20 THE SCIENTIFIC PAPERS OP 



from repeating the experiment, and in the meantime practical 

 engineers still continue to adhere to Watt's law. 



Shortly after the publication of Regnault's experiments by the 

 Cavendish Society, in 1848, the idea occurred to the author of 

 the present paper that their results might be brought to a 

 positive test by a simple apparatus, which he places before the 

 Meeting in operation, shown in Fig. 1, Plate 3. It consists of 

 an upright cylindrical vessel of tin-plate A, which is surrounded 

 by an outer vessel filled with charcoal B B, or other non-conducting 

 material. A steam-pipe C, with a contracted glass vein D, enters 

 the inner vessel in a slanting position, in order that the water of 

 priming from the boiler, and of condensation within the pipe, 

 may return to the former, allowing only a small jet of pure steam 

 to enter the vessel, where it suddenly expands and communicates 

 its temperature to the bulb of a thermometer E, which is inserted 

 through a stuffing-box from above. The lower extremity of 

 the inner vessel A is connected on the one hand to a mercury 

 gauge Gr, and on the other to a condenser, by means of a stop- 

 cock to regulate the pressure. The pressure and temperature of 

 the steam within the boiler being known, and the temperature of 

 the expanded steam observed, it will be seen whether that tempera- 

 ture coincides with the temperature which is due to pressure 

 indicated by the mercury gauge. If it did, then Watt's law 

 would be confirmed, but since the temperature rises higher than 

 is due to the pressure, it follows that the high-pressure steam 

 contains an excess of heat, which serves to super-heat the 

 expanded steam. All losses of heat from the apparatus would 

 tend to reduce the temperature, and be in favour of Watt's law ; 

 but it will be shown that those losses may be entirely eliminated, 

 and a true quantitative result be obtained. For this purpose the 

 pressure in the boiler should first be raised to its highest point, 

 and the indicating apparatus be well penetrated by the heat ; the 

 fire under the boiler should thereupon be reduced, and observa- 

 tions made simultaneously, and at regular intervals, of the 

 declining pressure within the boiler, and temperature of the 

 expanded steam of constant pressure. The pressures being nearly 

 equal, the fire under the boiler is again increased, and the 

 observations continued until the maximum pressure is once more 

 obtained ; and the loss of heat by radiation, &c., may be cor- 



