280 



THE BAROMETER, 



Tho eftecvs of suction by the mouth led, by a natural analogy, to suction by 

 artificial means. If a cylinder be open at both ends, and a piston playing in it 

 air-tight be moved to the lower end, upon immersing this lower end in water, 

 and then drawing up the piston, an unoccupied space would remain between 

 the piston and the water. " But nature abhors such a space," said the ancients, 

 " and therefore the water will not allow such a space to remain unoccupied : we 

 find, accordingly, that as the piston rises the water follows it." By such poetical 

 reasoning pumps of various kinds were constructed. 



The antipathy entertained by nature against an empty space served the pur- 

 poses of philosophy for a couple of thousand years, when it so happened that 

 some engineers employed at Florence in sinking pumps, had occasion to con- 

 struct one to raise water from an unusually great depth. Upon working it, they 

 found that the water would rise no higher than about thirty-two feet above the 

 well. Galileo, the most celebrated philosopher of that day, was consulted in 

 this difficulty, and it is said that his answer was, that " nature's abhorrence of a 

 vacuum extended only to the height of thirty-two feet, but that beyond this her 

 disinclination to an empty space did not extend." Some writers deny the fact 

 of his having given this answer ; others admit it, but take it to have been iron- 

 ical. It has been more generally taken as a solution seriously intended. It 

 appears, however, that Galileo, having his attention thus directed to the point, 

 soon saw the absurdity of the maxim that " nature abhors a vacuum," and sought 

 to account for the phenomenon in other ways. 



He attributed the elevation of the water to an attraction exerted upon that 

 liquid by the piston. This attraction he conceived to have a determinate inten- 

 sity, and when such a column of water was raised as was equal in weight to 

 the whole amount of the attraction, then any farther elevation of the water by 

 the piston became impossible. 



At a very remote period air was known to possess the quality of weight. 

 Aristotle and other ancient philosophers expressly speak of the weight of air. 

 The process of respiration is attributed by an ancient writer to the pressure of 

 the atmosphere forcing air into the lungs. Galileo was therefore fully aware that 

 the atmosphere possessed this property, and it is not a little surprising that 

 when his attention was so immediately directed to one of the most striking 

 effects of it, he was unable to perceive the connexion. 



Some writers affirm, we know not upon what authority, that Galileo, at the 

 time he was interrogated respecting the limited elevation of water in a common 

 pump, was aware of the true cause of the effect ; but that, not having thoroughly 

 investigated the subject, he evaded the question of the engineers, with a view 

 to conceal his knowledge of the principle until he had carried his inquiry to a 

 more satisfactory result. It does not, however, appear that he published his 

 solution of the problem. After his death, Torricelli, his pupil, directed his at- 

 tention to the same problem. He argued that whatever be the cause which 

 sustained a column of water in a common pump, the measure and the energy 

 of that power must be the weight of the column of water ; and, consequently, 

 if another liquid be used, heavier or lighter, bulk for bulk, than water, then 

 the same force must sustain a lesser or greater column of such liquid. By 

 using a much heavier liquid, the column sustained would necessarily be much 

 shorter, and the experiment in every way more manageable. 



He therefore selected for the experiment mercury, the heaviest known liquid. 

 The weight of mercury, bulk for bulk, being about 13^ times that of water, it 

 follows that the height of a column of that liquid which would be sustained by 

 a vacuum must be 13^ times less that the height of a column of water thus 

 sustained. 



Hence he computed that the height of the column of mercury would be 



