144 REPORT — 1894. 



Congress of the system of cloud nomenclature devised by Hildebrandsson 

 and Abercromby, and it is remarkable that so few actual measurements 

 have been carried out. 



So far as your Committee are aware, the only measurements of the 

 kind which have been systematically organised, at least in this country, 

 are those which were begun some years ago at Kew. 



Now it is not only important to have more observations, but it is 

 especially desirable to have them from other places than the vicinity of 

 London for comparison, and in the residence of the Secretary at Exeter 

 .such an opportunity is presented. 



In the course of experiments on methods of cloud photography it has 

 been found easy to secure well-defined images of clouds even when the 

 sun is in the middle of the field of view. If, then, two such photographs 

 are taken simultaneously by a pair of cameras at some distance apart, 

 there will be a displacement of the image relatively to that of the sun. 

 The amount of this displacement will depend upon a number of things, 

 but it will be increased by adding to the focal length of the lens and by 

 increasing the distance between the two cameras. By knowing these 

 values and the altitude and azimuth of the sun, the distance of the cloud 

 and its height above the ground may be calculated without diflBculty. 



The azimuth and altitude of the sun at the time of exposure may be 

 ascertained by direct observation, or it may be found by calculation, from 

 the known time at which exposure was made. There seems to be a 

 manifest advantage in thus using the sun as a fixed point of reference, 

 since it provides a means whereby any error in the observation of altitude 

 and azimuth may be effectively checked. 



Your Committee have therefore prepared a pair of cameras so con- 

 structed that they may be easily directed towards the sun. They are 

 provided with lenses of 18 inches focus covering a plate of whole plate 

 size, thereby giving a large displacement and allowing room for a displace- 

 ment of several inches. The lenses are provided with adjustable shutters, 

 which can be simultaneously freed by an electrical attachment. They are 

 placed on stands, which serve as cupboards for them when not in use. 



At present for purely trial purposes they are placed in the Secretary's 

 garden at a distance of 35 yards, yet even that short distance gives a 

 displacement of half an inch with clouds 3,780 feet distant. This, of 

 course, is too small for very accurate measurement, and would be far 

 smaller with high-level clouds, the determination of the altitudes of which 

 is most important. 



The intention of your Committee is to place them on a plot of level 

 ground by the side of the London and South- Western Railway near 

 Exeter. There is available a strip of waste ground, just over a quarter of 

 a mile in length, commanding an uninterrupted view of the sun from 

 sunrise until nearly sunset. The ground is level, and the cameras can be 

 placed due east and west, thereby greatly simplifying the reduction of 

 the observations. The directors of the London and South-Western Rail- 

 way have kindly consented to allow the ground to be used under conditions 

 which seem to your Committee quite satisfactory, but which involve the 

 payment of a nominal rent of 11. per annum ; and the cameras would 

 have been placed in position by the present time had it not been necessary 

 to get another meeting of the Committee to sanction the agreement. The 

 method is easy to apply, and promises to yield results at leas^ as accurate 

 as any which have yet been tried ; so your Committee ask for reappoint- 

 ment, with a grant of 10^. 



