NATURAL PHILOSOPHY. 135 



OX THE DIURNAL VARIATIONS IN THE DECLINATION OF THE 



MAGNETIC NEEDLE, ETC. 



i 



PROFESSOR W. A. NORTON, of Delaware College, has communi- 

 cated to Sillimari's Journal a long and interesting article upon this 

 subject, which is at present exciting much discussion. He says: 

 " In a former memoir I gave an exposition of a new theory of Terres- 

 trial Magnetism, of which the following are the fundamental princi- 

 ples. 1. Every particle of matter at the earth's surface, and to a cer- 

 tain extent below the surface, is the centre of a magnetic force exerted 

 tangentially to the circumference of every vertical circle that may be 

 conceived to be traced around it. 2. The direction of this force is 

 different, according as it solicits the north or south end of the needle ; 

 and it is always such, that to the north of the acting particle the ten- 

 dency is to urge the north end of the needle downward and the south 

 end upward, and to the south of the same particle it is the opposite. 

 3. The intensity of the magnetic force of a particle of the earth, at a 

 given distance, .is approximately proportional to its temperature, or 

 amount of sensible heat; and at increasing distances diminishes ac- 

 cording to some unknown law. From these principles I deduced three 

 simple formulas ; one, for the horizontal component of the directive 

 force of the needle, or the horizontal magnetic intensity of the place ; 

 a second, for the vertical intensity ; and a third, making known the 

 declination. These formula? were afterwards tested by numerous 

 comparisons with the results of observations made in every variety of 

 locality in the northern hemisphere of the earth. The agreement was 

 found to be very close, the differences amounting only to a few hun- 

 dredths for the horizontal and vertical forces, and less than 2 40', and 

 in most cases less than 1, for the declination. The positions of the 

 magnetic poles, the pole of maximum intensity, and the magnetic 

 equator, were also theoretically deduced, and shown to correspond very 

 closely with their observed positions. 



" In view of the whole discussion, the following great truths were 

 supposed to have been established. 1. All the magnetic elements of 

 any place on the earth may be deducted from the thermal elements of 

 the same ; and all the great features of the distribution of the earth's 

 magnetism may be theoretically derived from certain prominent fea- 

 tures in the distribution of its heat. 2. Of the magnetic elements, the 

 horizontal intensity is nearly proportional to the mean temperature, as 

 measured by a Fahrenheit 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 perpendicular to the isogeothermal line (that is, a line con- 

 ceived to be traced through all points at which the mean temperature 

 of the matter of the earth near its surface is the same as at the station 

 of the needle); and in general the direction of the needle is nearly at 

 right angles to the isogeothermal line. 3. As a consequence, the 

 laws of the terrestrial distribution of the physical principles of mag- 

 netism and of heat mu.st be the same, or nearly the same ; and these 

 principles themselves must be physically connected in the most inti- 



