Magnetic Intensity of the Earth 219 



From these results the following consequences may be drawn. 

 The intensity is more constant in June than in January. In 

 six summer 'months it has only varied between 830"96, and 

 829"24, but in the winter months between 825"27, and 

 829"98. But the regular daily variations from 10 a. m. to 



g 7 P . M#} are much greater in, June, when they amount to 



1"4, than in January, when they are not greater than 0"2. 

 In both these circumstances, these variations have a great re- 

 semblance to the variations of the barometer, of which the 

 regular daily variations are greater in summer than in winter, 

 the irregular greater in winter than in summer. I suppose 

 that both these phenomena, (the regular daily variations of the 

 barometer, and of the intensity and the magnetic declination,) 

 have a common cause, viz. a regular motion or stream in the 

 atmosphere, caused by the different effects of the sun-rays, 

 (proportioned to the sine of their angle of incidence,) upon 

 different places of the earth. This streaming, or transposition 

 of great quantities of air from one place to another, will affect 

 the barometer ; and different streams of air, of different tem- 

 ' peratures, at different heights, may perhaps produce a weak 

 electromagnetic effect, which may have some influence upon the 

 direction and rate of vibrations of the magnetic needle. Con- 

 sequently these regular variations may be much greater m the 

 summer, in proportion to the variations of the temperature 

 from noon to midnight. But these are only rude ideas, which, 

 on another occasion, I shall more fully explain. 2. The mean 

 between maximum and minimum of intensity is so very nearly 

 constant, that it is difficult to say whether it has decreased or 

 increased from 1819 to 1826. 3. The difference between sum- 

 mer and winter is very variable, and seems to have regularly 

 decreased from 1819 to 1826; but probably this difference 

 will again increase in the following years. In those days, 

 (from 1st to 3d June,) I have found the time of 300 vibrations 

 varying between 832"9 and 8S4"0 ; but as some few obser- 

 vations in the open air have given almost the same result as 

 the observations in June in the same spot in all the foregoing 

 yean, I fear that some change in the house may have produ- 

 ced a little local variation, which, by further observations, will 

 Ik- detected, 



