1891. Cloud Photography. 469 



low resistance, buried a few inches below the surface of the ground, 

 was laid between the two stands. Switches, attached to telephones 

 as well as to an electric battery, were fixed to these stands, and wires 

 were arranged on the cameras, so that the observers could either 

 communicate with one another, or work the exposing shutters of the 

 two cameras at will. 



Operations for the determination of cloud height and motion were 

 then carried out on suitable occasions, as follows : The two observers, 

 termed for convenience A and B, proceeded to their respective 

 stations, each provided with a box containing half-a-dozen dark slides 

 charged with sensitised plates, and also an adjusted watch. The 

 cameras were set up on the pedestals, levelled, and the connecting 

 wires joined up. Locking plates of peculiar construction were pro- 

 vided, which ensured that the zero points in azimuth of both cameras 

 were exactly directed to the same point of the horizon. 



The observer at A, when he saw B had reached his station and 

 placed his camera on the pedestal ready for use, attracted B's atten- 

 tion by means of a flag waved overhead, and directed him through 

 the telephone to set the instantaneous shutter of his camera, setting 

 that of his own camera at A at the same time. A then, making use 

 of the push, sent a current of electricity through the two cameras, 

 I ('Which should liberate both shutters at the same instant of time. An 

 I enquiry was immediately made through the telephone of B, and, if 

 the reply assured A that the shutters were working satisfactorily, the 

 I observers proceeded to the second stage of the observation, which 

 was as follows : 



A carefully examined the sky and, selecting a suitable cloud, 

 directed the sights on his camera towards it, making a convenient 

 setting of the horizontal and vertical circles, which he then read off. 



O l 



He then told B to set his camera to the same azimuth and altitude, 

 and insert a loaded plate-holder in its groove, repeating the circle 

 readings to ensure accuracy, and also at the same time to set his 

 shutter. A, whilst directing B through the telephone, conducted the 

 same series of operations at his own instrument, so that, as soon as 

 B telephoned that he was ready for action, A switched the battery on 

 to the line, and, watching the cloud for a favourable instant, touched 

 the push, whereby the two plates were exposed simultaneously, the 

 instant of the exposure being recorded by both observers in their respec- 

 tive note-books. They then quickly exchanged their plate-holders for 

 Dthers containing fresh plates, and again set the shutters, so that by the 

 Dime sixty or seventy seconds had elapsed since the first exposure was 

 made they were ready for a second, which was carried out as before 

 inder the directions of A, both observers again noting the time. 

 Aiter this. A, having switched on the telephones, enquired of B if he 



K obtained the two pictures. If the reply was in the affirmative, 

 )L. XLIX. 2 I 



