268 Dr. Faraday on Dr. Hare’s Second Letter, 
velocity of the piston (to recur to our old example) would in 
the first instance go on increasing, the resistance also increa- 
sing with it, at the same time that elastic force of the steam 
upon the piston is constantly diminishing; so that the two 
antagonist forces must gradually become equal. 
The piston, however, would still for some time continue to 
move forward by reason of the velocity it has acquired, so 
that ultimately the elastic force of the steam on the piston 
would be jess than the resistance of the atmosphere. Now it 
is obvious, if the jet be not large, that the resistance of the 
atmosphere, when perfectly free to expand in every direction, 
would not be very materially greater than the elastic force of 
the same fluid at rest; and we may thus see how possible it is 
that at a certain distance from the orifice the elastic force of 
the steam in the centre of the jet (and by consequence the 
lateral pressure) should be less than the pressure at the sur- 
face, which, as we remarked before, is equal to the atmo- 
spheric pressure. 
The same mode of explanation is applicable to another cu- 
rious and well-known phenomenon, I mean whereacircular dise 
is fixed at the end of a tube by which it is pierced centrically. 
In this case, as is well known, if a paper disc of the same 
size, or smaller than that which is fixed to the end of the tube, 
be brought near to the latter, it may be made to adhere to it 
by simply blowing in at the other end of the tube. ‘The ob- 
vious explanation is, that the pressure of the air between the 
two discs (or at least of a certain portion of it) is less during 
the motion than the pressure of the atmosphere in a state of 
rest. 
London, Jan. 28, 1843. R. M. 
XLVIII.—On Dr. Hare’s Second Letter, and on the Chemi- 
cal and Contact Theories of the Voltaic Battery. By M1- 
CHAEL Farapay, Esq., D.C.L., FBS. 
To R. Taylor, Esq. 
My pear Sir, 
OU are aware that considerations regarding health have 
prevented me from working or reading in science for the 
last two years. This will account to you for my ignorance 
of the circumstance that you had reprinted Dr. Hare’s se- 
cond letter to me*; and I believe I knew it only for the first 
time a week or two ago, on beginning to read up. As some 
persons think a letter unanswered is also unanswerable, I 
* Phil. Mag. 1841, vol. xviii. p. 465. 
