Mechanical Sciencei 195 



of 6 lines only, the rest of the water remained fluid. Another 

 shell was filled with water at 46° F., the aperture closed by a 

 metallic screw, havint^ a passaa^e 6 lines in diameter; it was ex- 

 posed to the air at 28° F., the shell burst into two pieces, one of 

 which was thrown 4 feet off, the thickness of the crust of ice was 

 13 lines, and the water within remained liquid. Another shell 

 was filled with water at the same temperature, closed by a solid 

 screw, and exposed to the air at 28° F. as before ; the shell was 

 burst into two parts, the smaller being thrown about a foot from 

 the place, and the thickness of the coating of ice within was 6 

 lines. — Bull. Univ. F. xiii. 314. 



5. New Hygroscope. — This instrument is the invention of M. 

 Benout, and although he calls it a hygroscope, it still has the 

 power to a certain degree of 9neasuring moisture present in air. 

 Its indications appear to be exceedingly delicate ; its action de- 

 pends upon the expansion and contraction of paper, when exposed 

 to moisture and dryness ; that used is what is known by the term 

 papier vegetal, which is exceedingly hygrometric, very thin and 

 very homogeneous. The principal piece in the instrument con- 

 sists of an exceedingly thin metallic plate about 10 inches long, 

 and -yj^ of an inch in width ; it is formed into a helix, and the 

 external part is covered with a plate of the paper above men- 

 tioned, of the same size, united to it by means of a cement unacted 

 upon by moisture. When this spiral is in moist air, its external 

 surface being hygrometric expands, whilst the inner does not, and 

 a motion of torsion takes place, which may be rendered evident 

 by a needle attached to the lower coil, and traversing over a 

 graduated circle. The contrary motion occurs as the instrument 

 becomes dry. The length of the spiral is conveniently made such 

 as to allow a motion of the needle, in one revolution from 60° to 

 100° on Saussure's scale. 



This instrument is liable to certain objections, amongst which 

 are the following : i. As the paper becomes moist, it expands, 

 overcoming the elasticity of the metal, but ultimately it becomes 

 so soft, that though it is becoming damper, the force of the metal 

 overcomes the paper, and the needle returns towards dryness; this 

 effect only happens near to extreme moisture, at 95° of Saussure's 

 scale, ii. The elasticity of the metallic plate varying with the 

 temperature, causes that changes in heat are sufficient, by in- 

 fluencing the quality, to produce motions in the needle, iii. The 

 two parts of the spiral, the metal and the paper, being of unequal 

 expansion by heat, the instrument acts like Breguet's thermometer. 

 The effect of heat is the same with increase of moisture, as to the 

 motion of the needle ; the efifect of cold is the same as increased 

 dryness. 



The sensibility of the instrument is very great ; an experiment 

 like one of Saussure's was repeated with it ; the instrument was 

 put under the receiver of a good air-pump, and when stationary, 



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