and -f28, so that if sights of greater inclination are to betaken 

 these extremes are to be laid off by noting some point in the 

 landscape; then, after setting the vernier back to zero, build up one 

 end of the instrument with any substantial bolster like a note 

 book and read the increment beyond the first limit. 



Such angles are used in connection with tables to calulate 

 difference in elevation by the hypsometric method. A constant 

 check should be kept on elavations, so determined, which in geo- 

 logical mapping should agree within 5 ft. ordinarily, or 10 ft. on 

 great distances. On large scale maps covering areas above one 

 mile in any direction, a correction for curvature of the earth is to be 

 applied to reduced horizontal distance by use of the table on p. 36. 



The standard of this instrument is so low (2-in.) that there is 

 not enough room for direct vision. The ocular is therefore sup- 

 plied with a permanently fixed total reflecting prism, and all ob- 

 servations are taken by looking straight down into the exit pupil. 

 This same idea is carried out in a large and better way with our 

 duplex diagonal eyepiece, as shown in Fig. 28 and made only by us. 



The adjustment and use of this instrument do not differ 

 materially from the larger models, except that every sight is to be 

 taken while looking downward into the fixed prismatic eyepiece. 

 The base rule is 2^ x 11 inches and may be divided into tenths and 

 fiftieths of an inch or in millimeters. The instrument is only 3^ in. 

 high and weighs 4^ Ibs. , complete in leather case with sling. 



The Traverse Alidade 



This is probably the highest grade sight-vane alidade extant. 

 It was made in 1907 to the requirements of L. C. Graton, now of the 

 Harvard Geol. Museum. The aluminum base rule is 7 in. long 

 and usually divided into decimal inches though frequently furnished 

 in the metric scale. 



The sight vanes are accurately and substantially hinged. As they 

 fold down one of them clamps the needle of the 3>i-in. declinatoire. 

 In April, 1913, we mounted, in an aluminum case, a small circular level 

 on the base rule at Mr. Graton's request. The instrument weighs 

 ^ o/. (140 grms), folds up very compactly and is packed in a leather 

 pouch which is to be slung at the hip of one's girdle. 



