A HISTORY OF SCIENCE 



It had long been suspected and believed that the 

 density of the atmosphere varies at certain times. 

 That the air is sometimes "heavy" arid at other times 

 "light" is apparent to the senses without scientific 

 apparatus for demonstration. It is evident, then, 

 that Torricelli's column of mercury should rise and fall 

 just in proportion to the lightness or heaviness of the 

 air. A short series of observations proved that it did 

 so, and with those observations went naturally the 

 observations as to changes in the weather. It was 

 only necessary, therefore, to scratch a scale on the 

 glass tube, indicating relative atmospheric pressures, 

 and the Torricellian barometer was complete. 



Such a revolutionary theory and such an important 

 discovery were, of course, not to be accepted without 

 controversy, but the feeble arguments of the oppo- 

 nents showed how untenable the old theory had be- 

 come. In 1648 Pascal suggested that if the theory of 

 the pressure of air upon the mercury was correct, it 

 could be demonstrated by ascending a mountain with 

 the mercury tube. As the air w r as known to get pro- 

 gressively lighter from base to summit, the height of 

 the column should be progressively lessened as the 

 ascent was made, and increase again on the descent 

 into the denser air. The experiment was made on the 

 mountain called the Puy-de-D6me, in Auvergne, and 

 the column of mercury fell and rose progressively 

 through a space of about three inches as the ascent 

 and descent were made. 



This experiment practically sealed the verdict on 

 the new theory, but it also suggested something more. 

 If the mercury descended to a certain mark on the 



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