METEOROLOGY. 327 



h abaci , 39 for St. Petersburg, 27 for Stony hurst. In general there is a 

 larger per cent, of suusliine in summer than in winter, or the short 

 winter days have more cloudiness than the long summer days; but 

 Allahabad shows the opposite, since there the months of longest days 

 are the rainy months, and the highest percentages of sunshine occur in 

 December and October. {Z. 0. G. M.^ xix, p. 326.) 



VI [.—(a) Winds and ocean currents; (h) Dynamic laws for 



MOVEMENT OF AIR, CONSIDERING THE ROTATION OF THE EARTH 

 AND THE LAWS OF THERMODYNAMICS. 



235. Dr. Woeikof has calculated for fifty stations in Siberia and Eus- 

 sia the diurnal i)eriod in the velocity of the wind, and shows its inti- 

 mate connection with the diurnal change of temperature and the uprising 

 currents as explained by Koppen. Where the surface-heating is slight 

 the diurnal amplitude is slight. {Nature, xxviii, p. 571.) 



236. The United States National Academy of Sciences has published 

 a report of its expedition to Caroline Island, in the South Pacific, for 

 the observation of the solar eclipse of May 6, 1883. The meteorological 

 results obtained by Prof. W. Upton give, among other things, a new 

 value of the atmospheric " constant of absorption of solar heat" and an 

 approximate determination of the heat reflected from our atmosphere 

 during totality; also a determination of the diurnal period in the force 

 of the wind deduced from hourly records of the velocity of the sailing 

 vessel as determined by the ship's log. During an interval of about ten 

 days the vessel sailed in a uniform trade-wind without a single change 

 in the trimming of the sails. Upton's results show a greater velocity 

 of wind by night than by day. 



237. [This exceptional result, if no flaw be discovered in the method, 

 may possibly be explained (1) as due to the fact that the height of the 

 center of wind pressure against the sail is about 40 feet above the sea, 

 and (2) by the assumption that above this elevation the wind has its 

 normal velocity at night, but a less velocity by day, owing to the upris- 

 ing of slowly-moving heated currents from the surface of the sea; in 

 other words, the diurnal periodicity of the wind, which over the land is 

 observed up to a height of many hundred and sometimes of several 

 thousand feet, is over the sea felt up to the height of only a few hun- 

 dred feet. Above that level its velocity is uniform ; immediately below 

 it the velocity is least in the middle of the day, while at the surface of 

 the sea the velocity is greatest in the middle of the day. The Espy- 

 Koppen theory has not yet been developed far enough to enable us to 

 state the relation between temperature of earth or sea surface, and the 

 height of the plane of no diurnal wind-period, but it is evident that the 

 height over the sea may really be as small as Upton's observations in- 

 dicate. This result explains also the peculiarities of the diurnal land 

 and sea breezes at sea-shore stations when the general winds are from 

 land or water.] 



