OF NATURAL PHILOSOPHY. 229 



same principle should not carry it up two, three, or 

 any number of feet ; none why it should suddenly 

 stop short at a certain height, and refuse to rise 

 higher, however violent the suction might be, nay, 

 even fall back, if purposely forced up too high. 



(245.) Galileo, however, at first contented himself 

 with the conclusion, that the natural abhorrence of 

 a vacuum was not strong enough to sustain the 

 water more than about thirty-two feet above its 

 level ; and, although the true cause of the pheno- 

 menon at length occurred to him, in the pressure of 

 the air on the general surface, it was not satisfactorily 

 demonstrated till his pupil, Torricelli, conceived the 

 happy idea of instituting an experiment on a small 

 scale by the use of a much heavier liquid, mercury, 

 instead of water, and, in place of sucking out the 

 air from above, employing the much more effectual 

 method of filling a long glass tube with mercury, 

 and inverting it into a basin of the same metal. It 

 was then at once seen, as by a glaring instance, that 

 the maintenance of the mercury in the tube (which 

 is nothing else than the common barometer) was the 

 effect of a perfectly definite external cause, while its 

 fluctuations from day to day, with the varying state 

 of the atmosphere, strongly corroborated the notion 

 of its being due to the pressure of the external air 

 on the surface of the mercury in the reservoir. 



(246.) The discovery of Torricelli was, however, 

 at first much misconceived, and even disputed, till 

 the question was finally decided by appeal to a cru- 

 cial instance, one of the first, if not the very first 

 on record in physics, and for which we are indebted 

 to the celebrated Pascal. His acuteness perceived 

 Q 3 



