ON ELECTRO MAGNETIC FORCES. 
105 
Mr. Pepys had taken advantage of this last 
circumstance, in inclosing fine cutting instru- 
ments in handles or cases lined with zinc. 
The author has not heard whether such ap- 
plications have succeeded, but he has made a 
number of experiments with a view to protect 
brass, iron, copper, &c., from tarnisli and cor- 
rosion in the atmosphere by means of zinc ; 
the results obtained, however, lead to the 
conclusion, that contact with zinc will not 
protect those metals in the atmosphere, the 
electricity thus produced, without the in- 
tervention of a fluid, being apparently too 
feeble to counteract the chemical action of 
air and moisture on the surfaces of the me- 
tals’^. 
EXPERIMENTAL RESEARCHES IN- 
TO THE LAWS OF THE MOTION 
OF FLOATING BODIES. By J. S. 
RUSSELL. 
It was the object of these inquiries to assist 
in bringing to perfection the theory of Hy- 
drodynamics, and ascertain the causes of 
certain anomalous facts in the resistance of 
fluids, so as to reduce them under the dominion 
of known laws. 
The resistance of fluids to the motion of 
floating vessels is found in practice to differ 
widely from the theory, being in certain cases, 
double or triple of what theory gives, and in 
other and higher velocities, much less. These 
deviations have now been ascertained to 
follow two simple and very beautiful laws : — 
1st. A law giving a certain emersion of the 
body from the fluid as a function of the velo- 
city. 2nd. A law giving the resistance of the 
fluid as a function of the velocity and magni- 
tude of a wave propagated through the fluid, 
according to the law of Lagrange. These two 
laws comprehend the anomalous facts, and 
lead to the following 
RESULTS. 
1. That the resistance of a fluid to the 
motion of a floating body will rapidly increase 
as the velocity of the body rises towards the 
velocity of the wave, and will become greatest 
when they approach nearest to equality. 
2. That when the velocity of the body is 
rendered greater than that due to the wave, 
the motion of the body is greatly facilitated: 
it remains poised on the summit of the wave 
in a position which may be one of stable 
equilibrium ; and this effect is such that at a 
velocity of nine miles an hour the resistance 
is less than at a velocity of six miles behind 
the wave. 
3. The velocity of the wave is independent 
of the breadth of the fluid, and varies with the 
square root of the depth. 
4. It is established that there is in every 
navigable stream a certain velocity at which 
it will be more easy to ascend the river against 
• [The negative results thus obtained by Mr. H. 
Davy, agree exactly with those of some trials 
which I have witnessed for protecting steel by 
this means.— E.W. B.] 
the current than to descend mt]\ the current. 
Thus, if the current flows at the rate of one 
mile an hour in a stream four feet deep, it will 
be easier to ascend with a velocity of eight 
miles an hour on the wave, than to descend 
with the same velocity behind the wave. 
5. That vessels may be propelled on the 
summit of waves at the rate of between twenty 
and thirty miles an hour. — Proceedings of the 
British Association at the Dublin Meeting, 
August, 1835. Lond. &; Edinb. Phil. Mag., 
vol. vii„ p. 302. 
ON AN ECONOMIC APPLICATION 
OF ELECTilO-MAGNK/nC FORCES 
TO MANUFACTURING PURPOSES. 
BY ROBERT MALLETT.— d'he separa- 
tion of iron from brass and copper filings, &c., 
in work-shops, for the purpose of the refusion 
of them into brass, is commonly effected by 
tedious manual labour. Several bar or horse- 
shoe magnets are fixed in a wooden handle, 
and are thrust, in various directions, through 
a dish or other vessel containing the brass and 
iron turnings, &c,, and when the magnets 
have become loaded with iron, it is swept off 
from them by frequent strokes of a brush. 
This is an exceedingly troublesome and 
inefficacious process. 
It appeared to the author that a temporary 
magnet of great power, formed by the circula- 
tion of an electric current round a bar of iron, 
might be substituted advantageously. The 
following is the arrangement which he has 
adopted. Several large round bars of iron are 
bent into the form of the capital letter U, 
each leg being about six inches long. They 
are all coated with coils of silk-covered wire 
in the usual way of forming electro-magnets 
of such bars, and are then arranged vertically, 
at the interval of five or six inches from each 
other. 
All the wires from these coils are collected 
into one bundle at their respective poles, and 
there joined into one by soldering, a large 
wire being placed in the midst of them and 
amalgamated. A galvanic battery is provided, 
which, if care be taken in making the junctions 
at the poles, &c., need not exceed four, or, at 
most six pairs of plates of from twenty inches 
to two feet square. The poles of this terminate 
in cups of mercury, which are so placed that the 
large terminal wires of all the coils can be 
dipped into them, or withdrawn easily. 
The rest of the arrangement is purely me- 
chanical. The required motions are taken 
from any first mover, usually a steam engine. 
The previously described arrangement being 
complete, a chain of buckets is so contrived 
as to carry up and discharge over the top of 
the magnets a quantity of the mixed metallic 
particles : most of the iron adheres to the 
magnets, while the so far purified brass falls 
into a dish or tray placed beneath to receive 
it. This latter is also one of a chain of dishes, 
the horizontal motion of which is so regulated 
that the interval between two dishes is im- 
mediately under the magnet.s, in the interval 
of time between two successive discharges of 
the mixed particles on the bars. 
At this juncture the communication between 
the galvanic battery and the magnets is in- 
