1 00 On the Laws of Terrestrial Magnetism 



to its interior, or to its surface ; it extends also into space, as 

 has been proved by Gay Lussac and mvself, in an aerostatic 

 ascent. It even appears, according- to our observations, that 

 the intensity of tliis action decreases slowly in proportion as we 

 remove from the terrestrial surface ; for we did not find any 

 vsensiblo diminution at the height to which we were elevated. 

 Probal)lv its diminution follows the general law of magnetic 

 actions; that is, the inverse ratio of the sipiare of the distance*: 

 if so, it will extend indefinitelv into space. Analogy leads us to 

 think that the moon, the sun, and the other celestial bodies, 

 are endowed with similar actions; and the more so, as the com- 

 position of aeroliths that have fallen on our globe have shown 

 us that the heavenly bodies contain, in like manner, magnetic 

 substances, such as nickel and ironf. The magnetic actions of 

 all these bodies must, therefore, according to their positions and 

 distances, influence the direction of the magnetic needle on 

 the earth's surface, as well as the absolute intensity of the di- 

 recting force: and as these positions and distances are con- 

 tinually changing, by the effect of the motion of the earth 

 and of all the planets, it must follow that there will also be per- 

 petual alterations in the magnetic forces. For example : If the 

 magnetic action of the sun and of the moon be sensible, the 

 motion of rotation of the earth round itself, and its motion of 

 revolution round the sun, must produce diurnal and annual os- 

 cillations in the magnetic needle. Now, not only do such mo- 

 tions exist, but their periods, having been established by a long 

 series of observations, agree with the cause which we have just 

 pointed out. At Paris, according to M. Cassini, the maximum 

 of the diurnal variation J appears to take place between noon and 

 three in the afternoon; then the needle becomes stationary; it then 

 approaches towards the terrestrial meridian, until about eight in 

 the evening ; then it stops, and remains stationary during the 

 whole of tlie night. But in the morning, about eight, it begins 

 again to recede from the meridian. If this second motion re- 

 move 



* This was the law of raagiietic attraction discovered by Coulomb; but 

 Dr. BiooiK Taylor found by experiments which he made to ascertain it, 

 that at sinali distances it was nearly in the inverse ratio of the square of 

 the distance: and futiher off it was inversely as the cube of tlic distance, 

 or even more tlian that. — Phil. Transactions, vol. xxix. 



t Ii is here supposed that these stones come to us from some of the 

 heavenly bodies, which has not yet been proved. 



I The diurnal variation was frrst observed by Mr. George Graham in 

 March, April, and May 1722, and published in the Transactions of the 

 Royal Society, No. 383. The greatest variation westward which he found 

 was Id" 45', and the least 13" 50', by a mean of about 1000 observations. 

 Generally speaking, it was seldom less than 14", or greater than 1'1."35'. 

 But the most detailed account of it is given by Mr. Canton, in the Philoso- 

 phical 



