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III. Magnetic Survey of the West of France, 1868. By the Bev. Stephen J. Perry, S.J., 
F.B.A.S . , Sfc. Communicated by the President. 
Received June 3 , — Read June 17, 1869. 
The Magnetic Survey of the West of France, of which the following is a report, was 
undertaken by the Kev. W. Sidgreaves and myself at the request of the authorities of 
Stonyhurst College, who generously undertook to defray all the expenses of the expedi- 
tion. A similar survey of the East of France will be made during the coming autumn, 
so as to complete the series of observations of the magnetic elements for the whole of 
France. 
The instruments employed were those which have been in constant use at Stonyhurst 
Observatory for the determination of the monthly absolute values of the Dip, Declination, 
and Intensity. They consisted of a dip-circle by Barrow, No. 32, a unifilar by Jones, 
and Frodsham’s marine chronometer, No. 3148. A beautiful transit-theodolite and an 
aneroid barometer were kindly placed at our disposal by the late Mr. Cooke. 
The dip-circle was provided with three needles of the ordinary construction, the length 
of each being 3‘54 inches, and the maximum breadth of Nos. 1 & 2 (P21 inch, whilst 
that of No. 3 was 032 inch. For the unifilar there Avere five magnets — two for decli- 
nation observations, a third, No. 7, for vibrating and deflecting, and the remaining two, 
Nos. 9 & 10, for suspension in the deflection experiments. The same declination magnet 
was used throughout the whole series of observations, and the only deflected magnet 
employed was No. 9. 
The reduced observations are given at considerable length in the following Tables, in 
order that the accuracy of the conclusions may be more reliable, and the results be more 
easily compared with those of past and future observers. 
The moment of inertia of the deflecting magnet, No. 7, with its stirrup, for different 
degrees of temperature, and the coefficients in the corrections required for the effects of 
temperature and of terrestrial magnetic induction on the magnetic moment of the magnet, 
were determined at the Kcw Observatory by the late Mr. Welsh. 
The moment of inertia of the magnet, with its stirrup, using the grain and foot as the 
units of mass and of linear measure, is 5 - 27303. Its rate of increase for increase of tem- 
perature is 0'00073 for every 10° of Fahr. 
The weight of the magnet, with its stirrup, is approximately 825 grains, and the length 
of the magnet is nearly 3 - 94 inches. The moment of inertia was determined, inde- 
pendently of the weight and dimensions, by the method of vibration, with and without a 
known increase of the moment of inertia. 
JIDCCCLXX. p 
