in the Magnetic Forces of the Earth. 351 



variations in question. It follows therefore from the conclusions 

 which have just been obtained, that the probable cause of the 

 secondary variations of the horizontal force is to be found in the 

 varying quantities of dew deposited at night ; from one hour to 

 another, and from one season to another. It is to be observed, 

 however, that it is not essential that the unequal losses of temper- 

 ature at night should be attributable entirely to the thermal influ- 

 ence of dew, to enable us to draw this inference. We have seen 

 that the results of observation and experiment conduct us to the 

 conclusion that the tendency of this influence is to produce the 

 inequalities which have been under consideration. This fur- 

 nishes sufficient ground for the inference ; but if the entire amount 

 ot the inequalities of the decrease of temperature be not due to 

 this influence, there is room to doubt whether the secondary vari- 

 ations of the horizontal force might not be attributable to the 

 action of the other unknown cause or causes tending to produce 

 unequal variations of temperature at night. In this event also, 

 in estimating the effect of the dew upon the horizontal force, 

 we could not be positively certain that the entire variations of 

 temperature at night were attended by corresponding variations in 

 the amount of heat in the stratum of variable temperature below 

 the earth's surface : unless, at least, it should be admitted that 

 the unknown cause must consist in some relation or phenomenon 

 external to this stratum. 



Having thus arrived, inductively, at the probable cause of the 

 secondary nocturnal variations of the horizontal force, let us see 

 how it applies itself to the detailed explanation of these variations. 

 It is abundantly evident, from what has gone before, that the 

 continual accumulation of dew, or condensed vapor, at the earth's 

 surface from evening till morning, must tend to diminish the rate 

 of decrease of the horizontal force ; and that the increase in the 

 quantity of dew that falls, from hour to hour, must tend to make 

 this diminution greater and greater, and that it is not improbable 

 that towards morning the effect of the increased quantities of 

 dew may prevail over the effect of the loss of heat, and thus that 

 the horizontal force may begin to increase. (See the July 

 No. of this Journal, p. 40, figs. 1 to 5.) The actual variations of 

 the horizontal force, it will be observed, are the result of two 

 antagonistic causes: the tendency of the uniform loss of heat, 

 from radiation, is to make the horizontal force decrease uniformly 

 during the night ; the tendency of the dew is to augment this 

 force, but it acts unequally, producing the greatest effect towards 

 morning, when it ordinarily prevails over the other cause. The 

 dew tends to augment the horizontal force in two ways ; by fur- 

 nishing a certain amount of heat to the earth, and thus diminish- 

 ing the loss of heat, and by adding to the amount of magnetic 

 natter at the earth's surface. The joint effect of these two 



