1872.] Technology. 277 
terrestrial magnetism. If an electrified body be placed near a moying con- 
ductor so as to induce an opposite charge in the moving body, this charge will 
move on the surface of the conductor so as to remain opposite the electrified 
body, whatever the motion might be. Suppose the moving condu¢tor to be an 
endless metal band running past a body negatively charged, the positive 
charge would be on the surface of the band opposite to the negative body; 
and here it would remain, whatever might be the velocity of the band. The 
effect of the motion of this negative electricity on the conductor would be the 
same as that of an electric current in the opposite direction to the motion of 
the band. If the moving body consisted of a steel or iron top spinning near 
the charged body, the effect of the electricity on the top would be the same 
as that of a current round it in the opposite direction to that in which it was 
spinning. It might be that the electricity in the inducing body would produce 
an opposite magnetic effect on the top; but even if this were so, its effect, 
owing to its distance, would be much less than that of the eledtricity on the 
very surface of the top. If no account were taken of the effect of the inducing 
body, the current round the top would be of such strength that it would carry 
all the electricity induced in the top once round every revolution. If the top 
were spinning from west to east by south it would be rendered magnetic with 
the positive pole uppermost, that is, the pole corresponding to the north pole 
in the earth or the south pole of the needle. In one of the experiments, to 
show that such a current might be produced, a glass cylinder, 12 inches long 
and 4 inches across, was covered with strips of tin-foil, parallel to the axis, 
with very small intervals between them. These strips were about 6 inches 
long and 4 an inch wide, and the intervals between them ;1,th of an inch. 
In one place there was a wider interval, and from the strips adjacent to this 
wires were connected, by means of a commutator, with the wires of a very 
delicate galvanometer. This cylinder was mounted so that it could be turned 
1200 revolutions in a minute, and brought near the conduétor of an ele¢trical 
machine. This apparatus, after it had been thoroughly tested, was found to 
give very decided results. As much as 200° deflection was obtained in the 
needle, and the direction of this deflection depended on the direction in which 
the cylinder was turned, and on the nature of the charge in the conductor. 
When this was negative, the current was in the opposite direction to that of 
rotation. It may be taken as experimental proof of the fact previously stated, 
that, if a steel top were spinning under the inductive influence of a body 
charged with negative eleGtricity, the effet would be that of a current round 
the top such as would render it magnetic. The cause of terrestrial magnetism 
has not been the subje& for so much speculation as many other more unim- 
portant phenomena, being regarded as a cause from which other phenomena 
might result, but not, as itself, the result of other causes, and yet when two 
phenomena have a relation to each other there is good reason for believing 
them connected in some way, either one being derived from the other, or else 
both springing from the same cause. The direction of the earth’s magnetism 
bears a marked relation to the earth’s figure, and yet it can have had no hand 
in giving the earth its shape, which is explained as the result of other causes. 
Assuming that the figure of the earth has something to do withits magnetism, 
its rotation, which keeps it in shape, causes it to be magnetic. We are led to 
believe the cause of this magnetism to be associated with the sun, from the 
influence it exerts on this magnetism, although the cause itself cannot be the 
result of either the sun’s heat, light, or attraction. The analogy between the 
magnetism produced by a spinning top by the induétive action of a distant 
body charged with electricity, and the magnetism in the rotating earth, probably 
caused by the influence of the sun, which influence is not its mass or heat, 
seems to suggest what its influence really is. If the sun were charged with 
negative electricity, it seems to follow, from Prof. Reynolds’s experiments, 
that the inductive effe& upon the earth would be to render it magnetic, the 
poles being as they are. 
TECHNOLOGY. 
Ordinary brick-dust, made from hard-burnt, finely pulverised bricks, apd 
mixed with common lime and sand, is universally and successfully employed 
