396 
MR. CHRISTIE ON THE MAGNETICAL OBSERVATIONS 
should not have been afforded him from that service, the Royal Navy, which has 
furnished, and still continues to furnish, abundant instances of zeal and intelligence 
in the prosecution of scientific inquiry. 
In order to deduce the relative magnetic intensities at the several stations from the 
results in the foregoing Table, it will first be necessary that a correction should be 
applied for the difference of temperature at which the observations were made. Im- 
mediately that I received the needles belonging to Captain Back’s instruments, I 
instituted experiments for the determination of the correction to be applied, for this 
purpose, to the observations with each needle. As the method I adopted was the same 
with all the needles, I consider that it will be better to give all the experiments with 
their results in one view, and shall therefore defer giving these experiments and de- 
ducing the intensities at the different stations from the preceding observations until 
I have given an account of the observations which were made by Captain Back with 
horizontal needles, for the same purpose. 
A small apparatus, on the plan first suggested by Hansteen, was employed for de- 
termining the horizontal intensity. It was furnished with two cylindrical magnetical 
needles, and a brass one of the same form and weight to divest the suspending silk 
fibres of torsion. As I had always found that pointed needles, whether of the form 
of a double segment of a circle or of that of a lozenge, vibrated more quickly than rect- 
angular needles of the same weight, I suggested the advantage of having such a needle. 
A needle of a lozenge form was in consequence added to the apparatus. The hori- 
zontal intensity was therefore, where time admitted of it, determined by the vibrations 
of these three needles, distinguished as No. 1, No. 3, and Lozenge Needle. The ob- 
servations appear to have been made in the usual manner, by noting the arc and 
time at the commencement ; the time at every ten vibrations ; generally the time and 
vibration at which the arc was diminished one half, and the terminal arc of vibration. 
The vibrations were, in general, continued until the arc was reduced to two or three 
degrees ; but it appears that only with one of the needles. No. 3, could the vibrations 
be counted, even in London, as far as three hundred. As the dip increased, the number 
of vibrations, within the same limits of arc, decreased, and was ultimately reduced, at 
Point Ogle, to forty or fifty. I do not consider that, in these observations, the time 
could have been determined with sufficient accuracy to admit of >the application of 
the method adopted by Hansteen, of taking a mean of several intervals, or that 
much advantage would arise from the application of a correction for the arc, to the 
time of vibration. I have not therefore thought it necessary to give the observations 
at length, but still I have, in the following Table, given the times in all eases where 
the arc was noted, in order that such a correction might be applied, if thought ne- 
cessary. This Table will, I consider, require little explanation. In each case, the 
number of vibrations, the corresponding arcs, where observed, and the times, are 
given in three consecutive columns ; and where the vibrations are continued beyond 
one hundred, the arcs and corresponding times are given in two subsequent columns, 
in which those corresponding to 100, 150, &c. vibrations are opposite to 0 50, &c. in 
the first column. 
