SURVEYING BY PHOTOGRAPHY. 
245 
matter rested. Last summer the lecturer used some of the new- 
gelatine dry plates in his camera, and found it easy to obtain by 
their means panoramic pictures containing upwards of 120° of 
horizontal angle (azimuth), and 30° above and below the horizontal 
line. 
From these views the altitudes and azimuths of all objects in 
the picture could be measured off by means of two simple scales, 
one of equal parts, each equal part being 1 degree of the circum- 
ference of the circle whose radius was the equivalent focal length 
of the lens, and the other scale, for altitudes, an ordinary scale of 
tangents constructed to the same radius. 
The system of surveying a given extent of country is as follows : 
Two stations are chosen at the same level, and the distance 
between them is measured by the chain. From each station a 
photograph is taken, including the other station and the objects 
whose relative positions are to be determined. From the two 
photographs, the azimuths, or lines of bearing of every object 
shown in both pictures, can be measured off by the plain scale, 
and plotted by a protractor from each end of the base. The inter- 
section of the two lines gives the position of the object, and as the 
length of the base is known, the distance of the object is capable 
of being measured. This distance being known, its actual height 
in feet or yards can be found by measuring its height above the 
horizontal line in the picture by the scale of tangents, in conjunc- 
tion with the known distance. By this means the heights and 
distances of all objects visible from both stations can be ascertained 
by measurement of the photographs in the quiet of the office, the 
actual operation in the field occupying only a few minutes. 
During the course of the lecture Mr. Tweedy exhibited the 
apparatus, and explained its action ; he also showed many photo- 
graphs taken with it, which were much admired for the great 
amount of subject they included, and their admirable rendering of 
instantaneous effects. 
