454 Rev. Mr. Lroyp on a New Method of Observation 
and, subtracting, 
og o= A-A,, (9) 
The last term in equations (8), or the correction for the variation of gravity, may 
be omitted when the places at which the magnetic force is compared do not differ 
considerably in latitude ; for in that case the quantity Me (cos 2,—cos 2X), which 
enters the logarithmic formula for the force, may from its smallness be disregarded. 
In applying this method of observation to the determination of the changes which 
the intensity of the magnetic force undergoes at the same place, it will be convenient 
to substitute an approximate formula for the preceding. To obtain this, we have 
only to differentiate the equations (8) and (9), and we find 
d 
i —tan 6 dé—cot (3—6) (d3—d0) + adr. 
in which we are to substitute for tan 0, cot (6—6), and a, their values belonging to 
the particular place and needle. If the needle be loaded so that 5—6 is very nearly 
equal to 90°, the term multiplied by cot (8—6) may be neglected, and we have 
= — — tan 0 d0+<adr. 
This seems to offer a very simple means of observing the diurnal and menstrual ya- 
riations of the total intensity, and of ascertaining the law of a phenomenon of which 
nothing certain is as yet known. 
I shall now adduce, in exemplication of this method, the results which I have ob- 
tained on the direction and intensity of the magnetic force in Dublin ; and shall com- 
pare these results with those of the received method, the observations being made, for 
the most part, at the same time. The needle employed (Needle IV) is one of those 
made expressly for the practice of this method ; and care has been therefore taken 
to preserve its magnetic state undisturbed. In each arm are drilled three small holes, 
close to one another, coinciding, as accurately as could be effected, with the axis of 
form of the needle, and distant from its centre by about two-thirds of the length of 
the arm. The weight is a small piece of brass wire, which is introduced into one of 
the holes on the southern arm,—the diameter of the wire corresponding accurately to 
that of the hole; the magnitude of the weight is such as to bring the needle into a 
position nearly perpendicular to the line of the dip. The length of the needle is 44 
inches. 
The following are the observations, and the results computed by the formule (3) 
(8) and (9); they constitute three distinct comparisons of Dublin and London, the 
first of which was made in the year 1834, and the other two in 1835. 
