8 



inch, which amounts to 2160 Ibs., or nearly a ton, upon 

 every square foot. We cannot therefore be surprised at 

 the effects of so elastic and compressible a body as air, 

 when it is set in motion ; the soft breeze of summer and 

 the furious hurricane of winter are instances of the effects 

 of its different velocities. 



On the Construction and Method of Using the Mercurial 

 Barometer. 



There are various forms of the Barometer, but the 

 one best suited for meteorological observations consists 

 of a tube about 33 inches in length, the extremity of 

 which is inserted into a small reservoir or cistern ; and 

 in order to maintain the mercury in the cistern always 

 at the same level, the cistern is constructed partly of 

 leather; that by means of a screw at the bottom, the 

 surface of the mercury in it may be so adjusted, as to 

 have it always at the place from which the scale com- 

 mences. Some barometers are furnished with a gauge 

 or float, that in great elevations and depressions the 

 observer may perceive when the mercury in the cistern 

 sinks too low or rises too high. 



Let a b, fig. 1, be the glass tube plunged into the 

 mercury in the cistern C, and D the surface-line of the 

 fluid in the cistern level with the commencement of 

 the scale, and adjusted to the particular height of the 

 mercury in the tube, which has been actually measured 

 from the surface of the cistern, in the construction of 



