530 



SCIENTIFIC RECORD FOR IS 



By studying the relative velocities and durations of the cloud motion 

 and the winds for each direction, Vettiu determines approximately the 

 motions of the masses and volumes of the atmosphere, as shown in the 

 following, Table III, where the figures represent the product of the per- 

 centage of duration of each movement by the total movement, but for 

 economy of space only the nearest hundreds are given: 



Table III. — Relative volume of air transported by each wind or cloud-direction. 



For the whole atmosphere the greatest average movement is from 

 the west; the least is from the east. In general as much air flows from 

 the north (the polar current) as from the south, the ratio being 19.1 to 

 18.1, and an exact equalization must therefore occur for a direction 

 about 2° east of north. This direction is subject to annual variation on 

 either side of the mean. (Z. O. O. M., xvn, pp. 267-351.) 



O. Jesse criticises Dr. Vettin's method of computing clond heights, 

 but apparently does not impugn the general accuracy of the conclusion. 

 {Z. O. G. M., p. 430, xvn.) 



In reply to some criticisms by Prof. O. Jesse, Dr. Vettin gives further 

 details as to the method of measurement, and shows that in the case of 

 the cirrus clouds his results are all confirmed by the separate consid- 

 eration of the cases in which he has determined the altitude by either 

 of the three methods, viz, the trigonometric, the sunset illumination, or 

 the projected velocity. Vettin also describes in detail the camera-ob- 

 scura and the formulae used iu the observation and computation of 

 cloud altitudes. The great need and general dearth of accurate cloud 

 observations should attract observers to the use of some equivalent 

 apparatus; that used by Vettin consisted of a camera box or tube 

 mounted with altitude and azimuth motion. The observer looking 

 upon the lower end sees the cloud image thrown upon a ground-glass 

 plate by the lens at the upper end of the camera; on this ground-glass 

 plate a divided circle is etched. The observation consists iu noting the 

 sides of this plate towards which the cloud-image moves, and also the 

 inclination to the vertical of the axis of the camera. The observation 

 is thus made as simple and speedy as possible, while the formulae and 



