296 
FORBES ON COMPRESSIBILITY OF WATER. 
and air are exhibited at once, under any pres- 
sure, that of the water being sensibly uniform 
for equal increments of pressure, whilst that 
of air rapidly diminishes. 
It is the very trifling compressibility of water 
^or any other liquid) which gives the value to 
^his application, and which seems to have been 
practically overlooked. 
The reason is obvious. The changes o^ 
volume produced by a pressure of only one or 
two atmospheres, in the case of air, are 
quantities very large in proportion to the 
primitive volume, so that, in the considera- 
tion of an additional change, we are obliged 
to take into account not merely the effect up- 
on the primitive volume, but upon the volume 
affected by the first unit of pressure. In other 
words, we are not at liberty to neglect quanti- 
ties of the second order, which we may safely 
do in the case of any known liquid. In the 
case of water, for instance, the variation of 
volume for one atmosphere does not exceed 
1 
of the whole ; so that the variation of 
20,000 
the variation is necessarily insignificant. 
All that we know of the constitution of liquids 
would lead us to infer, that such would be 
Mie case, and upon this circumstance depends 
the linearity of the expression, which coi\nects 
the volume of a liquid such as water, and the 
pressure to which it is subjected. 
Within ordinary practical limits, we may 
confidently anticipate the sensible proportion- 
ality of pressure and change of volume ; and 
this is fully borne out by a comparison of the 
best experiments on the compressibility of 
water made within great and within narrow 
limits. 
I did not hesitate, therefore, to recommend 
the trial of a manometer of water instead of 
air, for measuring the elasticity of gas up to 
40 atmospheres of pressure. 
The construction of such an instrument 
being almost like that of the common ther- 
mometer, is incomparably simpler than that 
of the other instruments above mentioned ; 
and almost the only practical difficulty is 
common to all these, namely, the accurate 
determination of the temperature of the fluid 
employed. 
It may be proper to remark, that Professor 
Oersted’s instrument for indicating the com- 
pressibility of water, consists merely of a 
very -sensible thermometer, constructed of 
water, and having the end of the tube left 
open. The tube being capillary, a short 
column of mercury rests on the surface of the 
water, indicating its volume at any moment ; 
and the whole is immersed in water contain- 
ed in a strong vessel, to which pressure is 
any how communicated, so that the thermo- 
meter-shaped vessel of glass being equally 
pressed within and without (the neck being 
open), is unaffected by pressure, and the true 
change is perceived in the volume of water 
which it contains. 
The applications of this form of instrument 
are very numerous ; we may take as examples. 
1 . The determination of the tension of gas 
or air in a compressed magazine, as I have 
just suggested. 
2. The measure of elasticity of high-pres- 
sure steam. 
3. The determination of the degree of com- 
pression under which bodies change their 
state, when such experiments can be per- 
formed in glass vessels, as in the case of 
the condensation of the gases into liquids, 
the pressures as stated by different authors 
varying extremely, and being confessedly im- 
perfect approximations. 
4. The ready determination, by inspection, 
of the pressure per square inch exerted by 
Bramah’s press at any instant. 
Nothing could be easier than to convert the 
instrument as above described into a self- 
registering one, by simply inserting an index 
of glass, which may be drawn back by the 
little mercurial column, just as in Six’s ther- 
mometer. We should thus be enabled to 
determine the operation of causes by their 
nature concealed from direct view ; as, 
5. The force exerted by water in the act 
of freezing, in a manner much more direct i 
and satisfactory than that of the Florentine I 
Academicians, because it would not be neces 
sary to cause the recipient to hurst, the ' 
maximum expansive force being indicated by I 
the register. 
6. The force of fired gunpowder : and even i 
of dead pressure and of percussion in a va- 
riety of cases. 
7. The depth of the ocean by the measure 
of the pressing column, the instrument being 
attached to the sounding-lead. I have been 
informed that the ingenious Mr. Perkins pro- 
posed this application of the compressibility 
of water which naturally arose from his method 
of ascertaining the fact of compression by 
using the pressure of the ocean, though no 
notice of this is taken in his paper in the 
Philosophical Transactions. The Piezometer 
there described was like Oersted’s instrument, ' 
intended for measuring compressibility not pres- 
sure. 
In these cases, a Register Thermometer 
would need to accompany the self-registering 
instrument. Probably no considerable error 
is to be feared from abrupt changes of volume 
to which the water might be subjected, for the 
coincidence of the velocity of sound in water, 
theoretically deduced from its modulus of elas- 
ticity, and experimentally by M. Colladon, 
seem to prove that little or no heat is deve- 
loped during its compression. 
The accompanying thermometer would, of 
course, require to be itself protected from the 
disturbing influence of pressure. 
The extensibility of the glass vessel contain- 
ing the water under pressure, might be applied 
to give an independent confirmation of the 
first result; and elegant practical construc- 
tions might be pointed out by which these sepa- 
rate results might be obtained, and also the 
effect of temperature eliminated.* 
* Jainesou’s Journal, No. 37. 
