Hardcastle. — On the Tarawera Eruption. 27f 



This theory was originated by the late Mr. Robert Mallet. 

 In working it out he, at some expense, had massive machinery 

 constructed by means of which he crushed small cubes of various 

 kinds of rock, and noted the force required to crush them. By 

 means of other apparatus he measured the amount of heat 

 generated in the fragments by the work of crushing. The 

 amount of heat developed, as was to be expected from the law of 

 conservation of energy, was always in proportion to the force 

 employed. Having ascertained from the researches of others 

 the annual loss of heat by the globe, he calculated the amount 

 of contraction due to such loss of heat, and from the results of 

 his crushing experiments calculated that the annual contraction, 

 in terms of descent of the crush, must furnish power enough to 

 crush an amount of rock that would be sufficient, and more 

 than sufficient, to yield all the heat required for the average 

 annual display of all kinds of volcanic activity. This calcula- 

 tion may have been perfectly correct ; but it will not apply to 

 particular cases. Professor Hutton points out the flaw in 

 Mr. Mallet's theory : So many times the amount of rock to be 

 fused must be crushed, and then all the heat developed must 

 be focussed in a small portion of the fragments, which is an im- 

 possibility. (It is some years since I read Mr. Mallet's treatise, 

 and I have only a few extracts and notes from it by me ; but I 

 think I have stated his theory and its defect fairly. Professor 

 Hutton's allusions to it corroborate my memory so far as they 

 go). Mr. Mallet made the mistake of reasoning directly from his 

 machinery to the volcano, and unfortunately mislaid the germ of 

 what I contend is the true theory, while attempting to bring the 

 matter as a whole into subjection to arithmetic. His experi- 

 ments were made with small cubes of stone, unsupported at the 

 sides, and the average force required to crush them was — I gather 

 from one of my notes — about 15 tons per square inch of the upper 

 surface. Had he reproduced as far as possible the conditions 

 under which rocks must be crushed in nature, when buried 

 under a few miles of other rocks — had he enclosed his cubes in 

 a strong steel box, or tried to crush the central portion of a slab 

 strongly bound round the edges, he would certainly have had to 

 apply much greater pressures to crush them, and would hare 

 obtained proportionately higher temperatures in the fragments, since 

 the heat developed is in proportion to the force employed. Pro- 

 fessor Hutton says ten volumes of rock must be crushed to 

 furnish heat enough to fuse one volume. Possibly that is a 

 conclusion from Mr. Mallet's experiments. Then, if the 

 crushing of a piece of rock unsupported at the sides deve- 

 lopes heat enough to fuse one-tenth of it, and if crushed 

 under other circumstances ten times the force is required, in 

 the latter case, ten times the heat being developed, the whole will 

 be fused. 



