370 NATURE AND MAN. 



means of more exactly determining the downward " pull " of a mass, 

 than any personal estimate of it could afford. Differences of 

 weight being thus determined between masses of the same size, 

 but of different kinds of matter— as, for instance, between a cube 

 of lead and a cube of stone, or between a cubic vessel of water 

 and a block of wood of the same dimensions — gave the notion of 

 differences of relative weight (or " specific gravity ") and the weight 

 of water being taken as the standard of comparison, a distinction 

 was drawn between " heavy " and " light " bodies. The floating 

 of a piece of wood on the surface of water, and its rising up from 

 the bottom when no longer held down, were rightly interpreted as 

 a consequence of their respective downward tendencies or relative 

 weights ; for since it could be shown by experiment that if equal 

 measures of wood and of water were put in the two scales of a 

 balance, the water would go down, it w^as seen that the earth must 

 have a greater attraction for it, and that the ascent of the wood is 

 brought about by the descent of the water to take its place. 



Now here we have a very simple case of what is commonly 

 called the " explanation " of a natural phenomenon. To those 

 who first reflected on the matter, the ascent of the solid wood 

 through the liquid water might seem an exception to the general 

 uniformity, for v/hich the philosopher of the time would be desired 

 to account. And he would do so by showing that it is really in 

 accordance with such uniformity. Further than this he could not 

 go ; and further than this no scientific explanation can go. As 

 J. S. Mill has truly said, " In science, those who speak of explaining 

 "any phenomenon mean (or should mean) pointing out not some 

 "more familiar, but merely some more general, phenomenon, of 

 "which it is a partial exemplification." — But our ancient philosopher 

 could ?iot have so explained the ascent of the smoke ; for he knew 

 not that both the atmosphere and the smoke have weight ; but 

 that the smoke, being the lighter of the two, ascends like a piece of 

 wood through a column of water ; and he could only account for 

 it by attributing to the smoke an exceptional "levity," which 

 made it ascend, whilst all other bodies descended. But he could 

 not really get any nearer to the "cause" of the general, than to 

 that of this exceptiotial phenomenon. As it is a " property," he 

 would say, of the earth to attract, and of bodies in general to be 



