in the Magnetic Forces of the Earth. 49 



Notwithstanding such variations of /, the horizontal force, as it 

 depends only on the absolute quantity of heat, would decrease 

 uniformly during the night. 



Having laid down these general principles, let us enter upon 

 the general enquiry to which the statement of these principles is 

 preliminary. If we compare the nocturnal variations of the hori- 

 zontal force with those of the temperature, as shown by the 

 curves, (see figs. 1, 2, &c. to 10,) we find that the deviations from 

 uniformity in the diminution of the horizontal force are attend- 

 ed with like deviations in the fall of the thermometer : — thus, 

 while the thermometer falls less and less rapidly as the night ad- 

 I vances, the diminution of the horizontal force becomes less and 



less ; also, while during the fall and winter months the fall of 

 I temperature during the night is materially less than in the spring 



I and summer months, the nocturnal diminutions of the horizontal 



force are less. It is true that the diminution of the horizontal 

 force gradually passes into an increase, but this is only the result 

 of a certain increase in the amount of the deviation from uni- 

 formity of diminution. The deviations therefore are of the same 

 character — lie continually in the same direction — for the tempera- 

 ture and horizontal force. They are also cotemporaneous ; they 

 differ only in proportionate amount. We have therefore to seek 

 for the cause of the secondary variations of the horizontal mag- 

 netic intensity of a place in the cause, whatever it maybe, of the 

 changes in the rate of diminution of the temperature during the 

 ^Jght, and from one season to another. The former cause is 

 probably identical or closely connected, physically, with the lat- 

 ter, but it is possible that the connection may be more or less re- 

 mote, or even that the correspondence between their variations is 

 accidental. Now the cause of the nocturnal inequality of tem- 

 perature must be found in some phenomenon or fact connected 

 either with the relations of the surface of the earth to the atmos- 

 phere, or with its relations to the matter below the surface. If it 

 be a meteorological phenomenon, we may suppose it to consist in 

 variations in the clearness of the sky, in the quantity of rain, in 

 the quantity of dew, or, speaking more generally, of vapor de- 

 posited in other forms than that of rain, in the direction and force 

 °f the wind, and in the amount of heat absorbed by the atmos- 

 here or exchanged with it. We may reject the phenomenon 

 a st mentioned at once ; for, as we have seen, in a calm clear 

 evening the nocturnal radiation is uniform at all temperatures, and 

 this radiation is the actual radiation, one element of which is 

 the exchange of heat with the atmosphere, and another the 

 absorptive action of the atmosphere. If it be surmised that it 

 is only the maximum radiation on clear evenings that is uniform 

 at all temperatures, such is not the statement of the law, and if 

 11 w ere, then the uniformity must be independent of the density 



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





