434 On M. Ampere s Rotaling Cylinder. 
much. improved by the construction explained in the letter, As it 
has not yet, | believe, been given in avy English work, it will, 
I am sure, be interesting to many of your readers. 
l remain, dear sir, 
Yours very truly, 
Royal Military Academy, June11, 1822. Perer BaRLow. 
‘ May 31, 1822. 
Sir, — Havine been lately employed in constructing for 
M:r.Barlow one of M. Ampere’s rotating cylinders, a new form of 
suspension suggested itself to my mind, which, upon trial, suc- 
ceeded admirably; and as it seems to add much to the interesting 
nature of the experiment, J have been induced, by the advice of 
the above gentleman, to give you the following description of it, 
under the hope that you may be disposed to give it a place in your 
valuable publication. I remain, sir, 
To Dr. Tilloch. Your obedient servant, 
James Marsa. 
The instrument alluded to is represented in Plate V. fig. 1 
being a perspective, and fig. 2 a section of it. ABCDisa 
cylinder of very thin copper, about one inch and a half high, 
and two inches in diameter; abcd is another copper cylinder 
of less diameter, soldered to the bottom of the former at dc, 
where there is a circnlar hole to receive it; so that within the 
space Aa, Dd, Bl, Cc, a quantity of diluted nitric or sulphuric 
acid may be introduced; ef gh is a very light hoop or cylinder 
of rolled zinc. ‘To the copper vessel acd is soldered a thin 
copper wire ai, having a small socket at its upper part 7, to 
receive the point proceeding from the other copper wire ekf, 
soldered at ef to the zinc cylinder. NS is a cylindrical mag- 
net, which is represented as broken in the figure, but which 
(when the instrument is used) has its lower end inserted in a foot 
or stand; at its upper end is a small agate cap to receive the 
point proceeding downwards from i. If now (the magnet being 
first placed vertical) the cylinders be suspended, as shown in the 
figure, and the copper cell ABCD be nearly filled with diluted 
acid, the two cylinders will begin to revolve; the one from left 
toright, and the other from right to left; the rotations under fa- 
vourab!e circumstances amountiug to 120 in a minute with the 
zine cylinder; but the motion of the copper cell, from its greater 
weight, is not so rapid.) With the north end of the magnet up- 
wards, the zinc cylinder revolves to the left, and the copper vessel 
to the right ; and if the magnet be inverted, the motions of the 
‘two cylinders will be inverted also. 
It is proper to observe, that M. Ampere’s construction is the 
same. 
