460 Analyses of Books, [Dec. 



0*123 M ; or taking; the intensity of the magnets 1, for each 

 degree of increase in temperature we should have a decrease of 

 intensity of 0*000564. iNow if the same, or nearly the same, 

 take place with all magnets, it is evidently necessary, in all 

 cases where the tjerrestrial magnetic intensity is to be deduced 

 from the vibrations of a needle, that great care should be taken 

 to make the observations at the same temperature ; or, the pre- 

 cise effect of change of temperature having been previously 

 ascertained, to correct the observations according to the differ- 

 ence of the temperatures at which they were made. I am not 

 aware that any one has yet attempted to make such a correction ; 

 but it is manifest from the experiments I have described, that it 

 is indispensible, in order to deduce correct results from the times 

 of vibration of a needle in different parts of the earth, where the 

 temperatures at which the observations are made are almost 

 necessarily different, that these temperatures should be regis- 

 tered, and the times of vibration reduced to a standard of tem- 

 perature. It appears to me, that the eftects will be the most 

 sensible in large and powerful needles ; and consequently, in 

 making use of such, the reduction for a variation of temperature 

 will be most necessary. There would be no difficulty in this 

 reduction, if we could give in terms of the intensity of any mag- 

 net the increment or decrement of intensity corresponding to a 

 certain decrement or increment of temperature at all tempera- 

 tures. To determine this accurately would however require a 

 great variety of experiments to be made with magnets of very 

 different intensities ; but as I have not made these, I must con- 

 tent myself for the present with pointing out some of the facts 

 which I have ascertained from more extended experiments than 

 those I have already given, reserving the detail of these experi- 

 ments for another opportunity, should they be deemed of suffi- 

 cient interest. 



" These experiments were made with a balance of torsion, the 

 needle being suspended by a brass wire ^-f^- inch in diameter : 

 by them 1 ascertained the following facts : 



'^ 1. Commencing with a temperature — 3° Fahrenheit, up 

 to a temperature 127*^, as the temperature of the magnets 

 increased, their intensity decreased. Owing to the almost total 

 absence of snow during the winter, I was unable to reduce 

 lower the temperature of the large magnets which I made use of; 

 but from an experiment I made at the Royal Institution, iu 

 conjunction with Mr. Faraday, in which a small magnet, enve- 

 loped in lint well moistened with sulphuret of carbon, was placed 

 on the edges of a basin containing sulphuric acid, under the 

 receiver of an air pump, I found that the intensity of the magnet 

 increased to the lowest point to which the temperature was 

 reduced, and that the intensity decreased on the admission of 

 air into the receiver, and consequent increase of temperature in 



