38 Browning's New Aneroid Barometer. 



BROWNING'S NEW ANEROID BAROMETER. 



A new aneroid, by Mr. Browning, was exhibited at several of 

 the scientific soirees of the season, and briefly noticed in the 

 last number of the Intellectual Observes. It was con- 

 structed with a view to two important qualities : great sus- 

 ceptibility to atmospheric changes of pressure, and power 

 of making small variations strikingly visible to the eye. 

 We shall endeavour to show how these requirements have 

 been fulfilled, and in order that those to whom the subject 

 is new, may follow our descriptions, we shall begin by con- 

 sidering what an aneroid barometer is. The term aneroid 

 literally means deprived of air, and we might substitute 

 for it the phrase, " vacuum chamber barometer." But 

 neither the original name, nor its English version, indicates 

 the peculiarity of the kind of instrument to which it is applied. 

 Water barometers, mercurial barometers, and so-called ane- 

 roids, all of them have vacuum chambers, more or less complete 

 in their freedom from air. To make a water barometer, a 

 quantity of that fluid is boiled until its air is expelled, and a 

 tube, closed at the top and about thirty-five feet long, is filled 

 with it. The lower end of the tube is immersed in a cistern 

 of water. Now the air presses upon the water in the open 

 cistern with a force of fifteen pounds per square inch ; but it 

 cannot press upon the water at the top of the closed tube — all 

 its pressure in that direction being intercepted by the tube 

 itself. Thus the water in the long tube falls until it exactly 

 counterpoises the air pressure on the open surface of the cis- 

 tern. In round numbers this takes place when the water 

 column in the closed tube is thirty-four feet high. But the 

 tube, according to our supposition, was thirty-five feet high, so 

 that when the water has fallen one foot, it has left a vacuum 

 chamber in the upper part ; not, however, a true and perfect 

 vacuum, because there will be a little vapour in it. Mercury 

 is so much heavier than water that one inch of it balances 

 about thirteen and a-half inches of water. From this it arises 

 that by substituting mercury for water, we can make a baro- 

 meter with a much shorter tube. We can, for example, fill 

 with mercury a tube, closed at the top, as in the preceding 

 case, but thirty-four inches long, and having immersed the 

 open end in an open cistern containing mercury, the column in 

 the tube will fall until its weight balances the atmospheric 

 pressure, and the space above the mercury in the tube will 

 be a vacuum chamber much more perfect than when water 

 was employed. In either case if the air grows heavier, the 



