THE SMITHSONIAN INSTITUTION. 



329 



During the period of the experiment, three minutes, in which the 

 gas was caught, the needle vibrated very little ; it receded regularly, 

 but the rate was at most 0°.5 in three minutes. The number of de- 

 grees of the table are the means of all the angles read from the begin- 

 ning to the end of tlie three minutes. 



The quotient obtained by dividing the quantity of gas for one min- 

 ute by the tangent of the corresponding angle of deflection should 

 be properly a constant quantity, indicating how much gas a current 

 develops per minute, which produces in the tangent compass a deflec- 

 tion of 45°, (because fang. 45° = 1). The following values of these 

 quotients were obtained from the different experiments : 



During the experiments the temperature of the room was 15° Cent. , 

 and the height of the barometer 744 millimetres. The gas was caught 

 in a graduated tube, and the surface of the water in the tube stood 

 about ten centimetres higher than that without, which is equivalent 

 to a pressure of seven millimetres of mercury. Hence the gas sus- 

 tained a pressure of 733 millimetres. Keduced to 0° Cent, and a 

 barometric height of 760 millimetres, the quantities of gas, 76.5 cub. 

 centimetres and 70 cubic c. , obtained from the observation at 15° Cent. , 

 and 733 millimetres, are respectively 69.94 and 64.01 cubic centime- 

 tres, or, in round numbers, 70 and 64. 



Thus a current which produces in the large comp;iss a deflection of 

 45° will yield 70 cubic centimetres per minute ; one producing in the 

 small compass the same deflection will yield 64 cubic centimetres per 

 minute of detonating gas, at 0° Centigrade, and under a pressure 

 of 760 millimetres. 



Hence, in chemical measure the force of a current which produces 

 a deflection of v° in the large tangent-compass is_, 



S =: 70 tangent v. 



A current producing a deflection of lo degrees in the small compass 

 has, in chemical measure, a force — 



S' = 64 tangent iv. 



The constant factor for the reduction of the reading of a torsion gal- 

 vanometer, a Weber's tangent-compass, or a compass of sines, may be 



