20 SEVENTH REPORT—1837. 
Six horizontal needles were employed for this purpose, and 
a number of observations were made with them at different 
dates at both places; the details are published in the Phil. 
Trans. for 1827. From these it appears that, if the horizontal 
intensity in London be designated as unity, the several needles 
gave its value in Paris as follows, viz. : 
Needle IV. = 1-0732 Needle XI. = 1:0723 
4 VIII. = 1:0675 Al oreo 
» =X. = 10726 of Bos 1 Onis 
Mean 1:0714. 
The observations were corrected for a small excess of tem- 
perature in the experiments at Paris over those in London, 
being, I believe, the first time in which a correction for dif- 
ference of temperature was introduced into any published re- 
sults of the variations of intensity at different stations. The 
places of observation were the magnetic cabinet of M. Arago at 
Paris, and the garden of the Horticultural Society at Chiswick, 
near London. 
In order to deduce the relative values of the total intensity 
from their observed horizontal components, we require the 
dip at the two stations as accurately as it can be inferred from 
nearly cotemporaneous observations. In August, 1828, the dip 
in the garden at Chiswick was observed by Mr. Douglas and 
myself, 69° 46°9. Phil. Trans., 1829. Ina paper of M. Han- 
steen’s, in the dnnalen der Physik, vol. xxi. p. 414, we find 
recorded the following observations at Paris, a part of which 
fall on either side of the London observation of August, 1828, 
Viz. : : 
PROS Arne WY) obs Se ETO 68 00 
1826 Humboldt and Mathieu YP OGT gore 
1827 Humboldt and Mathieu .. 67 58:0 
Daa ragorts she Pee . 67 41:3 
The months in which the observations were made are not 
named by M. Hansteen, but M. de Humboldt in a paper in the 
xvth vol. of the dann. der Physik mentions that those of 1825 
and 1826 were made in August and September, and I have 
taken those of 1827 and 1828 as corresponding to the same 
months. Allowing then an annual decrease of dip of 28 (dun. 
der Physik, vol. xxi. p. 419) we obtain the dip in Paris in Au- _ 
gust, 1828, as follows: Sei 
Uo Fal YOY Ae ea ie 67 51°6 
1826 Humboldt and Mathieu 67 50:9 
1827 Humboldt and Mathieu 67 55:2 
PSdO rapa eee ee es 67 46°9 
67 51-15 
I have therefore taken 67° 51':2 as the most satisfactory co- — 
tnt Rear s ay ta aeolian alata 
