136 ANNUAL OF SCIENTIFIC DISCOVERY. 



mate manner. 4. The principle of terrestrial magnetism, in so far as 

 the phenomena 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 mechanical 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 from a 

 very small number of magnetic data, determined by observation and the 

 mean annual temperature of the place. 



"From the theoretical investigation of the normal state of the ter- 

 restrial magnetic elements, I propose now to proceed to the discussion 

 in the light of the same theory of tlieir diurnal variations. This theory 

 furnishes as the following general principles as a basis for this discus- 

 sion. 1. The horizontal magnetic intensity is proportional to its tem- 

 perature. 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 horizontal and vertical 

 magnetic intensities must be linked to the variations of the tempera- 

 ture of the station of the needle and of the differences of -temperature 

 of places north and south of this, and that the variations of declina- 

 tion must be connected with the variations in the position of the ideal 

 line passing through all places which have the same actual tempera- 

 ture as the given place ; which line may be called the true isogeother- 

 mal 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 differences of temperature of places situated to the east and west 

 of the station of the needle." 



Professor Norton then gives a formula for the horizontal intensity 

 of a place, furnished by the above theory, which is equivalent to the 

 statement, that the mean horizontal magnetic force is proportional to 

 the mean temperature. "We have, therefore, to compare the diur- 

 nal variations of the horizontal force with those of the temperature of 

 the place. The theory strictly requires that the comparison should 

 be with the daily variations in the absolute amount of sensible heat 

 near the earth's surface, but from well-known facts it is evident that 

 a rise or fall of surface-temperature will, in general, indicate an in- 

 crease or decrease of the total amount of heat. This suffices for the 

 inquiry which first arises, viz., whether the horizontal force increases 

 and decreases with the total amount of heat." Professor N. then 

 compares some curves arranged so as to show the mean daily varia- 

 tion of the horizontal intensity with that of the temperature for the 

 year 1844. It is found that the horizontal intensity attains its maxi- 

 mum from 3 to 4 P. M., and that " the maximum temperature occurs 

 at the same hour; also that the intensity increases with the tempera- 

 ture in the forenoon after 10 o'clock, and decreases with it in the af- 

 ternoon and evening. The same correspondences are found in other 

 years and quarters of years, with the qualification that the maximum 

 horizontal intensity sometimes occurs an hour or two later than the 



