of. W. A. Norto 



213 



Horizontal Intensity. 



The formula which has been investigated, for the horizontal 

 component of the directive force, or the horizontal magnetic in- 

 tensity of a place, is 



Hor. intensity 



C'T 



(10.) 



in which C is an unknown constant, and T the mean annual 



temperature of a place ; understanding by the mean annual tem- 

 perature, the mean intensity of the heat of the earth at the place. 

 Temperatures are experimentally obtained by means of thermom- 

 eters, which give the height of the mercury above an arbitrary 

 zero, and by an arbitrary scale. It is not to be supposed, there- 

 fore, that any of those in use furnish the proportional absolute 

 amounts of heat. 



The only resource, then, is to add arbitrarily 



different numbers to the thermometric values of T until one be 

 found, if possible, which will give values of the horizontal inten- 

 sity that accord with observation. The following table gives 

 the results obtained for a number of places, by adding 15° (cen- 

 tigrade) to T° (centigrade.) The formula was 



Hor. intensity 



H 



£4-15° 

 T + 15° 



(11.) 



«| which II is the observed horizontal intensity for some partic- 

 ular place, T the temperature at this place, and t the temperature 

 at the place for which the horizontal intensity is required. For the 

 Western continent H= horizontal intensity at New York = -5291; 

 *or Europe, H= horizontal intensity at Paris = -513. The tem- 

 peratures, as in all other investigations in the present article, were 

 token from Mahlmann's table, or from Kaemtz, pp. 192, 193, and 

 reduced when necessary to the level of the sea. The "observed" 

 horizontal intensities were deduced from the observed total inten- 

 sities and inclinations ; these being derived from the same author- 

 jjjes as the declinations, with the addition of Sabine's Report on 

 jhe magnetic intensity of the earth. To avoid repetition, I will 

 h ere remark that the observations employed in the subsequent 

 FKto of this article were derived from the same sources. 



Table IV. 



Pla 



ce. 



Montreal, 



8 La" 3 "' 



- Louis, 



ha N Hill, 



!• Aii g „sti ne 



^yWest, ' 

 tquatnr, east . 



* rn coast of 



• America, 



Hor. Intensity. 



computed observed 



•509I 

 M09 

 •48 1 

 •482 

 •576 

 •5?6 

 •616 



,♦645 



• 7 53 



•82f 



-443 

 •5oo 

 •5oo 

 •571 



•5 7 5 

 •591 

 •640 



•770 

 •820 



•882 



•936 



Diff. 



+-o34 

 +•019 

 4-.018 



— *oo5 

 •001 

 02 5 

 — -oo5 



+■017 



— -OOJ 



o54 



Place. 



1 1 1 » r JiifHusny. 





Pans, 



Moscow, 



Christiania, 



Kdnigsberg, 



Berlin, 



Edinburgh, 



Gdttingen, 



London, 



Milan, 



Marseilles, 



Naples, 



Lat. 25°, W. coast 



of Africa, 

 Equator, do. 

 do. do. 



computed observed 



-5l8 



Diff 



•382 



•4lO 

 •422 



•474 

 .478 



•494 



•5o<5 

 •568 



•584 

 ♦632 



•754 



•854 

 •921 



•5o4 

 •43 7 



•479 

 •509 



■446 

 •5 10 



•485 



•571 



•543 



•65 7 



•814 

 •920 



•920 



•122 

 •027 



•o57 

 -•o35 

 -f-*o32 

 — 016 



-f--02I 



— -oo3 

 -f-*o4i 



— *025 







060 

 066 

 +*ooi( 



