158 
Sir Humphry Davy on a 
the slightest motion in a very light wheel hung on an axle ; 
and when fine powders of any kind were strewed upon the 
surface, they merely underwent undulations, without any 
other change of place ; and fine iron filings strewed on the top 
of the cone, arranged themselves in right lines at right angles 
to the line joining the two wires, and remained stationary, 
even on the centre of the cone. The effect, therefore, is of 
a novel kind, and in one respect seems analogous to that of 
the tides. It would appear as if the passage of the electricity 
diminished the action of gravity on the mercury. And that 
there is no change of volume of the whole mass of the mer- 
cury appears from the experiment, page 156 ; and this was 
shown likewise by enclosing the apparatus in a kind of ma- 
nometer, terminating in a fine tube containing air enclosed 
by oil ; and which, by its expansion or contraction, would 
have shown the slightest change of volume in the mercury : 
none however took place when the contacts were alternately 
made and broken, unless the circuit was uninterrupted for a 
sufficient time to communicate sensible heat to the mercury. 
This phenomenon, in which the same effects are produced 
at the two opposite poles, seems strongly opposed to the idea 
of the electro-magnetic results being produced by the transi- 
tion currents or motions of a single imponderable fluid. 
On the conjectural part of the subject I shall not however 
enter, for the reasons stated in the beginning of this paper ; 
but I cannot with propriety conclude, without mentioning a 
circumstance in the history of the progress of electro-mag- 
netism, which, though well known to many Fellows of this 
Society, has, I believe, never been made public, namely, that 
we owe to the sagacity of Dr. Wollaston, the first idea of 
