86 MR. BROOKE ON THE AUTOMATIC REGISTRATION OF MAGNETOMETERS, 
small weights have been made, each equal to the part of the whole weight. 
While the register is in action, one of these weights is placed on the torsion circle, 
and at an interval of time equal to that of one oscillation of the magnet, the second 
is added : if this be carefully done, the magnet will be very nearly at rest in its new 
position. After half an hour, or any convenient interval of time, the weights are re- 
moved in the saiue manner ; and this must be repeated sufficiently often to eliminate 
the error of reading by a finely divided scale the displacement of the register line. 
Half the scale reading of this displacement may therefore be taken as the value of 
0*00 1 of the whole horizontal force. By this process the necessity of making several 
accurate linear measurements of the apparatus, and the errors that might arise there- 
from, are avoided. 
The following is the proposed method of determining the temperature coefficient. 
Let two magnets, one of them having a known coefficient, and that of the other to be 
now determined, be suspended in the bifilar method, at a distance of 15 feet from 
each other, the line joining their centres being a normal to the plane of the magnetic 
meridian. The torsion of the double threads should be in opposite directions, so that 
when the magnets are duly adjusted in equilibrium, in the line joining their centres, 
the similar poles may be towards each other. 
The most convenient scale coefficient of the bitilar magnet, for the purpose of pho- 
tographic registration, appears to be that which renders the angular value of the 
ordinary diurnal range nearly equal to the range of the declination magnet ; the 
pulley over which the suspension thread of what may be called the standard magnet 
passes, being made of such a size as to give the required value to its scale coefficient, 
the distance between the threads of the other or trial magnet may be conveniently 
adjusted by a right- and left-handed screw (as in the suspension frame described 
in a former paper), so as to give a nearly equal value to its scale coefficient. The 
previously described arrangements for photographic registration being made, the 
registering apparatus is placed midway between the magnets, and by a few trials, 
the ratio of its distances from the magnets may be so arranged as to make their scale 
coefficients exactly equal. The magnets, having been previously protected by a coat 
of varnish, are suspended in water. The vessel made use of is a double zinc trough 
or box, the inner one being 18 inches long, 2 inches wide, and 4 inches deep, the 
outer one 3 inches longer and wider, leaving an equal space on all sides between 
the two, and half an inch deeper, with separate covers to each. For the standard 
magnet, the inner box only should be filled with water, the intervening space of air 
between the two tending to retard any variation of temperature of the water. For 
raising the temperature of the trial magnet, the outer box should be filled with 
warm water at such a temperature as will raise the water in the inner box to about 
100° Fahr. For lowering it to 32° Fahr., the outer box, or rather the space between 
the two, should be filled with a freezing mixture. The whole being allowed to cool 
gradually in the one case, and in the other to be raised gradually to the temperature 
of the atmosphere, the change of temperature will be found to be so slow, that the 
