585 
of Edinburgh, Session 1871 - 72 . 
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 he inelastic ( durs ), and points 
out that we ought not to suppose them to he permanently lodged 
in the heavy body ( entasses ), that we must rather suppose them 
to slip off; hut that being inelastic, their average velocities after 
collision must he less than that which they had before collision.* 
That these suppositions imply a gradual diminution of gravity 
from age to age was carefully pointed out by Le Sage, and referred 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. 
u Beponse. Concedo; mais pourvu que cet obstacle ne contribue 
u pas a faire finir le monde plus promptement qu’il n’auroit fini sans 
“ lui, il doit etre considere comme nul.”f 
Two suppositions may he made on the general basis of Le Sage’s 
doctrine:— 
ls£, (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 he traversed by ultramundane 
corpuscules; and a small proportion of the corpuscules coming 
from ultramundane space to suffer collisions with mundane matter, 
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 ;J that at a not infinitely remote past time they 
were set in motion for the purpose of keeping gravitation through¬ 
out the world in action for a limited period of time; and that 
* Le Sage estimated tlie velocity after collision to be two-thirds of the 
velocity before collision. 
f Posthumous. “ Traite de Physique Mecanique,” edited by Pierre Prevost. 
Geneva and Paris, 1818. 
i Newton (Optics, Query, SO Edn. 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. 
