The processes by which the measurements are worked out are laborious,[4] and consist of two parts, the first being the determination of the exact position of the sun from the date, hour, and latitude and longitude of the place, and the second, the determination of the position of the cloud. Two points which represent the same part of the cloud are selected, and their respective distances from the two lines drawn through the sun are measured as accurately as possible. Now, a certain distance on the negative corresponds with a definite angular displacement, and a scale can be constructed showing how much should be added to or subtracted from the sun’s position to get the exact position of the cloud. This being done, it is then a simple piece of trigonometry to deduce the actual height of the cloud above the place of observation. The work of computation, however, was greatly lightened by the fact that many of the pairs of negatives showed more than one layer of cloud; thus Plate 59, which is a fair specimen, shows three layers, and, consequently, one determination of the sun’s position sufficed for three distinct results. For the highest clouds the displacements were, of course, small, and could only be made with certainty of a correct result within about three hours of noon. Earlier than 9 a.m., or later than 3 p.m., the sun was too nearly in a line with the two stations, or too low in the sky, to give a sufficient displacement of image. A base line of 400 yards instead of 200 would have been better for the high clouds. But, on the other hand, when low level clouds are viewed from two different spots their outlines may seem so changed that it may be impossible to identify a pair of corresponding points, and the same difficulty may also arise when high clouds are seen through a gap in a lower stratum. The longer the base line the more frequent and more obtrusive would this perspective difficulty become, so the distance of 200 yards between the stations was adopted as a convenient mean. The method of making the observations was simple. Each observer was provided with some signal flags, by which the necessary communications were made in accordance with a simple code. Call the two observers A and B, and suppose A directed the operations. He watched the sky until a favourable opportunity seemed to be approaching. He then signalled to B, and both cameras were turned to the sun, the dark slides were inserted, the shutters set, and everything made ready. Signals were then interchanged, to signify that preparations were complete, and when A saw that the edge of the cloud had reached a suitable position to be in the same field of view with the sun, the contact key was pressed and the plates simultaneously exposed. At the moment when this was done the time was noted. Several observations were thus made in a short time.