NATURAL PHILOSOPHY. 209 



part of the heat acquired by condensation would be, that so long as the 

 matter continued in a fluid condition, the resistance to compression from 

 this cause would be very small. If, however, the fluid were on the point of 

 changing its state to that of solidity, the effect of the latent heat of fusion 

 which by hypothesis could not be emitted, would interpose a resistance of 

 great magnitude compared to that resulting from simple compression. The 

 fused matter of which the interior of the earth mo^t probably consists, would 

 be under conditions similar to those mentioned, from the slow conducting 

 power of the materials composing the earth, and from the pressure of all the 

 outermost strata of equilibrium of the fluid upon those near the centre, and 

 thus the influence of pressure in promoting solidification would be less than 

 at its surface. 



ON SIMULTANEOUS ISOTHERMAL LINES. 



Professor Hennessy, in a paper before the British Association, at its recent 

 meeting on the above subject, called attention to the importance of studying 

 the simultaneous distribution of temperature. The movements of the winds, 

 and in general almost all the daily perturbations of the atmosphere, depend- 

 ing much more on simultaneous conditions of temperature in different places 

 than on the mean amount, over long periods, it would be desirable to at- 

 tempt to trace the former class of lines when we attempt to obtain a complete 

 connection between the different classes of atmospheric phenomena. If 

 temperature were record 3d at every station on the surface of the earth at the 

 mean time corresponding to any given meridian, then a line traversing the 

 places where such temperature were found to be equal, would be a simulta- 

 neous isothermal line. The forms of such lines would manifestly depend on 

 the diurnal range of temperature at the several stations, as well as the seve- 

 ral physical conditions influencing mean temperature. If the earth were 

 absolutely at rest, and stripped of its fluid coverings, these lines would be 

 circles, having their planes perpendicular to a line joining the centres of the 

 earth and sun. This would be nearly the case in a body turning very 

 slowly on its axis, like our satellite, in which Prof. Hennessy anticipated 

 that observations would ultimately show a great diminution of heat radiated 

 from the edges compared to the centre. With a more rapid motion of rota- 

 tion, as in the case .of the earth, the isothcrmals would be elongated in a 

 direction parallel to that of the rotation. On introducing the influence of all 

 the actual motions of the earth, and of the emissive and absorbing powers of 

 the atmosphere, the ground, and the sea, the forms of these lines would be 

 considerably modified. The forms of these lines on the land and sea would 

 necessarily differ, and might be expected to present some important relations 

 to the directions of land and sea breezes. It appears, in general, far more 

 probable that a knowledge of the contemporaneous conditions of tempe- 

 rature at different places would assist in pointing to a connection between 

 these phenomena and atmospheric perturbations rather than a knowledge 

 of mean temperature. Atmospheric currents, whether vertical or parallel to 

 the earth's surface, depend upon contemporaneous differences of tempera- 



18* 



