38 Wm. A. Norton on the Variations 



from a very small number of magnetic data determined by obser- 

 vation, and the mean annual temperature of the place. 



From the theoretical investigation of the normal state of the 

 terrestrial magnetic elements, I propose, in the present article, to 

 proceed to the discussion in the light of the same theory, of their 

 Diurnal Variations. This theory furnishes us the following gen- 

 eral principles as a basis for this discussion. 1. The horizontal 

 magnetic intensity of a place is proportional to its temperature. 

 2. The vertical intensity is proportional to the difference between 

 the temperatures of two places situated at equal distances north 

 and south of the isogeothermal line, in a direction perpendicular 

 to it. 3. The direction of the needle is nearly perpendicular to 

 the isogeothermal line. From these general principles we may 

 draw the general conclusions, that the variations of the horizon- 

 tal and vertical magnetic intensities must be linked to the varia- 

 tions of the temperature of the station of the needle and of the 

 differences of temperature of places north and south of this, and 

 that the variations of declination must be connected with the 

 variations in the position of the ideal line passing through all 

 places which have the same actual temperature as the given place ; 

 which line may be called the true isogeothermal line. If the 

 latter conclusion be true, it may be added that the variations of 

 declination must also be connected with the variations in the dif- 

 ferences of temperature of places situated to the East and West 

 of the station of the needle. 



The data for the detailed discussion have been chiefly taken 

 from the Report of the " Observations at the Magnetic and Me- 

 teorological Observatory at the Girard College, Philadelphia, made 

 under the direction of A. D. Bache, LL.D., 1840 to 1845," and 

 the Report of the "Magnetical and Meteorological Observations, 

 made at Washington by Lieutenant J. M. Gilliss, U.S.N., dated 

 August 13th, 1838." The first Report contains a complete series 

 of Magnetic and Meteorological Observations, generally either bi- 

 hourly or from hour to hour, extending from June, 1840, to June, 

 1845, besides Term Day Magnetic Observations, and Extraor- 

 dinary Observations. Tables of the daily, monthly, quarterly, 

 semi-annual, and annual means, and of the hourly or bi-hourly 

 means for months, are given ; and curves traced exhibiting these 

 results to the eye, and showing the Extraordinary and Term Day 

 Observations. The second Report comprises a Journal of Mete- 

 orological Observations made at four different hours during the 

 day, (3 a.m., 9 a.m., 3 p.m., 9 p.m.,) kept from July, 1838, to 

 June, 1842; a set of bi-hourly meteorological observations and 

 observations of declination extending from June, 1840, to July, 

 1842 ; Term Day and Extraordinary Observations ; and occa- 

 sional observations of the dip of the needle. Tables of abstracts 

 are also given, and curves showing the variations of declination 



and temperature on the term days. 



