Magnetic Influence of Solar Light. 247 



dulums vibrating under the influence of gravitation, and 

 since in the latter case the squares of the times of perform- 

 ing a given number of oscillations are inversely proportional 

 to the force of gravity, it follows that in the former the cor- 

 responding law obtains likewise, hence then the higher the 

 intensity of the magnetic force that acts on a vibrating 

 needle, the more rapid will its oscillation be, and therefore 

 by comparing the times of its oscillations under different 

 circumstances, any variation of intensity that it may have 

 undergone, will immediately become apparent. According- 

 ly, by vibrating a needle before its exposure to the sun's 

 light, a distinct indication of its magnetic condition is obtain- 

 ed, while by vibrating it after its exposure, the change conse- 

 quent on this exposure becomes perceptible. There cannot 

 be a doubt of this being by far the best, and indeed I believe 

 the only sure method of judging of changes of magnetic 

 condition, and it is the employment of it that makes the ex- 

 periments of MM. Moser and Riess so much more decisive 

 than those of Mrs. Somerville or others, by whom a different 

 method, to be afterwards described, was employed. During 

 exposure, the needle was placed upon a table, and it ought 

 to be in a direction at right angles to the magnetic meridian, 

 since there is then the least possible chance of any inter- 

 ference of terrestrial magnetism with the results due to the 

 action of solar light. This was not done by me at first, as 

 the chance of error from the source alluded to, did not occur 

 to me ; but in the later experiments it was invariably attend- 

 ed to, and ought always to be borne in mind, since feeble 

 magnetism may be imparted to a steel needle held in the di- 

 rection of the magnetic meridian, especially if it approaches 

 the line of magnetic dip, by the mere inductive action of 

 the magnetism of the earth. While placed, therefore, in 

 this proper position, the direct light of the sun concentrat- 

 ed into a focus by means of a lens l-5th of an inch in dia- 

 meter and one inch focal distance, was made to traverse 



