1821.] Analyses of Books. 139 



immediately announcing it to the public, and pointing out the 

 importance of publishing it in an English translation. I shall 

 take a future opportunity of giving a detailed account of it to 

 the readers of the Annals of Philosophy. At present I shall 

 merely give the titles of the great divisions of the v^^ork, and add 

 Bome important facts and corrections w^hich Professor Hansteen 

 has pointed out to me in a letter, dated Christiania, Nov. 21, 

 1820. 



After an introduction of 14 i)ages, Prof. Hansteen treats of, 



1. Halley's Hues and of their motion between 1600 and 1800. 



2. Of the hues of inchnation, and of the magnetic intensity. 



3. The number, position, and peri odalgy ration of the magnetic 

 poles round the pole of the earth. 5. Calculation of Halley's 

 lines from the first imperfect theory of Euler. 5. Mathematical 

 theory of magnetism. 6. Apphcation of the theory of magne- 

 tism to the theory of the magnetical declination, inchnation, and 

 intensity, in a given place, whose geographical position is known. 

 7. A more accurate determination of the position of the magne- 

 tical axes, their size and relative intensities. 8. Of the daily 

 variation of the needle. The whole terminates with very ample 

 tables of the declination and dip in various parts of the earth. 



Prof. Hansteen requests me to inform the public that his 

 seventh chart, showing the dip of the magnetic needle, is erro- 

 neous, because, when projecting it, he was unacquainted with 

 the original observations made in the course of a voyage to the 

 Northern Pacific Ocean by Capt. Cook and Mr. WiUiam Bayley. 

 Led astray by some observations of Krusenstern, he was induced 

 in the corrections and additions to his work, p. 21, 22, to con- 

 tradict M. Biot, who insists, in his Traite de Physique, tom. iii. 

 p. 131, that the hne of no dip (magnetic equator) cuts the equa- 

 tor of the earth three or four times. " But," continues Prof. 

 Hansteen, " Biot is in the right, and the accompanying cor- 

 rected chart shows the dipping lines in the Pacific in their right 

 form. Your mentioning in your Annals , in a few words, that I 

 myself acknowledge my error, will be conferring a particular 

 obhgation on me." 



'' On observing the oscillations of a magnetical steel cylinder, 

 suspended by a silk-worm thread in a small box with glass 

 openings, I have made the curious discovery that the magneti- 

 cal intensity of the earth has a daily and annual variation. By 

 a chronometer of Arnold's, 1 observe five times every day, at 

 stated hours, the time required for completing 300 vibrations ; 

 and these observations 1 have already continued for nearly a 

 year. From early in the morning, the intensity is on the 

 decrease until between 10 and 11 o'clock in the forenoon, when 

 it has reached its minimum. From this time it increases, at 

 first slowly, afterwards more rapidly, till it reaches the maximum 

 at four o'clock afternoon in the winter, and between six and 

 eight in the summer. At times it does not reach this maximum 



