66 HISTORY OF MECHANICS. 



cause. But when urged as an ultimate principle, it 

 was not only unphilosophical, but imperfect and 

 wrong. It was unphilosophical, because it introduced 

 the notion of an emotion, Horrour, as an account 

 of physical facts ; it was imperfect, because it was 

 at best only a law of phenomena, not pointing out 

 any physical cause ; and it was wrong, because it 

 gave an unlimited extent to the eifect. Accord- 

 ingly, it led to mistakes. Thus Mersenne, in 1644, 

 speaks of a siphon which shall go over a mountain, 

 being ignorant then that the effect of such an in- 

 strument was limited to a height of thirty-four feet. 

 A few years later, however, he had detected this 

 mistake; and in his third volume, published in 

 1647, he puts his siphon in his emendanda, and 

 speaks correctly of the weight of air as supporting 

 the mercury in the tube of Torricelli. It was, 

 indeed, by finding this horrour of a vacuum to have 

 a limit at the height of thirty-four feet, that the 

 true principle was suggested. It was discovered that 

 when attempts were made to raise water higher 

 than this, nature tolerated a vacuum above the 

 water which rose. In 1643, Torricelli tried to pro- 

 duce this vacuum at a smaller height, by using ? 

 instead of water, the heavier fluid, quicksilver ; an 

 attempt which shows that the true explanation, the 

 balance of the weight of the water by another pres- 

 sure, had already suggested itself. Indeed, this 

 appears from other evidence. Galileo had already 

 taught that the air has weight; and Baliani, writing 



