NATURAL PHILOSOPHY. 173 



possible influence of high pressure on the temperature at which sub- 

 stances in a state of fusion solidity --an inquiry which was shown by 

 Mr. H., in a report recently presented to the British Association, to 

 have an important bearing on the questions of the original and pre- 

 sent state of the interior of the earth. It is well known that the tem- 

 perature of the earth increases as we descend, and it has been calcu- 

 lated that at the rate at which the increase takes place in such depths 

 as are accessible to us, the heat at a depth of 80 or 100 miles would 

 be such as to fuse most of the materials which form the solid crust of 

 the globe. On the hypothesis of original fluidity, and assuming that 

 the "rate of increase known to us by observation continues farther 

 down, and is not counter-balanced by a considerable increase in the tem- 

 perature of fusion occasioned by pressure, the present state of the 

 earth would be that of a solid crust of 80 or 100 miles in thickness 

 enveloping a fluid nucleus. Mr. Hopkins considers this state to be 

 inconsistent with the observed amount of the precession of the equi- 

 noxes, and infei'S that if the temperature of fusion be not increased 

 considerably by pressure, the hypothesis of internal high temperature 

 being due to primitive heat cannot be correct; whilst, on the other 

 hand, if the temperature of fusion be considerably heightened by 

 pressure, he considers the conclusion to be unavoidable, that the earth 

 must be solid at the centre. 



Mr. Hopkins is assisted in these experiments, which are carried on 

 at Manchester, by the well-known engineering knowledge of Mr. Fair- 

 bairn, and the equally well-known experimental skill of Mr. Joule. 

 The principal difh'culties attending the experiments with substances of 

 low temperatures of fusion have been overcome, and stixmg hopes are 

 entertained of success with substances of more difficult fusibility. The 

 pressures employed are from three to four tons to eight and ten tons 

 on the square inch The latter is probably equal to the pressure at 

 several miles beneath the earth's surface. 



HEAT OF THE SUX. 



M. SECCHI, of Rome, has made a series of photometric experi- 

 ments on the disc of the sun, by means of a thermo-electric pile. He 

 has found that the heat of the borders of the disc is nearly half that 

 of the centre, which confirms, as regards radiation of heat, what was 

 already known for light and chemical action. But he observed further, 

 that the heat was not the same at all points equi-distant from the 

 centre ; and that the place of maximum temperature was 3' above the 

 centre ; the isothermal curves were a species of parabola. The sun's 

 surface differs in temperature not only because of the absorption due 

 to its atmosphere, but also from certain inherent differences in the 

 surface itself. But M. Secchi also remarks that at the time of the 

 observations, the 20th, 21st, and 23d of March, the solar equator was 

 raised about 2.6' above the centre, and hence the inferior part of the 

 disc presented the south pole of the sun, while the north pole was con- 

 cealed ; and, moreover, the ascertained point of its greatest heat lies 



