128 
KIRTLEY F. MATHER 
of the vernier, wherever it happens to be, and determines the 
vertical angle by subtraction. He is then under no pressure of 
haste in reading the vertical arc and in centering the level bubble. 
To increased speed of geologic work is added thereby greater 
accuracy in instrumental observations. The record, then, in- 
cludes two angle readings, one on the target and one with level 
telescope; the sign of the angle, plus for stations above and minus 
for those below the altitude of the table; and the observed inter- 
Fig. 11. Explorers’ Alidade, with Vertical Arc Combined with Beaman 
Stadia Arc 
Courtesy of W. and L. E. Gurley, Troy, N. Y. 
cept on the rod. From these data the vertical distance may be 
computed at leisure. 
4 . Beaman stadia arc. The reading of vertical angles may be 
avoided by the use of the Beaman stadia arc, illustrated in 
figure 11. This is a specially graduated vertical arc which may 
be attached to the vertical limb of a transit or telescopic alidade. 
It carries two scales, of which the one nearer the adjustable 
index is known as the multiple scale because it indicates mul- 
