390 Royal Society. 



whether the action of steam upon the hull of an iron vessel affects a 

 compass, properly placed, in any degree that may be of practical 

 importance in its navigation ; and also whether the keeling of the 

 vessel produces any alteration in the deviations, or disturbs a com- 

 pass so placed to any considerable extent. The former question is, 

 from the results of these experiments, resolved in the negative ; but 

 with respect to the second, it ajjpears that the deviations produced 

 by keeling are very marked, and could not be safely disregarded. 

 These observations completely confirm those already made by Mr. 

 Walker and Commander Shaugh on board H.M. Iron Brig ' Recruit,' 

 Commander A. Slade, and they prove the necessity that exists for 

 ascertaining the deviations of the compass in all ships, not only at 

 the beginning and end of their voyage, but likewise at intermediate 

 stations ; as also constant observation of the course which the ship 

 may be steering. 



Jan. 6, 1848. — "On Terrestrial Magnetism." By William A. 

 Norton, A.M., M.A.P.S., Professor of Mathematics and Natural 

 Philosophy in Delaware College, United States of America. Com- 

 municated 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 terrestrial distribution of the 

 phj'sical 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 formulae. 



