PHYSICS OF THE EIGHTEENTH CENTURY. 



A reader unaccustomed to consider atoms and their motions of 

 greater or less amplitude, etc., may not improbably be at first unable 

 to reconcile the obvious and complete apparent absence of motion in 

 a body as a whole, with the possibility of complex and rapid motion 

 of its minute particles. He must remember that these particles or 

 atoms, being themselves by supposition altogether invisible, their mo- 

 tions must be invisible also. His conceptions may be aided by the 

 beautiful illustration which is given of this very subject by Lucretius, 

 who points out that a flock of sheep browsing on a very distant hill 

 will appear as a motionless white patch on the green slope, although 

 the individual animals are, in fact, freely moving in various ways. 



Nam ssepe in colli tondentes pabula laeta 



Lanigerse reptant pecudes,-quo quamque vocantes 



Invitant herbas gemmantes rore recent! ; 



Et satiati agni ludunt, blandeque coruscant, 



Omnia quas nobis longe confusa videntur 



Et veluti in viridi candor consistere colli. De Rer. Nat. II., 317. 



The idea of atoms and their invisible motions is, as indeed we have 

 already seen, by no means new to philosophy. The views enunciated 

 by Rumford and Davy as to the nature of heat may, however, be con- 

 sidered the first distinct expression of the Dynamical Theory of Heat. 



From the latter part of the eighteenth century dates a scientific dis- 

 covery or, perhaps more properly, an 

 invention which excited everywhere 

 the greatest surprise and admiration 

 in the public mind. It appeared also 

 to open up to the scientific explorer 

 regions which the most audacious of 

 his predecessors would have consi- 

 dered for ever inaccessible. Though 

 the great results which it seemed to 

 promise to science and to humanity 

 have not been realized, and the utility 

 of the invention has hitherto proved 

 as limited as the enthusiasm it first 

 excited was unbounded, it is never 

 even now seen in action without lively 

 interest. In 1782 the brothers STE- 

 PHEN and JOSEPH MONTGOLFIER 

 made their first experiments on bal- 

 loons; and in June, 1783, the ascent 

 of a balloon was for the first time FIG. 150. 



witnessed by the public at Annonay. 



(See Fig. 151.) The balloon was made of linen covered with paper, 

 and was of the kind called a fire-balloon. That is, it was provided 

 with the means of burning substances at the lower part, so that the 



