METEOROLOGY. 527 



Hon from tbe earth and the plants, frequently the latter aud occasion- 

 ally the former, is the only cause of the dew, but generally both act 

 together. Even in cloudy nights, radiation causes a precipitation at 

 the base of blades of grass, and in connection with the existence of the 

 coldest stratum of air near the earth, is the chief cause of dew. (Z. 0. 

 G. M., xvii, p. 113.) 



Prof. J. M. Pernter has given an exhaustive mathematical analysis of 

 the thermo dynamic laws of the cooling and condensation of vapor that 

 may occur when cold and warm air are mixed together according to 

 Hutton's theory of rain. Wettsteiu has contended that not the slight- 

 est ra : nfall can thus be produced. Hann had already shown by an 

 approximate computation of a special case that slight precipitation can 

 possibly occur. Pernter's formula and method are perfectly general, 

 although the computations are necessarily very tedious. He concludes 

 that only for large differences of temperature can any sensible precipi- 

 tation occur even sufficient to form cirrus clouds, and that the quantity 

 of precipitatiou computed bj< his formula is so small that it is impos- 

 sible to explain any heavy rainfall by Hutton's theory. (Z. 0. G. M., 

 xvii, p. 421.) 



Woeikoff has published a memoir on the cloudiness of the skies at 

 Russian stations based on the ten years 1870-'79, of which Koppen gives 

 a short summary. Dividing the Eussian domain into thirteen portions, 

 from the Baltic to the Pacific, it would appear that the maximum per- 

 centage of cloudiness generally occurs in November and amounts to 85 

 per cent, for the White Sea, but only 65 per cent, for central Siberia. 

 The annual means of cloudiness show the highest percentage (72) also 

 for the White Sea, and the lowest (31) for the Aral Sea, the next lowest 

 being 34 percent, for the trans-Baikal stations. In general the stations 

 may be divided into three groups: 1st, European Kussia and the Aral, 

 where the minimum is in summer and the maximum in winter ; 2d, from 

 Lake Baikal north aud west, where the maximum is in October or No- 

 vember and the minimum in March : 3d, the trans-Baikal and the eastern 

 coast of Asia, where the maximum is in summer and the minimum in 

 January. (Z. 0. G. M., xvii, p. 358.) 



Dr. F. Vettin, as the result of three years' continuous observations at 

 Berlin of the heights and velocities of clouds, publishes a very complete 

 analysis of cloud movements. He measured the angular velocity by a 

 fixed camera-obscura, on whose glass back the image of the cloud is 

 seen to pass over a system of graduation lines. The actual velocity 

 was measured by observations of the shadows of the clouds, as pro- 

 jected upon the earth. From these two data is deduced the actual alti- 

 tude. For other cases he observed the time at^hich the cloud was 

 first illuminated by the rising sun or last illumined by the setting sun, 

 whence he computed the actual altitude. He concludes that with in- 

 creasing altitude the cloud forms alter according to a very definite rule, 

 namely: (1) tbattbelowestcloud has indefinite boundaries similarto loose 



