If two cameras are placed at the opposite ends of a measured base line, whose direction is known, and if they are both pointed towards the sun, on making the exposures by electrical means at the same moment, the position of the image of the sun upon the plate gives the direction in which the cameras are pointed. It will be in the same direction as seen from both ends of the line.
Now, if we note the time at which the exposure is made, this with the date gives all that is required for ascertaining the sun’s position in the sky, and is, therefore, the only exact observation which need be made at the time of taking the photographs. Mistakes are almost impossible, as each plate contains its own record of the sun’s position, and even if some of the plates should get mixed the images of the clouds will generally suffice to pair them properly. For general measurements there is one grave defect in the method, and that is that it can only be used when the sun and cloud can be got into the same field of view. But with the higher varieties of cloud this is generally possible, and it was just these higher sorts about which knowledge was least certain, and which it was proposed to study.
An initial difficulty was the finding of a level site, flat land being very uncommon in Devonshire, but fortunately a suitable place was found in some artificially levelled ground close to Exeter, belonging to the London and South Western Railway Company. It was a stretch of ground intended to be covered with sidings, but had not been finished, and had become overgrown with grass, stunted sallows, and other wild plants. Being railway ground, it was, comparatively, though by no means entirely, free from mischievous and inquisitive people. The next point was a suitable camera. It must have fairly long focus in order to give a large image, and therefore large displacement; it must be capable of being pointed in any direction and clamped there; and it must be capable of standing considerable extremes of temperature and variations of dampness, as it was intended that they should be kept on the spot in wooden structures, which served for stands as well as to contain the apparatus.
The pattern finally decided upon is represented in Plate 58, which shows one of the cameras pointed up to the sky and standing on one of the stands. These cameras were to take plates of whole plate size, two double dark slides of the ordinary pattern being attached to each.
Plate 58.
CAMERA FOR MEASURING ALTITUDES.
