TRANSACTIONS OF THE SECTIONS. 3r. 
, “Trevandrum Observatory, July 10. 
« My prar Sir,—One of the first results which I obtained from the observations 
made in your observatory at Makerstoun, was the annual law of variation of the 
horizontal force of the earth’s magnetism; the Jaw, namely, that the force was a 
maximum near the solstices and a minimum near the equinoxes.. This law was 
communicated by me to the British Association in 1845,—confirmed by a discussion 
of the Toronto Observations for 1842; it was also confirmed by the results for other 
years at Makerstoun, in a paper read to the Royal Society of Edinburgh, January 5, 
1846. I afterwards further confirmed it by the results. of observations. made at 
Munich, by Dr. Lamont, in the years 1843-45. Persons interested in these questions 
who have examined plate 4 accompanying the paper already cited, must have re- 
marked the symmetrical and well-characterized curvés (No. 8), which represent the 
movement of the daily mean force from the beginning of January till the end of 
March, 1844. It must have appeared curious that Makerstoun should have been 
the only place where such curves have been produced, or, at least, that no other 
place has shown anything resembling them. Thé eXaibition of the annual law de- 
pends so much on a permanent magnet, an undisturbed instrument, and well-cor- 
rected observations, that it may have seemed less curious that different results for 
this period should have been obtained. As I was satisfied that the curves referred, 
to were real representatives of the daily variation of force, I was anxious to compare 
carefully other observations for the purpose of showing to what extent the law was 
similar at other places. After I had arranged anew the observatory of His Highness 
the Rajah of Travancore here, and established another on the highest peak of our 
Ghats, I commenced an examination of the observations made at Trevandrum in 
1844 by my predecessor, the late Mr. Caldecott. These observations had never been 
corrected for temperature, nor indeed had the coefficients been determined; and on 
my first researches, as the observations did not quite satisfy me, I put them aside, 
On discussing them carefully, however, about a year ago, having determined the 
necessary corrections myself, I was much gratified to find that the results at Trevan- 
drum in 1844 agreed perfectly with those at Makerstoun. This fact I announced in 
the following terms in a letter sent to England in the beginning of this year :—‘ The 
relative changes of mean horizontal force are the same all over the globe; and the. 
changes from day to day of the mean horizontal force at different places on the earth’s 
surface are nearly equal, the unit in each case being the whole value of the horizontal 
force at the place. In other words, the changes of mean horizontal force from day 
to day are in the same direction all over the globe, and are proportional to the hori- 
zontal forces at the places; the different effect of disturbance due to its diurnal 
period and the different directions of the secular change being allowed for.’ It was 
attempted in this paragraph to give a general statement of the result,—I shall attempt 
here to be somewhat more definite. The chain of observatories of the British 
Government confirmed and extended the result first due to Celsius and Graham, and. 
afterwards to Arago and Kupffer, that magnetic perturbations are felt simultaneously 
at places widely distant; but the conclusion did not go beyond this—it appeared 
that even for moderate disturbances of one element at one place, some element, it 
might not be the same, showed disturbance at the other place. Having satisfied 
myself of the great similarity of the variations of force at Trevandrum and Makers- 
toun, I could have no doubt but the same result would be obtained from moderately 
good instruments at other places; I accordingly undertook the discussion of obser- 
vations at Hobarton, Van Diemen’s Island, the Cape of Good Hope, and several 
other places. As I had to obtain the temperature coefficients and to correct the 
Observations by my own processes, the reductions were necessarily laborious. I first 
corrected the observations at Hobarton, and compared the monthly mean values at 
the two stations. I found not only that the annual law was the same at the two 
stations, Makerstoun and Hobarton, but that the changes of the individual monthly 
means followed generally the same law throughout the years 1844, 1845, 1846, 1847, 
and 1848, though the diurnal series of observations at Makerstoun were incomplete 
in the last three years. The secular change seems to obey the same law at both 
stations, but the amount is greater at Hobarton than Makerstoun. This will be 
seen by comparison of the monthly means in parts of the horizontal force at the res 
spective places for the year 1845 :— 
