Prof. W. A. Norton on Terrestrial Magnetism. 



217 



posing, what the table shows to be true, that the difference varies 

 from one latitude to another proportionally to 1 — cos 21at. 



Table VIII. 



Place. 



Lat. 25°, VV. coast of Africa, 



Christiania : 



Moscow, 



Edinburgh, 



Gottingen, 



London, 



Berlin, 



Paris, 



Milan, 



Marseilles, 



Lat. 35°, VV. 

 Lat. 30 c 

 Lat. 20° 

 Lat. 10° 

 Lat. 0° 



coast of A fi 

 do. clo 

 do. do 

 do. do 

 do. do 



(lor. Intensity 



rica 



Computed. 



•454 

 •5o4 



'464 

 •480 



•488 



'5io 



•523 



•53o 



-538 



-665 



•743 

 •878 



•961 



•991 



Observed 



•8x4 



•43 7 



•5o4 

 -446 



•5io 

 •485 

 •509 

 •5i8 

 •571 

 •543 

 •65o 



'746 

 •8 2 3 

 •900 

 •920 



Dirr. 



n 



+•017 

 •000 

 + 018 

 — -o3o 

 -f--oo3 

 -j-001 



-j~oo5 



— -o4i 

 — -oo5 

 +-oi5 

 — oo3 

 +-o55 

 +•061 

 +•071 



i-5 



1-5 



i-6 



1*75 



i-7 



1-6 



i'7 

 1-85 



195 



2' 

 2' 

 T 

 T 



The value of n was assumed the saiie for Moscow as for Chris- 

 tiania. A direct calculation from the observed temperature at 

 Moscow gives n — \ (nearly), and horizontal intensity = -6 (near- 

 ly)- For London n was assumed the same as for Paris. This 



accords with the 

 latitudes. Thp 



temperature 



temperatur 



and London is less than this law requires. 



The results which are given in Tables IV. to VIII, seem fully 

 to establish that the horizontal intensity is a function of the tem- 

 perature of the place ; and those of Table IV. indicate that it is 

 v ery nearly proportional to the temperature estimated from a point 

 JJJ the scale of the Centigrade thermometer 15° below the zero. 



£his point is within 2° of the zero of Fahrenheit. We have 

 therefore this curious and important result, viz. that the horizontal 

 ma gnetic intensity is nearly proportional to the mean annual tem- 

 perature of the place, as measured by a Fahrenheit's thermome- 

 [J. The zero of Fahrenheit is very nearly the temperature 

 *Wch obtains at the two cold poles. At these poles also the hor- 

 lz °Mal intensity is very feeble. The result just stated will then 

 a( *ord with the fundamental principle of our theory of the mag- 

 netic forces, that the magnetic intensity of each particle of the 

 earth's mass is proportional to the intensity of its heat, if we 

 JJake this modification in the statement of this principle, viz. 

 l nai the magnetic intensitv is nearly proportional to the intensity 

 01 the heat over and above that which obtains at the poles of 

 u —Jin cold. The real state of the case would probably be 

 ijttter represented by supposing that the magnetic intensity of 



le particles of the earth decreases more rapidly than the absolute 

 ^tensity of heat, and becomes comparatively feeble at the zero 



Fahrenheit, without becoming absolutely zero at this, or per- 



Btoxim 



81 



co «» S ERlEg , Vol. IV, No. 11.— Sept., 1847. 



28 



