12 Prof. Maxwell on the Theory of Molecular Vortices 



Finally, comparative observations made simultaneously on the 

 borders of the lake, within a few feet of the water, gave the fol- 

 lowing result : — 



Temperature of the surface of the gravel . . . 990 C. 

 Temperature of the air 3 inches above the ground. 10*40 

 „ „ 6 feet „ . 10-55 



„ 15 feet „ . 10-62 



showing that the immediate neighbourhood of the water is suf- 

 ficient to modify the results generally obtained on land. 



The following conclusions may, I think, be safely drawn from 

 the foregoing observations : — 



1 . The gradual increase of temperature occurring on ascending 

 through the lower strata of the atmosphere, which appears con- 

 stantly to prevail on land about and after sunset, is not apparent 

 above a large surface of water. 



2. The immediate vicinity of a large sheet of water is suffi- 

 cient to modify to a considerable extent the effects of the noc- 

 turnal radiation of the earth, and thereby materially diminish 

 the increase of temperature observed under ordinary circum- 

 stances on ascending above the surface of the ground. 



3. One cannot help being struck by the great difference 

 (amounting to between 2 and 3 Centigrade degrees) constantly 

 observed between the temperature of the atmosphere a few feet 

 above the ground, and that of the air at the same height above 

 a large sheet of water. 



III. On Physical Lines of Force. By J. C. Maxwell, F.R.S., 

 Professor of Natural Philosophy in King's College, London*. 



Part III. — The Theory of Molecular Vortices applied to 

 Statical Electricity. 



IN the first part of this paper f I have shown how the forces 

 acting between magnets, electric currents, and matter capa- 

 ble of magnetic induction may be accounted for on the hypothesis 

 of the magnetic field being occupied with innumerable vortices 

 of revolving matter, their axes coinciding with the direction of 

 the magnetic force at every point of the field. 



The centrifugal force of these vortices produces pressures di- 

 stributed in such a way that the final effect is a force identical 

 in direction and magnitude with that which we observe. 



In the second part J I described the mechanism by which 

 these rotations may be made to coexist, and to be distributed 

 according to the known laws of magnetic lines of force. 



* Communicated by the Author. 



t Phil. Mag. March 1861. Phil. Mag. April and May 1861. 



