488 
THE ASTRONOMER ROYAL’S EXPERIMENTS 
portion for comparison, without asserting that it is accurate. All results obtained for 
the coil without core ought therefore to be divided by 120, to make them comparable 
with the other results. 
The results obtained for the direction of magnetical force now consisted of lines drawn 
upon paper. Upon examining these, some very small irregularities were found — gene- 
rally of systematic character, partly arising from minute failure in the neutralization of 
terrestrial magnetism, partly from a difference in the intensity of the poles of the great 
magnet ; these were eliminated by the following graphical process : — The paper was 
bent upon its longitudinal axis, and exposed to a strong light passing through the two 
folds of paper ; the lines drawn upon both sides of the magnet or coil were visible, and 
a mean line bisecting the small angle between each pair was easily drawn. Then the 
paper was unfolded and was bent upon its transversal axis, and a similar operation was 
performed upon the mean lines mentioned above. Thus for one fourth part of the cir- 
cumference of the magnet a series of lines was obtained representing the mean of the 
four parts ; these mean lines, repeated for the four divisions of the magnet’s or coil’s 
circumference, are alone used in further graphical deductions and in the subjoined 
figure. 
The results, however, for magnitude of force were obtained in numbers. The means 
of these were taken in an analogous order — first taking the sums of those on opposite 
sides of the magnet or coil, then taking the sums of the last-found sums for opposite ends. 
The division by 4 was omitted ; and thus the numbers in the Table below give the value 
of 400 X cotangent of deviation. At two stations the proximity of the coil-terminals 
made it difficult to obtain actual observations ; but there was no difficulty in supplying 
them conjecturally, with great confidence in their accuracy. 
The diagram below was drawn carefully to represent the positions taken by the small 
magnet when the edge of the large magnet is presented to the small magnet. The same 
diagram will serve, almost without perceptible error, for the case when the fiat side of 
the large magnet is presented to the small magnet, or for the case of a galvanic coil 
inclosing an iron core ; but it will not apply to the case of a galvanic coil without an 
iron core : for that case the axis of the small magnet in the positions numbered 35, 5, 
