468 
THE ASTRONOMER ROYAL ON THE COMPARISON OE 
the revolution of the barrel, gives ample information whereby a time-scale can be laid 
down applying to every part of .the photographic curve. 
The zero of galvanic measure is obtained by simply interrupting the wire-circuit. 
I desire to state that great attention has been given to every one of these adjustments, 
more especially to those which relate to time. At an early epoch in the comparison of 
the two classes of measures, it became obvious that questions of simultaneity would 
arise. Great care has been taken by Mr. Glaisher in the formation and application of 
all the time-scales, and I am confident that I can answer for the simultaneity or non- 
simultarieity of the phenomena, to one or two minutes of time. 
Before entering into details of our observations, I will remark that it is perfectly 
certain that atmospheric influences have produced no effect on our records. This 
appears in two ways. First, that the Croydon currents (which convey almost the whole 
of the terrestrial galvanism) have, at all times, an aspect totally different from that of 
the Dartford currents ; secondly, that whenever a wire is broken, at whatever place on 
the wire, the galvanometer-needle then remains motionless at the zero point. In one 
or two instances the wire has been broken at a considerable distance from the Observa- 
tory, and has fallen so that its end lodged in the ground; and then it has conveyed a 
very good terrestrial current. 
We thus obtain simultaneous galvanic indications from the two instruments, measurable 
by the parts of scales of sensibly equal value. I proceed to explain the method by which 
they have been treated. 
From the Map which accompanies this communication, it appears that the azimuth of 
X If 
