will show how our vial mount is guarded against compound flexure 

 by the spherical washers and open bosses. 



The vial itself is held in position by spring tongues only, and 

 sealed at the ends with cork in preference to plaster-of-paris, which 

 offers resistance to ordinary expansion and interferes with field 

 repairs. 



Wherever the construction of the instrument is such that the 

 extension brackets are required to facilitate reading the bubble, 

 we are in the hahit of substituting them, as indicated in Fig. 49. 

 This is especially necessary where the vertical limb is covered with 

 a closed guard. Our vertical arcs are designed to permit an unob- 

 structed view of the bubble between the legs of the standard, as indi- 

 cated. in Fig. 42. 



There are two or three excellent reasons why the Reversion 

 Bubble should be employed when the transit is much used for 

 leveling purposes: 1st, the visibility; 2nd, the beneficial effects upon 

 the telescope axis, and 3rd, the availability for rapid parallelism 

 tests, as described on p. 70. The extension brackets touched upon 

 above improve conditions without perfecting them and add nothing 

 to the artistic mechanical treatment of the question. 



If the circle is entirely covered, as usually prefered for mining 

 or tunnel surveys, or for work in humid climates, the bubble cannot 

 be seen from the left side of the instrument unless at the top of the 

 telescope. A rather desperate alternative in such a case is to adjust 

 the ordinary bubble upside-down so that it may not be used at all 

 except at the top. With a control bubble to regulate the vertical 

 circle, as shown in Fig. 38, such a plan is not without commendation. 



In such flat country as skirts the Gulf sea board we find engi- 

 neers taking all observations between the limits of, say, 5 above and 

 15 below the horizon with the result that in the course of time the 

 telescope axis becomes worn and rlatened in this segment, rv 

 for lack of oil. If the surveyor would use his telescope reversed in 

 altitude for half of his work, he would at least double the life of his 

 telescope axis, and a reversion bubble would contribute materially 

 to this plan. 



93 



