Royal Society, 71 



a certain time is easily calculated ; and again, from the dimensions 

 of the earth's orbit, the space through which the sun draws the 

 earth in the same time is also found. These data and the well-known 

 law of gravitation, that the attractive power varies as the mass of 

 the attracting body divided Jby the square of the distance, assigns 

 the proportion of the sun's weight to the earth's weight. A similar 

 method applies to all the planets which have satellites ; and those 

 which have none are determined, though more imperfectly, by the 

 effects they produce in disturbing other bodies, planets or comets. 

 The satellites of Jupiter are weighed by their mutual disturbances. 

 The mass or weight of the moon is approximated to by several in- 

 dependent methods which agree well together. The author con- 

 cludes in the following words ; — " I shall now repeat what I said in 

 commencing this course of lectures, that I fully believe there is no 

 part whatever of these subjects of which thQ principle cannot be well 

 understood, by persons of fair intelligence, giving reasonable atten- 

 tion to them ; but more especially by persons whose usual occupa- 

 tions lead them to consider measures and forces ; not without the 

 exercise of thought, but by the application only of so much thought 

 as is necessary for the understanding of practical problems of mea- 

 sures and forces." — P. 247. 



XIV. Proceedings of Learned Societies. 



jf ROYAL SOCIETY. ,2^; 



ri< [Continued from vol. i. p. 574.] '' 



ll^pril 10, 1851 — Sir Philip de Malpas Grey Egerton, Bart., V.P., 



in the Chair. 



ri'MlE following communication was read :— Extract of a letter 

 X from Professor Kamtz to Lieut. Colonel Sabine, on " Correc- 

 tions of the Constants in the general theory of Terrestrial Magnet- 

 ism." Received April 3, 1851. 



Translation. 



Dorpat, y% January 1851. . 



From the active zeal with which you pursue the phenomena of 

 terrestrial magnetism, and collect all the facts which can conduce to 

 the elucidation of this difficult subject, I think that some researches 

 with which I have occupied myself will not be wholly uninteresting 

 to you ; and I therefore address you the following lines, which I have 

 also permitted myself to write in my own language. 



Some years ago I employed myself in endeavouring to correct 

 the constants which Gauss has given for the earth's magnetism. 

 The process I adopted was by considering the horizontal and verti- 

 cal components separately ; but when I learned that Erman had the 

 same work in hand, I left mine unfinished. I did not then possess 

 the Reports of the British Association, as it was not until this last 

 summer (1850) that they were obtained here, and when I had seen 



