MAGNUS ON THE EXPANSIVE FORCE OF STEAM. 227 
come constant, air let out until its expansive force is equal to 
that of the steam. In this manner I have determined the ex- 
pansive forces above 100°C. I was however compelled to ar- 
range the caoutchouc-tubes so that they should not be blown 
out by the inner pressure. With the employment of the above- 
mentioned gauge, 0 p q, it would only have been possible to 
observe up to an expansive force of somewhat more than two 
atmospheres. To measure still higher expansive forces, it was 
only requisite to exchange the manometer for one in which the 
pressure is indicated by the changes in volume of a mass of air; 
it was also requisite to substitute solid metallic connections for 
those of caoutchouc and glass. The tube a 4d in which the va- 
pour is generated, as well as the tube of the manometer, can 
always be procured of sufficiently strong glass. This arrange- 
ment would then, in my opinion, deserve the preference to that 
employed by the French academicians* to measure the expansive 
forces at high temperatures ; for this latter has, if I mistake not, 
yielded no perfectly accurate results, because the vapour, not- 
withstanding the measures of precaution taken, could not pos- 
sess so high a temperature there, where it exerted its pres- 
sure, as in the boiler; but if the temperature of a space con- 
taining vapour differs at various places, its expansive force will 
always correspond only to the lowest, or at least to nearly the 
lowest temperature; and yet in the investigation by the acade- 
micians, only the temperature in the boiler, consequently only 
the highest existing, could be observed and laid down as a basis. 
Probably the differences between their results and those subse- 
quently made by the Committee of the Franklin Institute in 
Pennsylvania}, are owing to this circumstance, although even 
in these latter the temperature in the boiler appears alone to 
have been measured. 
The calculation of the expansive forces of steam from the ob- 
servations obtained by means of the above-described apparatus 
is so simple that it scarcely requires mentioning; I will, how- 
ever, give one example. 
On the 16th of June, 1843, the barometer in the glass case 
HK L indicated 759°4™™ at 20° C., and the gauge 436:1™™; 
the difference consequently was 323°3™™, These reduced to 0° 
give 322-26™™ for the expansive force of the rarefied air. In 
the case the mercury in the arm of the tube a 4 d, in which 
* Ann. de Chim. et de Phys., vol. xliii. p. 274. 
t Encyclopedia Britannica, yol. xx. p. 588, Steam. 
