The two camera stands were placed 200 yards apart, and were connected by a line of telegraph wire carried on short poles. At each end of the wire an insulated connecting piece was brought down to the camera stand, and to the batteries and other apparatus. The current which was sent through this wire by pressing a contact at one end of the line did not directly make the exposures; but two similar relays were brought into action, and each of these sent the current from a local battery of Leclanché cells through the electro-magnet on the camera and made the exposure. After development the two negatives showed the image of the sun, not far from the centre of the field of view, and the cloud whose altitude was required. Since this was taken from two different points of view, the negatives were not alike, but the distances between the centre of the sun’s disc and any special point of the cloud were different. For instance, if the cloud were east of the sun, with its edge just apparently touching the solar image as photographed from the eastern station, then the negative taken from the western end of the base would show an interval of clear sky between the two, which would be greater as the cloud was lower. It often happened that after developing the plates the image of the sun was lost in a black blur, but it was easy to reduce this part of the image by local application of a reducing agent[3] by means of a paint-brush, until the disc became clear enough. Two lines were then drawn on the negative, one vertical and the other horizontal, intersecting each other at the centre of the sun’s image. These lines served as the starting-points for exactly measuring the distance from their point of intersection to any selected point of the cloud. The distances could generally be determined to a fiftieth or a hundredth part of an inch, and their difference was, of course, dependent upon the direction of the sun relative to the base line and the altitude of the cloud, but for low level clouds the difference was sometimes so great that no pair of corresponding points could be detected, while it was often as much as an inch. With higher clouds the differences were smaller, but unless the sun was very low in the sky, either east or west, the displacements of the cloud image were great enough to give reliable measures. Specimen prints from pairs of negatives are shown in Plates 59 and 60. [Illustration: Plate 59. PRINT FROM A NEGATIVE USED FOR MEASURING ALTITUDE.] [Illustration: Plate 60. PAIR OF PRINTS SHOWING THE DISPLACEMENT OF THE CLOUD.]