MECHANICAL PRINCIPLES OF FLUIDS. 347 



action of a pair of bellows, its drawing water if immersed in water, 

 its refusing to open when the vent is stopped up. The action of a 

 cupping instrument, in which the air is rarefied by fire ; the fact that 

 water is supported when a full inverted bottle is placed in a basin ; or 

 when a full tube, open below and closed above, is similarly placed : 

 the running out of the water, in this instance, when the top is opened ; 

 the action of a siphon, of a syringe, of a pump ; the adhesion of two 

 polished plates, and other facts, were all explained by ikefuga vacui. 

 Indeed, we must contend that the principle was a very good one, inas- 

 much as it brought together all these facts which are really of the 

 same kind, and referred them to a common cause. But when urged 

 as an ultimate principle, it was not only unphilosophical, but imper- 

 fect and wrong. It was unphilosophical, because it introduced the 

 notion of an emotion, Horror, as an account of physical facts ; it was 

 imperfect, because it was at best only a law of phenomena, not point- 

 ing out any physical cause ; and it was wrong, because it gave an un- 

 limited extent to the effect. Accordingly, 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 instrument 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 horror 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 produce 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 pressure, had already suggested itself. Indeed, 

 this appears from other evidence. Galileo had already taught that 

 the air has weight; and Baliani, writing to him in 1630, says, 2 "If 

 we were in a vacuum, the weight of the air above our heads would be 

 felt." Descartes also appears to have some share in this discovery ; 

 for, in a letter of the date of 1631, he explains the suspension of. 

 mercury in a tube, closed at top, by the pressure of the column of air 

 reaching to the clouds. 



2 Drinkwater's Galileo, p. 90. 



