deg. 53 min. 15 sec.

A level and a compass are also mounted on the lid as shown. To use the
instrument the observer stands so that the object the range of which
is required lies on his right hand, and looking into the left-hand
corner of the upper prism views it there by double reflection from the
internal faces of the prism. At the same time looking through the
opening shown in the lid below the prism he selects some object, which
appears nearly in line with the image seen in the prism. He then
shifts his position till these two images coincide, in which case
lines joining him with the two objects will make right angles with
each other. In Fig. 2, O is the object whose range is required, D the
object seen by direct vision, and A the position of the observer. The
observer now marks his position on the ground, and shifting the
instrument looks into the left-hand corner of the second prism, when
he again sees the image of the object, whose range is required, by
double reflection, but lying now to the right of the object, D. He
then retires, keeping in line with A and D, till he reaches B, when
the two images again coincide; the lines joining them and the observer
now make an angle of 88 deg. 51 min. 15 sec. Then in the triangle,
OBA, OA = tan 88 deg. 51 min. 15 sec. X A B = 50 AB. The length AB is
easily paced, and the distance OA is 50 times this length.

A longer base, and probably greater accuracy, can be obtained by using
the second prism only, as indicated in Fig. 3, in which case the
distance of the object is 25 times the distance BC. This second prism
is, however, best adapted for predicting the range of moving objects.
Three observers are required. Two of them have finders, while the
other measures the distance between the two. The first two observers
separate, and No. 2 takes a position such that the object is reflected
to one side of observer No. 1, whom he views by direct vision. As the
object continues to move, its image gets nearer and nearer No. 1, who
during the whole of the time moves a little to one side or the other,
so as to keep the image of the object constantly in line with No. 2.
Just as the image of the object gets very near No. 1, No. 2 calls out
"Ready," the distance between the two observers is taken by the third,
and when the image of the object actually falls on No. 1 its distance
is just 25 times the distance between them, and the guns set to this
range are fired by word of command from No