328 Sir William Thomson on the Lltramundane 



Indeed there exists a dissertation by Segner on this subject*. 

 But science took another course, and works of this nature gradu- 

 ally lost appreciation. Le Sage has never failed on any occasion 

 to call attention to the system of Redeker, as also to that of 

 Fatio " f. 



Le Sage shows that, to produce gravitation, those of the ultra- 

 mundane corpuscules which strike the cage-bars of heavy bodies 

 must either stick there or go away with diminished velocities. 

 He supposed the corpuscules to be inelastic (durs), and points out 

 that we ought not to suppose them to be permanently lodged in 

 the heavy body (entasses), that we must rather suppose them to 

 slip off, but that, being inelastic, their average velocities after 

 collision must be less than that which they had before collisionf. 



That these suppositions imply a gradual diminution of gravity 

 from age to age was carefully pointed out by Le Sage, and re- 

 ferred to as an objection to his theory. Thus he says, "... Done, 

 la duree de la gravite seroit finie aussi, et par consequent la duree 

 du monde. 



Reponse. Concedo ; mais pourvu que cet obstacle ne contribue 

 pas a faire finir le monde plus promptement qu'il n'auroit fini 

 sans lui, il doit etre considere comme nul"" §. 



Two suppositions may be made on the general basis of Le 

 Sage's doctrine : — 



1st (which seems to have been Le Sage's belief). Suppose 

 the whole of mundane matter to be contained within a finite space, 

 and the infinite space round it to be traversed by ultramundane 

 corpuscules, and a small proportion of the corpuscules coming 

 from ultramundane spaceto suffer collisions withmundane mattter, 

 and get away with diminished gravific energy to ultramundane 

 space again. They would never return to the world were it not 

 for collision among themselves and other corpuscules. Le Sage 

 held: — that such collisions are extremely rare that ; each collision, 

 even between the ultramundane corpuscules themselves, destroys 

 some energy || ; that at a not infinitely remote past time they were 

 set in motion for the purpose of keeping gravitation throughout 

 the world in action for a limited period of time ; and that both by 



* De Causa gravitatis Redekeriana. 



t Le Sage was remarkably scrupulous in giving full information regar- 

 ding all who preceded him in the development of any part of his theory. 



% Le Sage estimated the velocity after collision to be two thirds of the 

 velocity before collision. 



§ Posthumous Traite de Physique Mecanique, edited by Pierre Prevost, 

 Geneva and Paris, 1818. 



|| Newton (Optics, Query 30, ed. 1721, p. 373) held that two equal 

 and similar atoms, moving with equal velocities in contrary directions, 

 come to rest when they strike one another. Le Sage held the same ; and 

 it seems that writers of last century understood this without qualification 

 when they called atoms hard. 



