129 
Temperature in shade 4 feet 6 inches above ground. 
Time of greatest obscuration 32° 
At the end of eclipse 34 -5 
Temperature on ground. 
Greatest obscuration 29° 
End of eclipse 31 *5 
“ On an Apparatus for determining the Horizontal Mag- 
netic Intensity in Absolute Measure/’ by J. P. Joule, 
LL.D., F.R.S., &c., V.P. 
Many years ago Weber described a small portable appara- 
tus for ascertaining the horizontal component of the intensity 
of the earth’s magnetism. In it a magnetic bar, about four 
inches long, was vibrated under the influence of the earth’s 
magnetism, and afterwards employed to deflect a compass 
needle. The moment of the deflecting bar was determined 
from its dimensions and weight, the compass needle was 
supported on a point in the ordinary manner, and no allow- 
ance was made for the influence of the earth in increasing 
the strength of the magnet while vibrating in the magnetic 
meridian. Notwithstanding these defects, the results obtained 
were of considerable accuracy, and justified the employment 
of the portable apparatus by travellers. I have endea- 
voured to improve Weber’s apparatus, so as to give it an 
accuracy comparable to that of the instruments employed in 
stationary observatories, without increasing its expense or 
diminishing its portability. I will, with these preliminary 
observations, at once commence by describing the various 
parts of the apparatus I have constructed for my own use. 
I employ two deflecting bars, for reasons which will after- 
wards appear. It is important that the length of the bars 
should be decreased as far as possible. By so doing, there 
