320 



NATURE 



{August 4, 1887 



extremities of a suitable base. The new base at Upsala 

 has a length of 4272 feet ; the old one was about half 

 the length. The result of the change has been that the 

 mean error of a single determination of the highest 

 clouds has been reduced from 9 to a little more than 

 3 per cent, of the actual height. At the same time 

 the difficulty of identifying a particular spot on a low 

 cloud is considerably increased. A wire is laid between 

 the two ends of the base, and each observer is provided 

 with two telephones — one for speaking, the other for 

 listening. When an observation is to be taken, the con- 

 versation goes on somewhat as follows : — First observer, 

 who takes the lead : " Do you see a patch of cloud away 

 down west } " " Yes." " Can you make out a well- 

 marked point on the leading edge.'"' "Yes." "Well 

 then ; now." At this signal both observers put down 

 their telephones, which have hitherto engaged both their 

 hands, begin to count fifteen seconds, and adjust their 

 instruments to the point of cloud agreed on. At the 

 fifteenth second they stop, read the various arcs, and the 

 operation is complete. 



But when the angles have been measured the height 

 has to be calculated, and also the horizontal and vertical 

 velocities of the cloud by combining the position and 

 height at two successive measurements at a short interval. 

 There are already well-known trigonometrical formulae 

 for calculating all these elements, if all the observations 

 are good ; but at Upsala they do far more. Not only are 

 the observations first controlled by forming an equation 

 to express the condition that the two lines of sight from 

 either end of the base should meet in a point, if the angles 

 have been correctly measured, and all bad sets rejected ; 

 but the mean errors of the rectangular co-ordinates are 

 calculated by the method of least squares. 



The whole of the calculations are combined into a 

 series of formulte which are necessarily complicated ; and 

 even by using logarithms of addition and subtraction, 



a 

 and one or two subsidiary tables — such as for log. sin- - 



2' 



specially constructed for this work — the computation of 

 each set of observ^ations takes about twenty minutes. 



Before we describe the principal results that have been 

 attained, it may be well to compare this with the other 

 methods which have been used to determine the height of 

 clouds. A great deal of time, and skill, and money, have 

 been spent at Kew in trying to perfect the photo- 

 graphic method of measuring the height of clouds. Very 

 elaborate cloud cameras, or photo-nephoscopes, have been 

 constructed, by means of which photographs of a cloud 

 were taken simultaneously from both ends of a suitable 

 base. The altitude and azimuth of the centre of the 

 plate were read off by the graduated circles which were 

 attached to the cameras ; and the angular measurements 

 of any point of cloud on the picture were calculated 

 by proper measurements from the known centre of the 

 photographic plate. When all this is done, the result 

 ought to be the same as if the altitude and azimuth of the 

 point of the cloud had been taken directly by an ordinary 

 angle-measuring instrument. 



It might have been thought that there would be less 

 chance of mistaking the point of the cloud to be mea- 

 sured, if you had the pictures from the two ends of the 

 base to look at leisurely, than if you could only converse 

 through a telephone with the observer at the other end 

 of the base. But in practice it is not so. No one who 

 has not seen such cloud-photographs can realize the 

 difficulty of identifying any point of a low cloud when 

 seen from two stations half a mile or a whole mile apart ; 

 and for other reasons, which we will give presently, the 

 form of a cloud is not so well defined in a photograph as 

 it is to the naked eye. 



At Kew an extremely ingenious sort of projector has 

 been devised, which gives graphically the required height 



of a cloud from two simultaneous photographs at opposite 

 ends of the same base, but it is evident that this method 

 is capable of none of the refinements which have been 

 applied to the Upsala measures, and that the rate of 

 vertical ascent or descent of a cloud could hardly be 

 determined by this method. But there is a far greater 

 defect in the photographic method which at present no 

 skill can surmount. 



We saw that the altazimuth employed at Upsala had no 

 lenses. The meaning of this will be obvious to anyone 

 who looks through an opera-glass at a faint cloud. He 

 will probably see nothing for want of contrast, and if any- 

 thing of the nature of a telescope is employed, only well- 

 defined cloud outlines can be seen at all. The same loss 

 of light and contrast occurs with a photographic lens ; 

 and many clouds that can be seen in the sky are invisible 

 on the ground glass of the camera. Cirrus and cirro- 

 stratus — the very clouds we want most to observe — are 

 always thin and indefined as regards their form and con- 

 trast against the rest of the sky ; so that this defect of the 

 method is the more unfortunate. 



But even when the image of a cloud is visible on 

 the focusing glass, it does not follow that any image 

 will be seen in the picture. In practice, thin high white 

 clouds against a blue sky can rarely be taken at all, or 

 only under exceptional circumstances of illumination. 

 The reason seems to be that there is very little light re- 

 flected at all from a thin wisp of cirrus, and what there is 

 must pass through an atmosphere always more or less 

 charged with floating particles of ice or water, besides 

 earthy dust of all kinds. The light which is scattered 

 and diffused by all these small particles is also concen- 

 trated on the sensitive plate by the lens, and the resulting 

 negative shows a uniform dark surface for the sky without 

 any trace of the cloud. What image there might have 

 been is buried in photographic fog. 



In order to compare the two methods of measuring 

 clouds, I went out one day last December at Upsala with 

 Messrs. Ekholm and Hagstrom when they were measuring 

 the height of some clouds. It was a dull afternoon, a low 

 foggy stratus was driving rapidly across the sky at a low 

 level, and through the general misty gloom of a northern 

 winter day we could just make out some striated stripes of 

 strato-cirrus — low cirro-stratus — between the openings in 

 the lower cloud-layer. The camera and lens that I use 

 habitually for photographing cloud-forms — not their 

 angular height — was planted a few feet from the altazimuth 

 which M. Ekholm was observing, and while he was 

 measuring the necessary angles I took a picture of the 

 clouds. As might have been expected under the circum- 

 stances, the low dark cloud came out quite well, but there 

 was not the faintest trace of the strato-cirrus on the nega- 

 tive. MM. Ekholm and Hagstrom, however, succeeded 

 in measuring both layers of cloud, and found that the low 

 stratus was floating at an altitude of about 2000 feet high, 

 while the upper strato-cirrus was driving from S. 57^ W. 

 at an altitude of 19,653 feet, with a horizontal velocity of 

 81, and a downward velocity of 7 "2 feet per second. This 

 is a remarkable result, and shows conclusively the_ 

 superiority of the altazimuth to the photographic methc 

 of measuring the heights of clouds. 



Whenever opportunity occurs, measures of clouds aij 

 taken three times a day at Upsala, and it may be well 

 glance at the principal results that have been obtaine<] 

 The greatest height of any cloud which has yet be€ 

 satisfactorily measured is only 43,800 feet, which is rathe 

 less than has usually been supposed ; but the highest 

 velocity, 112 miles an hour with a cloud at 28,000 feet, 

 is greater than would have been expected. It may be 

 interesting to note that the isobars when this high velocity 

 was reported were nearly straight, and sloping towards the 

 north-west. 



The most important result which has been obtained 

 from all the numerous measures that have been made is 



