730 



vations at tlie same place show how little correctness the mean of 

 them, or the result, however obtained, can pretend to. 



The results of the recent observations, with which the author has 

 been furnished by various navigators and by the Hydrographer's 

 Office, have been obtained by throwing the observations into curves, 

 according to methods formerly used and described by the author. 

 This labour has been carefully performed by Mr. D, Ross of the 

 Hydrographer's Office. 



January 6, 1848. 

 GEORGE RENNIE, Esq., Treasurer, in the Chair. 



"On Terrestrial Magnetism." By William A. Norton, A.M., 

 M.A.P.S., Professor of Mathematics and Natural Philosophy in De- 

 laware College, United States of America. Communicated by Lieut. - 

 Colonel Edward Sabine, R.A., For. Sec. R.S. 



The object of the author in the present memoir is to show that, 

 by adopting certain fundamental conceptions with respect to the 

 terrestrial magnetic forces, the magnetic may be deduced from the 

 thermal elements of the earth. The following are the propositions 

 which he considers he has established by his inquiries. 



1 . All the magnetic elements of any place on the earth may be 

 deduced from the thermal elements of that place ; and all the great 

 features of the distribution of the earth's magnetism may be theore- 

 tically derived from certain prominent features in the distribution of 

 its heat. 



2. Of the magnetic elements, the horizontal intensity is nearly 

 proportional to the mean temperature, as measured by Fahrenheit's 

 thermometer ; the vertical intensity is nearly proportional to the 

 difference between the mean temperatures, at two points situated at 

 equal distances north and south of the place, in a direction perpen- 

 dicular to the isothermal line ; and, in general, the direction of the 

 needle is nearly at right angles to the isothermal line, while the pre- 

 cise courses of the inflected line, to which it is perpendicular, may 

 be deduced from Sir David Brewster's formula for the temperature, 

 by differentiating and putting the differential equal to zero. 



3. As a consequence, the laws of the ten^estrial distribution of the 

 l>hysical principles of magnetism and heat must be nearly the same ; 

 and these principles themselves must have towards one another the 

 most intimate physical relations. 



4. The principle of terrestrial magnetism, in as far as the pheno- 

 mena of the magnetic needle are concerned, must be confined to the 

 earth's surface, or to a comparatively thin stratum of the mass of 

 the earth. 



5. The mathematical theory of terrestrial magnetism which has 

 been under discussion must be true in all its essential features. 



6. We may derive the magnetic elements by very simple formulse, 

 and with an accuracy equal to that of Gauss's formulse, from a very 



