MAGNUS ON THE EXPANSIVE FORCE OF STEAM. 219 
the pressure of steam are founded either on the determination of 
the temperature at which water boils under various pressures, or 
on the bringing of water into a perfect or nearly perfect vacuum, 
exposing it to different temperatures and measuring the pressure 
which its vapour exerted. 
The first of these two methods has been executed sometimes 
by observing the temperature at which water boils on high 
mountains, sometimes by boiling it under the recipient of the 
air-pump. This latter mode of inquiry, according to which ex- 
periments have been made by. Achard*, Gren +, and likewise by 
Dalton {, can only yield approximately accurate results, since 
when water boils under the air-pump its temperature constantly 
decreases, as well as the expansive force of the vapour. The 
observation of the boiling-point on high mountains may on the 
contrary lead, when carefully performed, to perfectly certain de- 
terminations. However, of this kind we unfortunately possess 
only older observations, with the exception of a very recent 
treatise by Forbes§. The first who made such observations 
was probably Le Monier||, who at Perpignan, in 1739, im- 
mersed a thermometer graduated at 28 inches 2 lines in boil- 
ing water on the Canigou in the Pyrenees. The barometer stood 
only at 20 inches 23 lines, and corresponding, the thermometer 
likewise stood 15° Delisle’s or 9° R. lower than at Perpignan. 
Subsequently, in the years 1762 and 1770, De Luc** made 
several series of such observations on the Alps. He employed 
a thermometer which had been graduated at a barometric height 
of 27 inches. Gren+t+ has on that account reduced the state- 
ments to a thermometer graduated at 28 inches. These observa- 
tions, however, as well as a couple by Saussure {{, made in his 
journey through the Alps, have not been performed with all the 
requisite measures of precaution, and consequently have not led 
to any accurate results. 
Most of the investigations on the expansive force of steam 
* Achard, Sammlung physikalischer und chemischer Schriften, i. 213; like- 
wise in the Schriften der Berliner Akademie, 1782, p. 3. 
+ Neues Journal, i. 184. 
t Memoirs of the Literary and Philosophical Society of Manchester, vol. v. 
part 2. p. 535. 
§ Transactions of the Royal Society of Edinburgh, vol. xv. part 3. p. 409. 
|| As stated by Cassini de Thury in the Mémoires de l’ Académie, 1740, p. 92. 
** Recherches sur l’ Atmosphere, ii. 285. and iv. 143. 
++ Gren’s Neues Journal der Physik, i. 179. 
tt Saussure, Voyage, § 1275 and 2011. 
