MR. HENNESSY’S RESEARCHES IN TERRESTRIAL PHYSICS. 
503 
would then tend to change the acquired densities of the cooled matter. The as yet 
uncooled fluid would have its temperature reduced from contact with the cold par- 
ticles from above, and it would tend to change its position in a similar manner to 
the portions at first cooled. As each cooled portion of the fluid descends, the three 
following causes would therefore impede its descent. 
1. From the arrangement already indicated of the denser strata about the centre 
of the mass, and from the nature of the law of density of the strata, each stratum 
into which the cooled fluid Avould descend, would be denser than the preceding. 
2. If the general effect of the refrigeration be to increase the density of the cooled 
matter, each stratum would have its density augmented by the passage through it of 
the cooler matter from above. 
3. The descending portions will have their densities diminished by the increase in 
their temperature. 
During this process of circulation the interior matter in a state of fusion will be 
losing some of its heat by conduction. It must be evident however that if the tem- 
perature of the fluid be very high, the cooling would be carried on at first chiefly by 
the process of circulation. If, as analogy seems to point out, the conducting power 
of the fluid were small, the process of circulation would long continue to perform a 
principal part in the refrigeration of the fluid. 
Although from the imperfect state of our knowledge with respect to the cooling 
of fluids it would be impossible to arrive at the precise laws and nature of the pro- 
cess of circulation, it yet seems probable from the three impeding causes which 
have been adduced, that the energy of this process would diminish from the surface 
of the spheroid to its centre. The 'principal oscillations of the fluid produced by this 
cause would therefore he at first confined to the vicinity of the surface. A solid crust 
may therefore be formed at the surface, long before the process of circulation could 
have extended to any great depths. We should thus have an external solid crust 
with strata of the imperfect fluid which may have been subjected to the process of 
circulation below it, and in the centre a mass of matter still retaining a high state of 
fluidity. The existence at the centre of the spheroid of a solid nucleus of small mass 
compared to the entire mass of the earth, cannot, as will hereafter appear, exercise 
any important influence on geological phenomena. If from compression, or any 
other cause, this solid nucleus should exist, it is evident that solidification would 
proceed much more slowly upon it upwards, than upon the interior surface of the 
shell downwards ; or in other words, the increase of its mass after its first formation 
would be small compared to the increase of the mass of the shell subsequent to its 
first solidification. Whether such an internal solid nucleus exists or not, certain 
phenomena will attend the formation of the shell which it may be important to 
remark. From what is known respecting the solidification of fused substances, it 
may be inferred that at the moment when a portion of the fluid assumes the solid 
3 T 2 
