Lovering and Bond on Magnetic Observations at Cambridge. 75 
The buildings which contain these instruments are made of wood 
with copper nails. Iron has been carefully excluded from every 
part. In order that the corrections for temperature may be small 
they are furnished with a soapstone stove, having a copper funnel.* 
The instruments rest upon blocks of red sandstone, which are firm- 
ly set on the ground, insulated from the vibrations of the floor, 
and known to be free from magnetic influence. It is of the first 
importance to secure the several Magnetometers from mutual inter- 
ference. They are all too distant from the Gauss Magnetometer at 
C' to give or receive any disturbance in that quarter. The mutual 
action of two magnets diminishes as the cube of the distance, and 
greater security has been placed in this mode of reducing it than 
more complex ones. Thus the distance from b to ¢ is 36 feet 9 
inches ; from 6 to d is 36 feet 2 inches; from ¢ to d 40 feet 6 inch- 
es. The distances of the fixed telescopes e and f from 6 and ¢ re- 
spectively are 7 feet; from a to 6 it is 30 feet, and the width of the 
building is 6 feet. Biot, Gauss and Lloyd have investigated the 
problem of the mutual action of a system of magnets; and_posi- 
tions have been found for a limited number of bars in which the 
disturbance should be nothing or constant; in the latter case, a 
simple correction applied to all the observations or which might be 
made at once in the construction of the scale, if the arrangement 
were previously known, is all that is necessary. Out of several 
combinations of this kind for three magnets, which the general theo- 
ry discloses, we have selected the one recommended by Weber. 
The line a b of course is the direction of the magnetic meridian; a c 
is perpendicular to it; then the point c is so selected that the an-< 
* In the magnetic Observatory at Munich, the instruments are placed in a 
room, 13 feet below the surface of the ground, where the temperature varies 
but little through the year. 
