6-1 



ANNUAL OF SCIENTIFIC DISCOVERY. 



pressure in A would cease to remain uniform with the pressure in B, and Ilic 

 mercury column woul.l at oii'v l-ill, and thus indi'-atc the difference. The 

 ::,- tantuur'>i:s change of the position 01' the iiK-ivury is the indication of the 

 point at whi'-h the temperature iu the bath corroponds with the saturation 

 point of the steam in A. 



To show the delicacy of this test, I may instance that, at two hundred and 

 ninety decrees Fahrenheit, the meivury column would rise nearly two inches 

 for every decree of temperature above the saturation point, as the increase 

 of pressure arising from vaporization is twelve times that arising from ex- 

 pansion in superheating at that point, and a similar difference exists at other 

 temperatures. 



The apparatus, as employed for experiment, vanes according to the pres- 

 sure and other circumstances of its use. Fig. 2 represents one of the 



arrangements which has been employed 

 with success. It consists of a glass globe 

 of about seventy cubic inches capacity, in 

 whk'h is placed, after a Torricellian vacuum 

 has been formed, the weighed globule of 

 water. The globe, with the stem, is shown 

 at A ; this is surrounded by a copper boiler, 

 B B, prolonged by a stout glass tube, C, 

 enclosing the globe stem. This copper 

 boiler forms the water and steam bath 

 through which the globe is- heated, and, in 

 fact, corresponds to the second globe, B, in 

 the former figure. The fluctuating mer- 

 cury column, or saturation gauge, is placed 

 at the bottom of the tube, C, and the satu- 

 ration point is indicated by the rise of the 

 inner mercury column, a, and the fall, at 

 the same time, of the outer mercury col- 

 umn, b. As soon as the whole of the 

 water in the globe A is evaporated, there 

 is an instantaneous rise of the inner mer- 

 cury column to restore the balance of pres- 

 sure, and that progressively with the rise 

 of temperature. 



As an auxiliary apparatus, the boiler is 

 provided with gas-jets, E, to heat it, and 

 with an open oil bath, G, to retain the 

 glass tubes at the same temperature as the 

 boiler; and this oil bath is placed on a 

 sand bath and also heated with gas. A 

 thermometer, D, registers the temperature, 

 and a pressure-gage, F, the pressure of the 

 steam; and a blow-off cock, H, serves to 

 iv.'.ucc the temperature when necessary. 

 A number of results have already been 

 obtained, but they arc not yet sufficiently 

 advanced to be made public. The follow- 

 ing numbers have been, however, approxi- 

 mately reduced from the theoretical formula above, and the experimental 



