Correspondence — Mr. R. Lydekker — Rev. Dr. Irving. 479 

 coiaS/iEsiE^oisriDsisroE. 



THE STUDY OF MAMMALS. 

 Sir, — I am indebted to the reviewer of this work in your last 

 number for pointing out that the family Tritylodontidce occurs twice 

 in the systematic table. Its second occurrence is not, however, as 

 he supposes, an inadvertent repetition, but a " misprint" for Tricono- 

 dontida. Another slip occurs on p. 99, where in giving the range 

 of Oryx Persia stands instead of Syria. E. Lydekker. 



DYNAMIC METAMORPHISM "AGAIN." 

 Sir, — Personally I am son'y to be called upon to point out briefly 

 that Mr. Fisher (Geol. Mag. Sept. 1891, p. 430) has made the 

 mistake of substituting exclusion (or " outness ") in space for logical 

 exclusion of one term from a series of other terms used in a train of 

 reasoning, and that this misconception seems to run through the 

 whole of his letter except the last paragraph. It is not Mr. Fisher's 

 (P - W) w, but the " last term " of the four lohich I had just 

 enumerated (p. 299), which is logically outside the other three. To 

 say that it is outside the cubic unit (not "element") of the mass, 

 on which the work is done, is something to which I am unable to 

 attach any meaning at all. The energy, to which the motion of the 

 train (in my illustration) is due, is dissipated (not annihilated or 

 necessarily " converted " into some other form of energy) according 

 to the ordinary laws of thermodynamics, having been obtained as 

 heat from the potential energy of the fuel and atmospheric oxygen, 

 and utilized, while in ' a condition of high intensity (the H^O being 

 the carrier of the energy), to move the piston of the engine with the 

 load attached. In running down from a state of high intensity 

 (in which work can be got out of it) to a state of low intensity (in 

 which it is either absorbed by surrounding bodies, or passes off by 

 radiation into the general entropy ^ of the universe), there is no 

 destruction, there is only dissipation, of energy ; and when it is 

 thus dissipated, you cannot get any more work out of it. If energy 

 were (under the conditions specified) " stored up in the train," after 

 it had come to a 'standstill (the idea which was before my mind, 

 though, I fear, not explicitly stated), the train would be, after 

 translation, in a position of advantage loith respect to motion, as 

 compared with its position before translation, which is absurd. 

 Certainly during the accelerating stage of translation energy is 

 being stored in the train, just as you store energy in the weight of 

 a clock in winding it up ; but the same amount of energy is taken 

 out of the train in bringing it to a standstill, just as it is taken out 

 of the clock-weight, when it runs down. There is therefore no 

 more energy stored in the train, after it has come to rest, than there 

 is in the weight and works of a clock after it has run down. So in 

 the case of the rock-mass under consideration, the source of the 

 energy is gravitation. The work done on the rock is only a case of 

 1 ' ' Entropy ' ' in the sense in which the word was first used by Clausius. 



