160 ANNUAL OF SCIENTIFIC DISCOVERY. 



condensers of an engine. A barometer is made in the same simple 

 manner, by completely exhausting the air from the coiled tube', and 

 hermetically closing it. As the pressure of the atmosphere varies, so 

 does the tube give under it in proportion ; and thus, with the greatest 

 accuracy, such an instrument indicates the smallest variations of 

 atmospheric pressure, and forms one which, from its solidity and sim- 

 plicity, we consider to be preferable to any sort of barometer as yet 

 produced. 



To form a thermometer the same shaped metal is employed ; or if 

 for indicating the temperature of liquids, the tube is generally twisted 

 in its whole length, instead of being coiled, and, being filled with alco- 

 hol, it is well closed ; and thus as the temperature varies so will the 

 alcohol in the tube expand or contract, and force the tube to observe 

 its action, and indicate the variations of temperature, either of the 

 surrounding atmosphere, or of any body of fluid into which it may be 

 plunged. 



Steam pressure gauges, constructed upon M. Bourdon's principle, 

 have been ordered by the French government to be applied to all loco- 

 motive engines in that country. 



ALARM BAROMETER. 



A BAROMETER has been constructed by Captain Ericsson, so adapted as 

 to give warning itself of any remarkable atmospheric change. The 

 principle of its construction is as follows : when the column descends 

 in the tube beyond any given altitude, the falling of the mercury 

 causes a gong to be sounded, by means of a hammer impelled by a 

 spring. The mercury, in its descent into the cup, disturbs the equi- 

 librium of a lever, which disengages a catch connected with the ham- 

 mer. This barometer will be of special benefit at sea, where a 

 remission of watchfulness may peril the safety of the vessel. 



HEIGHT OF THE ATMOSPHERE. 



SIR JOHN W. LUBBOCK, according to the hypothesis adopted by him 

 in his Treatise on the Heat of Vapors, shows the density and temper- 

 ature for a given height above the earth's surface. According to that 

 hypothesis, at a height of fifteen miles the temperature is 240 6' Fahr. 

 below zero ; the density is .03573 ; and the atmosphere ceases alto- 

 gether at a height of 22.35 miles. M. Biot has verified a calculation 

 of Lambert, who found, from the phenomena of twilight, the altitude 

 of the atmosphere to be about eighteen miles. The condition of the 

 higher regions of the atmosphere, according to the hypothesis adopted 

 by Ivory, is very different, and extends to a much greater height. 



MICROMETRIC APPARATUS. 



ONE of the most original contributions to the " Great Exhibition," 

 was the so-called " Micrometric Apparatus," contributed by Messrs. 

 Whitworth. It is the first attempt yet made in the "mechanical world 

 to establish a uniform standard of magnitude in machinery, so that 



