; and if the presence of sulphureted hydrogen is suspected, then, cotton satu- 

 rat.-d with vinegar of lead, placed in the box, will prove a preventive against the 

 tarnishing of solid silver graduations. 



Transportation of Instruments. 



DURING the progress of field work the more ordinary and portable transits and 

 levelling instruments, etc., can generally be carried on their tripods for ease and 

 dispatch. Nothing in the way of precise instructions, however, as to the best me- 

 thod of carrying an in-t rument : whether on the tripod, in the arm without the 

 tripod placing the hand beneath the leveling base or in the box, can be sug- 

 gested here. The nature of the ground, the surroundings, the size and weight, and 

 the distance to be traveled over, and last but not least t he fineness of the instru- 

 ment, will dictate to the engineer the best means of conveying it from point to point 

 in order to protect it from injury, and its adjustments from derangement. 



