SCIENTIFIC SUMMARY. 
485 
that increase was at a compound rate. Precisely in conformity to the law 
which applied to compound interest and the increase of the successful mer- 
chant’s capital is the increase of electricity by this machine. Given the 
smallest quantity of electricity, and the instrument increased it at the rate of 
compound interest, and this increase went on at a perfectly uniform rate. 
But just as the capitalist finds that he cannot always go on getting higher 
and higher interest for his money, but must ultimately, perhaps, be content 
with per cent, instead of 5, so was it to some extent with thi3 machine. 
When a very high charge was reached, the increase of the quantity of avail' 
able electricity was not so great, owing to sparks passing in various parts of 
the machine, preventing the operator from retaining the full quantity of 
electricity which was got by it. There is great necessity for an easy- 
going electric machine, and that shown fulfilled this condition. 
How the Earth's Rotation affects Gunnery. — Some may be found to doubt 
that the movement of the earth affects the direction of a ball expelled from 
a cannon ; nevertheless, the fact is correct. In the Astronomical Register 
Mr. Kincaid says that a simple illustration of this effect may be made by 
attaching to the same axis two wheels of different diameters, so that both 
shall rotate together. If the one have a diameter of 3 feet, and the other of 
1 foot, it is evident that any point on the circumference of the larger will, 
during a revolution, move through three times as much space as a similar 
point on the periphery of the lesser circle, and will, therefore, move with 
three times the velocity. The figure of the earth may be considered as 
made up of an infinite number of such wheels, diminishing in size from the 
equator to the poles, and all revolving in 24 hours. Now, if a gun be fired 
from the equator in the direction of the meridian, which is obviously that 
of maximum deviation, at an object nearer the pole, it is plain that that ob- 
ject, being situated on a smaller circle than the gun, but revolving in the 
same interval of time, will move, during the flight of the projectile, through 
less space eastwards than the shot, which will have imparted to it the 
greater velocity of the larger circle from which it started, and the latter 
will therefore tend to strike eastwards from its butt. — Astronomical Re- 
gister, August. 
The Physics of a Soap-bubble. — That the physics of a soap-bubble is a diffi- 
cult problem to work out has been shown by Sir David Brewster’s repetition 
of Plateau’s experiments. The changes of form which the films undergo are 
especially difficult to explain, the ordinary theory being insufficient. Sir 
David Brewster thinks that the colours of the soap-bubble are not produced 
by different thicknesses of the film itself, but by the secretion from it of a 
new substance flowing over the film, expanding under the influence of 
gravity and molecular forces into coloured groups of various shapes, and 
returning spontaneously when not returned forcibly into the parent film. 
Production of Inductive Currents . — It has been discovered by Signor 
Blaserga, of Padua, that inductive currents are not produced instantaneously. 
Their development requires a definite period of time, which he has estimated 
at about ^ of a second. He also states that when established an appreciable 
time is occupied in arriving at the maximum. 
The Luminosity of Phosphorus. — Dr. Moffat read a paper before the 
British Association, and describes a number of experiments on the above. 
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