

I7i! Lieut.-GentT.ii Str.-iehey and Mr. \Vliij)j)l.-. [A\> 



The necessary measurements were made on a scale of millimeters, 

 and the computations carried out by the help of logarithms. 



The operations thus described have lately been much abbreviated 

 in various ways. First, it has been found possible to carry out the 

 superposition of the pictures by means of the negatives only, and to 

 work without either employing 'positives or depending on the identi- 

 fication of a few selected points whose positions were transfern l 

 a receiver. 



A frame has been constructed which carries the glass negative 

 plates upon sliders in grooves running in parallel planes, one imme 

 diately over the other, but arranged so as to travel at right angles 

 one another, the lower moving towards and away from the observe 

 whilst the upper traverses from right to left. A mirror, either 

 silvered or an opal plate, is employed to reflect the light of the si 

 upwards to the eye through the negative photograph when tl 

 apparatus is placed upon a table in front of a well-lighted window. 

 Stray or diffused light is excluded by placing a box, darkened on it 

 inner surface, over the negatives, and the observer views the com- 

 bination through a tube fixed perpendicularly upon the top of the box. 

 The two photographs to be compared are placed one in each of tl 

 sliding frames, which are first so adjusted that the fiducial lines whic 

 follow the direction of the base pass exactly over one another. Nei 

 the bottom or backwards-and-forwards slider is moved until the clouc 

 pictures, say a pair marked A and B, are seen to coincide, and tl 

 distance between the intersections of the cross lines on the two plat 

 representing the zenith points, which is the parallax, is then measui 

 by means of a pair of compasses ; but a scale could readily be fix* 

 on the slides from which the parallax could be read off withov 

 measurement. 



In order to avoid calculations, a standard curve has been drawi 

 (see fig. 2), from which the height of the cloud may at once 

 graphically determined from the distance between the intersections 

 the cross lines or parallax of the base as thus measured. 



On the axis of abscissa? of this curve are marked off the heights 

 a scale which makes 2400 feet, the length of the base, equal to tt 

 focal distance of the camera, and at regular intervals along t-hi< lii 

 ordinates are drawn of the length, as measured on the photograph!: 

 of the parallax corresponding to the several heights. Through t\ 

 extremities of these ordinates a curved line is drawn, which gives tl 

 locus of the equation 



h = p cot IT, 



the lengths It and p being both expressed on the scale just 

 tioned. 



The same operations are next performed with pictures Aj and 



