TRANSACTIONS OF THE SECTIONS. 
17 
4. It is established that there is in every navigable stream a cer¬ 
tain velocity at which it will be more easy to ascend the river against 
the current than to descend with the current. Thus, if the current 
flow at the rate of one mile an hour in a stream 4 feet deep, it will 
be easier to ascend with a velocity of 8 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 20 and 30 miles an hour. 
On a Species of Balance and its Application to the Measurement of 
Electrical Repulsion . By W. Snow Harris. 
The principle of this instrument depends on the reactive force 
imparted to two parallel silk threads without torsion, from which is 
suspended a horizontal needle or other body. If a needle be sus¬ 
pended by tw r o parallel and vertical filaments of silk without torsion, 
equally distant from the centre, its position of rest will be horizon¬ 
tal, and in the vertical plane passing through the silk filaments. 
When the needle is turned through any given angle, the centre of 
gravity of the mass is raised, so that the needle will, when aban¬ 
doned to the force of gravity, continue to oscillate, and will be in 
the state of a body falling down a small circular arc. Mr. Harris 
has examined the law of this force imparted to the threads, and finds 
it as the weight and square of the distance between the threads di¬ 
rectly and as the length indirectly, and that it is exactly proportionate 
to the angle of deflection of the needle. Upon these principles 
Mr. Harris has constructed a balance, which he exhibited to the 
Section, and by which he can estimate any forces of repulsion in 
.electricity however small. The instrument is not liable to many 
difficulties which embarrass the use of the torsion balance, and may 
be employed with advantage in several branches of experimental 
physics. 
On Electrical Attraction . By W. Snow Harris. 
The object of this paper was to examine the operation of attrac¬ 
tion in electricity, and the law r s and differences between the force of 
attraction actually exerted between two bodies, and the force excited 
in a neutral uninsulated body, by the influence of a charged body 
acting upon it at a distance. The author endeavoured to show that the 
former force varied in an inverse ratio of the distance simply ; that 
the law of the inverse square of the distance, which is the general 
law for the former force, does not obtain at all distances between 
bodies, except one of them be uninsulated and neutral; and that in all 
cases of attraction there are tw r o previous forces to be considered, 
1st, the force directly induced in the neutral body ; 2nd, the effect 
of this induced force upon the charged body; which last he called 
c 
