ill the Magnetic Forces of the Earth. 



353 



inferred from the following considerations. In the first place the 

 dew deposited at night is evaporated during the morning hours. 

 In the next place the greater part of the evaporation of the rain 

 that falls during the night and the latter part of the day, will have 

 place during the forenoon of the following day; except when the 



rain is a heavy one, or the ground was previously quite wet, in 



which case, in the same state of the sky, the evaporation will be 

 most abundant during the warmest part of the day. The average 

 amount of rain that falls during a single night is considerably 

 greater than the average amount of dew. The average quantities 

 of rain that fell during the different quarters of the year, at Phil- 

 adelphia, according to the observations for the years 1842-3-4, 

 vary from five inches to sixteen inches, which is equivalent to from 



in -03 to in *09 (nearly) in a single night. It is to be expected, 

 therefo 



really attributable to evaporation, there will be variations in the 

 amount of the decrease connected with the variations in the 

 quantity of rain. That such a connection really exists will be 

 manifest on consulting the following table, showing the quan- 

 tities of rain, and the average decrease of the horizontal force for 

 the different quarters of the years 1842-3-4. 



re, that, if the morning decrease of the horizontal force be 



1st Quarter, 

 2d 



3d 

 4th 



u 



it 



Rain. 



1842. 



Force 



1843. 



10 6 

 11-4 



1-8 



limn. 





3 

 4 

 575 



3-7 :> 



Force 



10-3 



15-7 



8-7 



1844. 



Rain. 



325 



4-75 

 2 



7'3 



5*2 

 10-8 



8 



Force. 



2 5 

 35 

 3 



The following table of averages will make the connection in 



question still more evident. 



1st Quarter, 

 2d 



3d 



4th 



.. 



« 





6-8 



8-7 



126 



8-2 



Ratios 



1-3 

 1-45 



Force. 



25 

 325 



4*66 



292 





Rati 



in HL 



1-28 



1-43 



•63 



The correspondence between the ratios is remarkably close, 

 and would seem to indicate that, in the average of years, the 

 diminution of the horizontal force in the morning is mainly due 

 to the evaporation of the water that has fallen in rain and is but 

 slightly effected by the variations in the rise of temperature, and 

 in the amount of dew. 



Diurnal Variations of the Vertical Magnetic Intensity. 



, According to our general theory the vertical intensity is propor- 

 tional to the difference of temperature of two places situated at 

 e iuai distances to the north and south of the station of the needle, 

 a ^d on a line perpendicular to the isogeothermal line. We have 

 then to enquire whether the diurnal variations of the vertical in- 

 tensity are proportional to the diurnal variations of the difference 



