MIRACLES AND SPECIAL PROVIDENCES 29 



depend. The mind is, as it were, a pliotographic plate, 

 which is gradually cleansed by the effort to think rightly, 

 and which, when so cleansed, and not before, receives 

 impressions from the light of truth. This passage from 

 facts to principles is called induction; and induction, in 

 its highest form, is, as I have just stated, a kind of in- 

 spiration. But, to make it sure, the inward sight must 

 be shown to be in accordance with outward fact. To 

 prove or disprove the induction we must resort to deduc- 

 tion and experiment. 



Torricelli reasoned thus: If a column of water thirty- 

 two feet high holds the pressure of the atmosphere in 

 equilibrium, a shorter column of a heavier liquid ought 

 to do the same. Now, mercury is thirteen times heavier 

 than water; hence, if my induction be correct, the atmos- 

 phere ought to be able to sustain only thirty inches of 

 mercury. Here, then, is a deduction which can be imme- 

 diately submitted to experiment. Torricelli took a glass 

 tube a yard or so in length, closed at one end and open 

 at the other, and filling it with mercury, he stopped the 

 open end with his thumb, and inverted it into a basin 

 filled with the liquid metal. One can imagine the feeling 

 with which Torricelli removed his thumb, and the de- 

 light he experienced on finding that his thought had fore- 

 stalled a fact never before revealed to human eyes. The 

 column sank, but it ceased to sink at a height of thirty 

 inches, leaving the Torricellian vacuum overhead. From 

 that hour the theory of the pump was established. 



The celebrated Pascal followed Torricelli with another 

 deduction. He reasoned thus: If the mercurial column be 

 "supported by the atmosphere, the higher we ascend in the 

 air, the lower the column ought to sink, for the less will 



