356 Wm. A. Norton on the Variations 



If now we compare the curves of vertical intensity for the dif- 

 ferent quarters of the year 1841, and other years, we find that the 

 variations are generally less for the first and last than for the other 

 two quarters of the year. But, so far as the calculations of differ- 

 ence of temperature have been made, there does not appear to be 

 an equal proportionate difference in the curves of difference of 

 temperature. It is barely possible that this apparent discrepancy 

 may be attributable to the fact that the data are not precisely 

 those which the theory calls for, and that the variations of the 

 vertical force are really the joint effect of the variations of the 

 difference of temperature of all places situated on lines drawn 

 through the station of the needle and at equal distances from this 

 station ; but, in all probability the principal cause is to be sought 

 elsewhere. The first inquiry that naturally arises is whether it 

 may not be found in the fact that, instead of taking the difference 

 between the temperatures at the earth's surface, we should take 

 the difference between the average temperatures of the stratum 

 just below the surface, which is subject to a daily variation of 

 temperature. In fact it is easy to see that if we make this cor- 

 rection, the vertical force ought to be less for the same difference 

 of temperature, during the cold than during the warm months; 

 for, the daily variation of temperature being then less, the stra- 

 tum of sensible daily variation of temperature will be of less 

 thickness. The freezing and thawing which take place in the 

 colder months will also have the effect to diminish this thickness ; 

 since when the earth freezes at night sensible heat will be given 

 out, which will make the cooling less than it otherwise would 

 be, and when it thaws during the day sensible heat will be ab- 

 sorbed, which will have the effect to diminish the rise of tem- 

 perature, and these effects are not confined to the surface of the 

 earth, but extend to a certain depth below it. The rising and 

 falling of vapor during the twenty-four hours will have little or no 

 sensible effect upon the intensity of the vertical force, (unless we 

 suppose that the vapor acts magnetically only when it is in con- 

 tact with the earth's surface,) since it is chiefly the matter at a 

 distance that is concerned in the vertical action upon the needle, 

 and the tangential force of any particle of matter thus situated 

 will be sensibly vertical for considerable distances both above and 

 below the needle. The evaporation which has place during the 

 day, and the deposition of dew during the night can have then 

 (except upon the above supposition) little or no sensible effect 

 upon the intensity of the vertical force, in any other way than 

 by the heat evolved and absorbed ; and this has already been 

 tacitly allowed for, for the actual difference of temperature de- 

 pends upon the deposition of dew and the evaporation, as well as 

 upon the heating power of the sun and the radiation into space 

 and the atmosphere. 



