ON THE MAGNETIC INTENSITY OF THE EARTH. 21° 
temporaneous result that I can obtain for Paris, all the obser- 
vations being made in M. Arago’s magnetic cabinet. It appears 
therefore, that about the period in question, the dip in London 
exceeded that in Paris by 115'"7; preserving this difference in 
- the dips at the two stations when reduced to the period of the 
horizontal observations in 1827, and combining them with the 
observed horizontal intensities, we obtain 1°018 as the value of 
the total force in London to unity in Paris. 
Such being the case, if any other number than unity be 
taken for the measure of the force in Paris, the correspond- 
ing value in London will be the product of that number multi- 
plied by 1:018. By the observations of M. de Humboldt al- 
ready described, the intensity at Paris to that of a place in 
Peru, where the needle had no dip, was found to be as 1°3482 
to 1000. As at that period it was supposed that an equal in- 
tensity, being the minimum on the surface of the globe, pre- 
vailed at all places where the needle had no dip, the station 
at which M. de Humboldt had observed in Peru appeared the 
proper unity of the system of intensities. Subsequent ex- 
perience, however, has shown that the intensity lines follow a 
very different course from the dip lines; and in retaining the 
expression of unity for the force observed by M. de Humboldt 
in Peru, we are necessitated to employ terms less than unity 
to express the force in many other of the inter-tropical parts 
of the globe, and even in one quarter beyond the tropic. The 
scale is therefore purely arbitrary; but it is in general. use, 
and will probably continue to be employed till experiments 
(perhaps those of M. Gauss) shall have determined an abso- 
lute value for the magnetic intensity at some one station; when 
all the relative intensities may be converted into the corre- 
sponding absolute intensities. Suchis the origin of the num- 
ber 1°3482 employed by observers generally as expressing the 
force at Paris. In assuming a constant expression for the force 
at any station on the globe for any considerable number of 
years, we are of course subject to error resulting from the 
secular change in the intensity ; of the amount of which we have 
as yet no definite knowledge. 
The force in London relatively to the above value of the 
force at Paris is 1:3482 x 1:018 = 1°372. 
In the spring of 1828 two of the needles used in this com- 
parison were interchanged between M. Hansteen and myself, 
for the purpose of determining in a similar manner the ratio 
of the horizontal intensity at London and Christiania. The 
observations are detailed in the Journal of the Royal Institu- 
