Terrestrial Magnetic Intensity. 

 [TABLE IV. continued.'] 



167 



19. In the case of No. 1, the magnetism has been consi- 

 dered as stationary throughout the period 1832-1835, with 

 which we are now concerned. In the case of the Flat Needle, 

 this cannot be assumed, nor can we admit the change to have 

 been uniform. It seems probable that much movement, and 

 especially alternations of temperature, accelerate the loss of 

 magnetism, that loss having been greatest in 1832, when most 

 of the following observations were made. This is t^nfirmed 

 by a more minute inspection. Observations were made at 

 Geneva on the 20th August 1832, and again on the 10th 

 November, and between these dates the whole of the alpine 

 series is contained : Now the variation of the logarithms for 

 that period is no less than '00452, or at the rate of '02001 

 per annum ; whilst we have seen that during the period from 

 June 2, 1832, to May 7, 1833, which includes the above, the 

 mean change was only at the rate of '00736 per annum. It 

 is clear then that, in order to render the observations of 1832 

 comparable with one another, we must assume a much higher 

 rate than the mean, for the months from June to November. 

 Admitting some little doubt as to the Geneva comparisons 

 due to the monthly change of intensity, and the great differ- 

 ence of temperature in the two cases ; I think that I shall best 

 satisfy the conditions by assuming the log. time to have in- 

 creased by '00100 for each month from June to November, 

 leaving -00684. — '00500 = '00184 for the whole of the remain- 

 in which they came from Norway. This arrangement I have not changed, 

 but in packing them I have taken pains to place the opposite poles nearest 

 one another, an arrangement which seems to have been attended with 

 good effect; and to show that needles may lie within an inch or two of one 

 another without material injury, when we see the stationary condition of 

 No. 1, and the diminishing rate of variation of the "Flat" Needle. 



