268 PROCEEDINGS OF THE AMERICAN ACADEMY. 



between latitudes 36° and 34° north, show a rapid fall of temperature 

 with height (7.8° per 1000 meters) which is fastest (0.92° per 100 

 meters) within the first 500 meters. The relative humidity rises 

 nearly to saturation within the first 500 metres, but does not attain 

 this point, the flat cumulus clouds found in the region being probably 

 formed by the condensation of water-vapor rising from the ocean 

 through a slow motion of diffusion. The wind, which is generally 

 northeast, changes its direction but little with altitude, although it 

 increases in velocity above 500 meters. The same features are shown 

 by the only two observations south of the northeast trade, in about lat- 

 itude 10° north. "Within the trade-wind region, between latitudes 31° 

 and 15°, the vertical distribution of temperature and moisture is quite 

 different. Near its origin the trade has the character of a descending 

 current, that is, small vapor contents and little cloud, which is the flat 

 cumulus tjrpical in northern regions of high pressure and descending 

 air. In approaching the equator, that is, south of latitude 24° north, 

 the trade presents characteristics of an ascending current, the relative 

 humidity increases much with height, the sky is cloudy, and there are 

 frequent rains, often accompanied by thunder-storms. The decrease 

 of temperature within the trade region is less than 1°C. in 1000 meters, 

 there being a fall of 0.58° in the first 500 meters and a rise of 0.56° 

 in the next 500, on account of the inverted temperature-gradients 

 which occur near 1000 meters at the upper limit of the trade-wind. 

 Its depth varies from day to day between 300 and 1500 meters, and ap- 

 pears to be greatest in the afternoon and least at night. The upper por- 

 tion is damp, with cumulus and strato-cumulus cloud, above which the 

 wind falls light and the relative humidity sinks nearly to zero, coincid- 

 ing with the rise in temperature, which frequently carries it much 

 above that at sea-level. With increasing altitude there is a gradual 

 shifting of the wind, when facing it, to the right, accompanied by an 

 accelerated velocity, up to at least 1000 meters. The conditions at 

 greater heights were deduced by Mr. Clayton as follows : Above the 

 surface-trade is a current some 2000 meters in depth, varying in 

 direction between northeast and northwest, but coming always from 

 a direction to the left of the lower wind when facing it. This current 

 is extremely dry and potentially warm, and its velocity usually much 

 exceeds that of the lower wind. The third stratum, which begins at 

 a height of about 3000 meters, moves fi'om east, south or southwest, 

 being generally from the east in equatorial regions, and from the 

 south between latitudes 15° and 30° north (see Table II). As observed 

 on the Peak of Tenerifife this stratum was dry in its lower portion, but 

 with a larger vapor contents than the air immediately below. Alto- 



