198 NINTH ANNUAL REPORT OF 



feels warmer on this account than it really is. The temperature 

 within this cloud being higher than that on the outside in consequence 

 of tlj(^ evolution of latent heat, the passage of a thunder-cloud over 

 any place almost invariably disturbs the air at the surface of the 

 ground. The extrication of heat is a motive power which con- 

 stantly causes the air to ascend in the front of the storm as the 

 cloud drifts along in the upper current. Squalls are produced in the 

 same way. Professor Espy, in his Report on Meteorology, says : 

 " Low clouds are constantly forming in the front of squalls by the 

 upward motion of the moist air as fast as their hinder parts are 

 swept down by the falling rain, and thus they appear constantly just 

 in f-ont of the squall, for it is only in front that there is an up-moving 

 column of air from below." 



We shall enter a little more particularly into this subject, to show the 

 analogy between these isolated disturbances arising from the formation 

 of cloud and precipitation of rain and the land and sea breezes, hur- 

 ricanes of the West India islands, thunder storms of America and 

 Europe, and also some of the great snow and rain storms of both con- 

 tinents. 



In Humboldt's celebrated voyage from Europe to South America he 

 relates that, on reaching the latitudes of the trades, the "wind fell 

 gradually the further we receded from the African coast ; it was some- 

 times smooth water for several hours, and these short calms were 

 regularly interrupted. Black, thick clouds, marked by strong outlines, 

 rose in the east, and it seemed as if a squall would have forced us to 

 haul our top-sails ; but the breeze freshened anew, there fell large 

 drops of rain, and the storm dispersed. Meanwhile it was curious to 

 observe the effects of several black, isolated, and very low clouds 

 which passed the zenitli. We felt the force of the wind augment or 

 diminish progressively according as small bodies of vesicular vapor 

 approached or receded. It is by the help of these squalls, which 

 alternate with dead calms, that the passage from the Canary islands to 

 the Antilles or southern coast of America is made in the months of June 

 and July." 



From this description we gather that there was an upper current 

 prevailing from the east, in which the clouds drifted, bearing them over 

 the calm air resting on the ocean. The air in front of the cloud is dis- 

 turbed as soon as the cloud approaches, and a violent squall prevails so 

 long as it is overhead. . This shows the intimate connexion between 

 the upper and lower currents of the atmosphere when clouds are 

 passing. 



Unless clouds were constantl}^ replenished by vapor from below, it 

 is physically impossible that they could continue to throw down such 

 great quantities of rain over long tracts of country as they are some- 

 times known to do. At no time, even within the tropics, is the air over 

 one spot capable of precipitating more than three inches of rain. 



In the squalls described by Humboldt we observe that there was 

 ■calm both before and after the passage of the cloud. The motion of 

 the air at the surface of the sea is much the same as if the whole por- 

 tion of it under the cloud were in a slate of vertical rotation around a 

 horizontal axis. You will perceive that there is a resemblance in the 

 motion of the air under isolated storm-clouds to that of the air in land 



