470 
THE ASTRONOMER ROTAE ON THE COMPARISON OE 
preliminary trials, I had four proportional compasses constructed by Mr. Simms, by means 
of which I took the proportional part (as expressed by the factor e, or f, See.) of the ordi- 
nate to which that factor applied, laid it down on a new ordinate, and laid, at the top 
or bottom of the ordinate, the other part which was to be added or subtracted, and 
thus formed curves for Magnetic Tendencies without the use of any numerals whatever. 
But this method is not suited to a junior computer. After having satisfied myself that 
there was good ground for proceeding further, I arranged, for the work of the young 
assistants, the following process. Four pasteboard scales were prepared, with different 
graduations : — 
“Scale E for Dartford,” with graduation =~X graduation of Horizontal-Force-Mag- 
netometer, measuring negatively ; 
“ Scale F for Croydon,” with graduation 
netometer, measuring negatively ; 
r. X graduation 
of Horizontal-Force-Mag- 
Scale G for Dartford,” with graduation 
netometer, measuring positively ; 
Scale H for Croydon,” with graduation 
X graduation 
X graduation 
of Horizontal-Force-Mag- 
of Horizontal-Force-Mag- 
netometer, measuring negatively. 
Then, the times for which the measures were to be taken having been previously pre- 
pared, the ordinate D' was measured with scale E, and the ordinate C' was measured 
with scale F ; the numbers were set down in parallel columns and added together, and 
the sum was used to form a new ordinate of Magnetic Tendency to N. Similarly D' was 
measured with scale G, and C' with scale H, and their numbers were added to form a 
new ordinate of Magnetic Tendency to W. At every point where the zero of ordinate 
had been obtained by breaking the wire-circuit, the ordinate was measured in the same 
way. 
In this manner were formed all the curves, exhibiting Magnetic Tendency to N. and W. 
inferred from Galvanic Measures, which I now exhibit to the Society. (Plates XXV. 
to XXX.) 
For the other curves, that which exhibits Magnetic Force to the North is simply a 
copy, on an altered scale, of the record of the Horizontal-Force-Magnetometer; and 
that which exhibits Magnetic Force to the West is derived from the record of the 
Declination-Magnetometer, the angular deviations being converted into proportionate 
parts of horizontal force, and laid down on the same scale as the ordinates of the Hori- 
zontal-F orce-Magnetometer. 
The days were selected as exhibiting rather bold curves. I was prevented from taking 
several which I should have thought preferable, from some scruples (which afterwards 
proved to be entirely unfounded) about the accuracy of the time-scale. 
I now proceed to remark on the comparison of the two systems of curves. 
It is impossible, I think, to see these curves without being struck with their general 
