in the Magnetic Forces of the Earth. 41 



ference from the first mentioned fact, is that if the daily variation 

 of temperature is one cause of the daily variation of the hori- 

 zontal force, there must also be some other cause in operation 

 besides the mere variation of temperature. Is this cause one en- 

 tirely unconnected with the variation of temperature, or is it some 

 indirect effect of this variation ? The Newtonian principle of not 

 multiplying causes would prompt us to try the latter supposition. 

 Besides a connection exists between the time of the secondary 

 maximum of horizontal intensity and the time of maximum tem- 

 perature, which serves to render it probable that this supposition 

 is the true one. This connection may be inferred not merely 

 from their coincidence in the mean curves for the year, but also 

 from their approximate coincidence in the different curves for the 

 quarters of the year (see figs. 1, 2, 3, &c, to 10) ; — in other 

 words, from the fact that the time of the secondary maximum of 

 horizontal intensity moves forward and backward, during the 

 [ year, with the time of sunrise. It should be observed, however, 



that this fact is less distinctly shown by the curves for some years 

 than for others. It has been recognized by Professor Loyd, in 

 his observations at Dublin. He says, " The epoch of the morn- 

 ing maximum moves forward as the time approaches the winter 

 solstice, appearing to depend upon the hour of sunrise which it 

 precedes by a short interval." The manifest inference from this 

 connection is that the increase of the horizontal force during the 

 latter part of the night, when the temperature is on the decrease, 

 I and the decrease of this force for several hours after sunrise, 



i when the temperature is on the increase, are in all probability in- 



| direct effects of the change of temperature. While making a 



\ hasty comparison of the daily variations of the horizontal force 



with the theory that its intensity varies with the temperature, 

 about a year since, it occurred to me that the secondary changes 

 of this force, just noticed, were probably due to the deposition of 

 condensed vapor from the atmosphere during the night, and the 

 evaporation which immediately succeeds in the morning. These 

 are well established effects of the daily fall and rise of tempera- 

 ture. The tendency of the deposition of vapor that goes on 

 while the temperature is falling, must be to augment the horizon- 

 tal force, and the tendency of the evaporation of the dew that 

 falls at night, produced by the heat of the sun in the morning, 

 must be to diminish this force. The deposition of vapor must 

 tend to increase this force in two ways : viz., by the heat given 

 out in the act of condensation, and by adding to the amount of 

 latter at the earth's surface which acts upon the needle. The 

 evaporation must also tend to diminish the force in two ways ; 

 v iz., by the loss of sensible heat accompanying the vaporization, 

 and by the loss of a certain amount of matter, from the earth's 

 surface, whose horizontal magnetic action had formed a part of 



Second Series, Vol. VIII, No. 22.— July, 1849. 6 





