465 
NATURAL-PHILOSOPHICAL COLLECTIONS. 
On the Elastic Power of Steam.—Report of the Commission appointed by the 
Academy of Sciences of Paris. 
In the name of a commission, composed of MM. Arago, de Prony, Ampere, 
Girard, and Dulong, the last mentioned gentleman presented a report, which was 
entitled :—An account of inquiries made by order of the Academy of Sciences, 
for the purpose of determining the elastic powers of the vapour of water at high 
temperatures. 
The decree which fixes the measures of security to which the construction of 
steam-engines is to be subjected, prescribes, among others, as a necessary pre- 
caution, the employment of metallic rondelles fusible at temperatures, which 
surpass by from 10° to 20° the temperatures corresponding to the elasticity of 
vapour in the ordinary state of the machine when at work. 
Before such a condition can be complied with, it is necessary to know the tem- 
perature which corresponds to a given elasticity of steam. Now, science does not 
possess this knowledge, and the commission above named was directed to fill 
up the void which existed on this subject. MM. Dulong was particularly charged 
with the construction of the apparatus, and the direction of the experiments. M. 
Arago also co-operated with him, and made a great number of experiments in 
common. 
The commission, desirous of making its investigation as perfect as the present 
state of science demands, and presuming that an opportunity might not soon oc- 
cur of recommencing ahd carrying to the same length observations of the same 
kind as those with which they were engaged, rejected as inaccurate the procedure 
which consists in estimating the elastie power of steam by means of a valve 
loaded with weights, and resolved, notwithstanding the difficulty of such an 
enterprise, to have recourse to the direct measurement of the column of mercury 
which forms an equivalent to the elasticity of the steam. 
When this elasticity does not exceed a certain number of atmospheres, the di- 
rect measurement of the fluid column which it can support, presents no diffi- 
culty ; but in such cases, as when the pressures of from twenty to thirty atmos- 
pheres are to be valued, a column of mereury of from 70 to 80 feet high has to 
be contained in a tube; and it is necessary that the tube be of giass, to be easily 
observable in every part. The difficulties connected with the execution of such 
a project may readily be conceived. They were, however, surmounted, and the 
apparatus was finally reared, with the permission of the administration of civil 
buildings, in the old square tower, known by the name of Tour de Clovis, the 
only remains of the ancient church of St. Genevieve. 
The apparatus consisted essentially of two parts: a kettle or caldron intended 
for supplying the steam, and a glass tube employed for supporting the column 
of mercury. But it was to be feared that the too rapid augmentation of the 
power of the steam, and especially the instantaneous diminution which would 
necessarily follow the opening ef the safety-valve, would occasion shocks similar 
to those of the hydraulic ram ; which might endanger the more fragile parts, and 
produce the effusion and loss of a great mass of mercury. It was, therefore, ne- 
cessary to guard against accident. 
To accomplish this object, it was resolved not to put the column of mercury 
immediately in connection with the steam; but to make use of an intervening 
instrument, a kind of manometer, which should give precisely the same indica- 
tions as the column of mercury, without being productive of the same inconve- 
niences. 
The operations relative to the measurement of the tension of the stream were, 
therefore, preceded by an operation, which consisted in ascertaining the precise 
degree of elasticity acquired by air, whose volume was reduced in a determinate 
VOL. I. 3N 
