in the Magnetic Forces of the Earth. 43 



the horizontal force. It would seem that the increase of the 

 horizontal force during the night and decrease in the morning, 

 cannot be attributable solely to the heat given out by conden- 

 sation and abstracted by vaporization, since, notwithstanding the 

 deposition of vapor at night and the evaporation in the morn- 

 ing, the temperature continues to fall until morning, and after 

 that rises steadily during the forenoon : — unless it should chance 

 that a considerable portion of the heat evolved by the condensa- 

 tion penetrates below the surface, so as to augment the average 

 temperature of the stratum which is subject to daily variations of 

 temperature, and that, in like manner, the cooling due to evapo- 

 ration lowers the average temperature of this stratum, at the 

 same time that the surface temperature rises. I am disposed, 

 therefore, to attribute the secondary variations of the horizontal 

 intensity, under consideration, chiefly to the variations in the 

 quantity of condensed vapor at the earth's surface, attending the 

 fall and rise of temperature. But, whether the morning maxi- 

 mum and minimum are principally effects of variations in the 

 quantity of magnetic matter in action, or of variations in the ab- 

 solute amount of sensible heat due to the fall and rise of vapor, 

 the effect in a given time, will on either supposition, be propor- 

 tional to the amount of vapor deposited or of water evaporated. 

 When we consider the entire secondary variations during the 

 night, it is to be observed that, upon the view which I have 

 adopted, the fall of vapor has two effects ; it diminishes the rate 

 of cooling of the earth, by the heat evolved, which makes the 

 decrease of horizontal force less, and it augments the quantity of 

 magnetic matter in action, which makes the diminution of this 

 force still less, or converts the decrease into an increase, according 

 to the amount of vapor deposited. In like manner the evapora- 

 tion after sunrise has two effects. If any heat be evolved or ab- 

 stracted, in addition to that connected with the variations of tem- 

 perature at the surface produced by the rise and fall of vapor, it 

 will conspire with the changes in the quantity of condensed vapor 

 acting upon the needle, to produce an augmentation of the hori- 

 zontal force during the forenoon. 



Before presenting the considerations which 1 have to urge in 

 support of the theory which I have now advanced, let me con- 

 sider an objection which has probably occurred to the reader; 

 viz., that the quantity of vapor that falls during the night is alto- 

 gether too trifling to produce an effect so considerable— to more 

 than neutralize the effect of the decrease of temperature; and, 

 in like manner, that the loss of matter from the surface of the 

 earth, by reason of the evaporation in the forenoon, is too small, 

 m comparison with the whole amount of matter which has a 

 variable action upon the needle, to have the effect attributed to it. 



