IS Prof. Leslie on Heat and Climate. [July, 



whirled it some minutes about my head. In one experiment, the 

 contained mercury was heated to 2-1°, and in another 3°. The 

 ancients were well acquainted with this fact, but exaggerated it 

 so much, that the moderns have treated the whole as a fiction. 

 The Babylonians are said by Suidas * to have roasted eggs by 

 whirling them in slings ; and Virgil, with the licence of a poet, 

 represents a leaden ball as melted by the violence of the throw : 



Et media adversi liquefacto tempora plumbo, 

 Diffidit, ac multa porrectum extendi t arena. 



jEn. ix. 588. 



The work of Lucretius is didactic, and may, therefore, be 

 presumed to adhere closer to truth ; yet this author attempts to 

 explain the origin of thunder from the principle alluded to, which 

 he cites as a fact well known and established : 



Mobilitate sua fervescit ; ut omnia motu 

 Percalefacta vides ardescere — — ^— - 



But unfortunately he is led by a hasty analogy into a false 

 conclusion : 



' pluinbea vero 



Glans etiam longo cursu volvenda liquescit. Liv. vi. 179. 



This last clause suggests the way in which this curious conse- 

 quence was drawn by the ancients. The slingers usually threw 

 leaden bullets, which, being sometimes picked up immediately 

 in order to be returned by their antagonists, would give a sensa- 

 tion of heat. Perhaps an opinion which, in a certain degree, 

 is true, that the heat acquired is proportional to the length of 

 the track, was the source of the hyperboles which cloud the 

 subject. 



But to estimate the precise effects, it will be necessary to 

 bestow a closer attention. The density of the air at the surface 

 of the earth is on an average equal to what would be produced 

 by the pressure of a column of air 28,000 feet high, and of the 

 same uniform density. Hence it may be computed that the 

 compression of a whole atmosphere would project the aerial 

 particles with the velocity of 1340 feet in a second. In other 

 cases, the velocity of the stream will be in the subduplicate ratio 

 of the difference of density. By reversing the supposition, we 

 may conclude that when a flat body is carried directly through 

 the atmosphere with the celerity of 500 feet in a second, the 

 stratum of air on its anterior surface will suffer a condensation 



equal to (-jvjT)) or '1391. Consequently the temperature of this 



portion of the fluid must be augmented by 125° x -1391 ( ' -) 

 or about 16°. The velocity just stated may be produced by aa 



