Aofw. 435 



V. 



On the History and Construction of the Barometer. 



THE barometer takes its origin from the experiment of Torri- 

 celli, who in consequence of the suggestion of Galileo with regard 

 to the ascent of water in pumps, proceeded in 1643 to make experi- 

 ments with a tube filled with mercury, conjecturing that as this 

 fluid was about thirteen times heavier than water, it would stand 

 at only one thirteenth of the height to which water rises in pumps, 

 or at about thirty inches. He, therefore, filled a glass tube about 

 three feet long with mercury, and upon immersing the open end in 

 a vessel of the same fluid, he found that the mercury descended in 

 the tube, and stood at about twenty-nine and a half Roman inches, 

 and this vertical elevation was preserved, whether the tube was 

 perpendicular or inclined to the horizon, according to the known 

 laws of hydrostatical pressure. This celebrated experiment was 

 repeated and diversified in several ways with tubes filled with 

 other fluids, and the result was the same in all, allowance being 

 made for difference of specific gravity, and thus the weight and 

 pressure of the air were fully established. Such, however, was the 

 force of prejudice that many refused to yield their assent till, at the 

 suggestion of Pascal, the experiment was performed at different 

 heights in the air with such results as left no longer any doubt upon 

 the subject. 



Great care is necessary in the construction of the barometer. 

 The tube after being cleansed as perfectly as possible, is to be grad- 

 ually heated, and to be kept at a pretty high temperature for a con- 

 siderable time, for the purpose of expelling all moisture that may 

 be found adhering to it. The mercury is then to be introduced ; a 

 small quantity only is first poured in by means of a fine funnel and 

 thoroughly boiled in order to free it from air ; then another portion 

 is added, and so on till the tube is filled. It is afterward to be care- 

 fully inverted, and the open end immersed in a cistern of boiled 

 mercury. 



The tube and cistern is enclosed in a metallic or wooden frame- 

 work, containing the graduations and some necessary appendages. As 

 the mercury rises and falls in the tube by the fluctuations of the 

 atmosphere, its surface varies also in the cistern. But the gradual 

 tion is intended to mark the exact length of the column, reckoned 

 from this variable surface. If a horizontal section of the tube and 

 cistern have a constant ratio to each other throughout the extent 



