331 \ 
and by a window at each end. It contains six stone pillars 
for the support of the instruments: these are imbedded in 
solid masonry beneath the floor, and the supports of the 
floor are framed around them, so that they are completely 
insulated. 
The elements on which the determination of the earth’s 
magnetic force is usually based are, the declination, the in- 
clination, and the intensity. If a vertical plane be conceived 
to pass through the direction of the force, that direction 
will be determined when its inclination to the horizon is 
given, as well as the angle which the plane itself forms with 
the meridian; and if, in addition to these quantities, we like- 
wise know the number which expresses the ratio of the in- 
tensity of the force to some established unit, it is manifest 
that the force is completely determined. 
For many purposes, however, and especially in the deli- 
cate researches connected with the variations of the magnetic 
force, a different system of elements is preferable. If the 
intensity be resolved into two portions in the plane of the 
magnetic meridian, one of them horizontal and the other 
vertical, it is manifest that these two components may be 
substituted for the total intensity and the inclination ; while, 
at the same time, their changes may be determined with far 
greater precision. The former variables are connected with 
the latter by the relations 
; X=Rcosu, Y=Rsinu; 
in which r denotes the intensity, x and y its horizontal 
and vertical components, and w the inclination. It will be 
easily seen that the variations of u and R are expressed in 
terms of the variations of x and y by the following formule : 
au = sinweos u (= —*); 
x 
oR re |) 
— = cos*u — + sin?’u —. 
R x Y 
Mr. Lloyd then proceeded to describe the instruments 
