STORMS AT SEA, &C. 187 



I apprehend the squall to be occasioned by a wind which The squall is ^ 

 blows immediately downwards. If we suppose a cloud to be ^^i!|',j ' ° 

 suddenly condensed into drops (no matter by what process of 

 chemistry, electricity, or other general cause, to us but little 

 known), the falling drops or masses of water will occasion a 

 descending current of air by their impulse against those portions — pro.lnced by 

 of the atmosphere through which they pass. This kind of blast J3^,^i!,"j^;ain! °^ 

 was formerly used in the water-blowing engine, an apparatus It resembles 

 which acted with a force capable of supporting a pressure of j.^„ ^^^,^1,^^^ 

 about three feet of water.* But the tropical rains frequently 

 descend with velocities much exceeding any that could be pro- 

 duced by engines of this kind, and their effects are much more 

 striking. 



The cloud which affords the descending stream is seldom The clouJ, 

 stationary, but in general carried along by the common wind ^he'biast moves 

 or lower current of the atmosphere ; and the stream of de- horizontally, 

 scending water is certainly and in all cases affected with the ^"J^^. ^i^'^j Ij^q^g 

 same horizontal motion. The descending wind, which might impulse to the 

 otherwise have struck the water perpendicularly, is by this^*^"' 

 cause made to take an oblique direction, and runs rapidly along 

 that surface, diverging from its principal place gi descent in 

 all directions, but most particularly in the direction which the 

 V'ind already possesses. We must therefore be careful to dis- The velocity of 

 tinguish two kinds of velocity belonging to the squall or de- that orthe^ 

 scending current; namely, first, the velocity of the current clouds are dis- 

 itself, which is greatest at the place of descent, and diminishes ' ^ J*^ • > 

 in receding from that place ; and, secondly, the velocity of the 

 cloud, or blowing apparatus, which is carried along by the com- 

 mon hoiizontal current or prevailing wind. The first of these — t^e first prc*- 



, . . ., , .,..,.. , , , . . duces a loral 



two velocities constitutes, within Jts limitted sphere ot activity, storm, and the 



a storm ; and the other velocity measures the progress with second carries 

 which the storm is horizontally conveyed along. Let us suppose wards aloMgthe 

 a stone to be thrown into smooth water; its action will cause sta. 

 a wave to be propagated horizontally to great distances around : 

 In the same manner we can conceive a wave or a swell to be 

 raised by the impulse of descending air, and propagated around - . 

 in all directions, though modified by the winds it may meet. 

 This swell, moving uniformly, will not be propagated as speedi- 

 ly as the descending current, where this last is most rapid ; but 



* Lewis, ill his Philos. Commerce of Arts. 



B b 2 - at 



