232 episti.es to students. 



ways seen to descend from a black cloud, sometimes with a velocity 

 of halfaniile in two seconds. Now, as Prof. Espy observes, this velo- 

 city precludes the possibility of this visible spout having fallen by grav- 

 ity, for, in that time, if its specific gravity were ten thousand times 

 greater than it is, it could not fall more than sixty-four feet in two sec- 

 onds. Besides, if the cloud were very heavy and descended, the neces- 

 sary consequences would be that trees, barns, ^houses, &c. would be 

 crushed by its immense weight, and their fragments would be left upon 

 the ground where they before stood. But every fact connected with 

 the phenomena of such storms not only disproves^ such a result, but 

 also the idea of 7nere horizontal centrifugal force, as advocated by a 

 certain celebrated meteorologist. At the very moment that the cloud 

 appears to be sinking lower and lower, the up-moving current is be- 

 coming stronger and stronger, and its velocity and force are soon ex- 

 hibited by the rapidity with which heavy bodies ascend. 



If the cloud pressed downwards, and there werfe no other forces 

 except the centrifugal, why do the leaves and trees Jly up ? Why do 

 the roofs fly up, and the walls of buildings /Zi/ apart, as if iy explosion? 

 Why are shingles and other light bodies carried twenty and thirty miles 

 off and then descend in a hail storm? And why are hail-stones fre- 

 quently picked up containing particles of sand and other matter imbed- 

 ded within them ? 



These things are all satisfactorily explained by Prof. Espy's-theory. 

 Indeed the great beauty of this theory is that it is a philosophical de- 

 duction from existing facts, and is based upon well established principles 

 of science. He has nothing to do with conjecture ; he assumes no hy- 

 pothesis ; he begins, goes on, and ends with facts, demonstrating their 

 truth as he proceeds, and explaining tlie laws which govern them. A 

 theory, thus founded, must be correct in all its essential features, even 

 should some of the minor phenomena be misinterpreted. 



epistles to students. no. vii. 

 My Young Friends : 



Although some time has elapsed since my last communication, it 

 does not indicate an indisposition on my part to continue the letters. 

 Letter-writing is not always the work to which there is a strong ten- 

 dency, and indeed there is frequently greater neglect in complying with 

 duty in this, than in almost any thing else amongst the minor moralities 

 of life. So strong is my inclination to do you good, if not by the com- 

 munication of new truth, yet by stirring up your minds by way of re- 



