202 DESCENT OF VAPOUR. 



the surface, and mounts up through the rest of the 

 mass. If the air were perfectly still (which it never 

 is), and the dense smoke of a furnace rose up with 

 perfect uniformity (which that also never does), we 

 should see the hyperbolic column mounting and 

 swelling till the upper part of it became so thin as to 

 be invisible, and it seemed to melt away into the 

 uir, both upward and laterally, with the most finely 

 melting shade imaginable. And even as it is, al- 

 though the smoke is always irregular in its quantity, 

 and though those very irregularities produce little 

 currents in the air, which throw the smoke into 

 curling volumes, an eye well disciplined in the ob- 

 servation of forms can trace the hyperbola in its 

 general outline, even when it is blown aside by a 

 pretty smart breeze. The side opposite the wind is 

 always more bent than the windward side, so that 

 the column broadens as it gets distant from the 

 chimney; and we have only to imagine it to be 

 raised straight and the inequalities of the ends to be 

 arranged, in order to have a very clear notion of 

 what it would be if the causes of disturbance were 

 removed. 



The descent of a cloud, of a column of cold air, or 

 of any thing else that can be so dispersed through the 

 atmosphere, is just the reverse of its ascent ; and 

 therefore its form, if it were visible and undisturbed, 

 would be hyperbolic, only with a downward motion, 

 in place of an upward one, as in the former case. 

 The motion would, however, be more rapid ; and for 

 that reason the descending hyperbolic mass would 

 converge, or come together, more rapidly than the 

 ascending one spreads. It is true that as it descended 

 it would meet with more resistance from the denser 

 air ; and also from the upward current of air and of 

 heat from the earth's surface, if the place under it 

 happened to be warm ; but still the weight of the 

 descending matter itself, the velocity it had acquired 

 in descending, and the attraction of cohesion be- 



