Disturbance of the Earth’s Magnetism. 147 
Observations were also made on the 20th and 2Ist, but no distur- 
bance was again noticed ; the intensity had resumed its former state. 
The following table exhibits the observed times of three hundred 
vibrations, with the mean temperature and aspect of the weather 
during each observation : 
Time of 300| Mean tem- ANvoath on 
vibrations. | perature. ‘ 
April 19th, 12 h. noon. | 980/.75 663° |Cloudy, rain a. mM. 
Pe both. 26. bh. p. im.) 9687 65 61° Clear. 
Sot LOrbe pai... 9821.20 ale Broken clouds. 
= 20th, 6 bh. p.m: | 978.638 | - 514° -..\Clear. 
-DAY. 
The above observations may be reduced approximately to the 
uniform temperature of 60°, by the formula, 
T=T" [110.000165(¢’tt) |, * 
(T being time, ¢ temperature in degrees of Fahrenheit,) which 
was deduced from experiments on a similar needle. The relative 
intensities may also be readily calculated, since they are reciprocal- 
ly as the squares of the times of the vibrations. In this way, by 
assuming as unity the time observed on the 20th, we have the fol- 
lowing results: 
Time of 300 vibrations 
DAY. at temperature of 60: Relative intensities. 
April 19th, 12h. noon. | 9797.94 1.00022 
Oth... 6..H. p- m. 968”.49 1.02401 
« 19th, 10h. p. m. 983.50 0.99299 
OPO, to kt p- m. 980.05 — 1.00000 
From the mean of several observations made with this needle in 
April, I consider its time of three hundred vibrations for this month, 
and in an undisturbed state of terrestrial magnetic intensity, to be 
nine hundred and seventy nine seconds. ‘The accidental errors in the 
above observations do not probably exceed, in any case one second. 
At the time of registering the above observations, I had not seen 
the following remark of Prof. Hansteen, which was subsequently 
met with in the 12th volume of the Edinburgh Philosophical Jour- 
nal:—‘ A short time before the aurora borealis appears,” says 
Prof. Hansteen, “the intensity of the magnetism of the earth is 
apt to rise to an uncommon height; but so soon as the aurora be- 
gins, in proportion as its force increases, the intensity of the mag- 
* This formula was obtained by Hansteen. 
