84 



SCIENCE. 



[Vol. V., No. 104. 



incredible transgression of an older mass upon 

 a newer one, now reported, has few parallels, 

 unless one may be found in the famous over- 

 turning of the Windgallen Alps, studied out 

 by Escher von der Linth, and confirmed by 

 Heim. In the face of such an example, so 

 utterly beyond explanation without the aid of 

 irresistible lateral compression, we feel that the 

 contractional hypothesis gains new support ; 

 and against the English school of physi- 

 cal geologists, who claim to show its insuf- 

 ficiency, the conclusion of Heim may be now 

 quoted with new force : more may be learned 

 of the earth's structure from critical observa- 

 tions on its surface than from calculations 

 founded on physical assumptions concerning 

 its interior. Besides these extraordinary facts 

 of motion, the production of chemical changes 

 during the mechanical stresses and distresses 

 of the Highlands is hardly less remarkable. 

 Sandstone passes into gneiss, and gneiss 

 acquires schistosity, in a new direction in 

 obedience to distorting forces. All this is 

 beautifully confirmatory of Lehmann's conclu- 

 sions from his researches in Germany during 

 the past few } r ears. Mechanical metamorphism 

 thus gains the support of a series of facts that 

 chemical metamorphism can ill afford to lose. 



The article on this subject, contributed to 

 the present number of Science b} T a well-known 

 writer on these matters, contains certain state- 

 ments to which exception may be taken. The 

 questions raised with regard to the New-Eng- 

 land rocks here referred to certainly cannot 

 be considered ' settled ' in the manner implied 

 by our contributor, as was very evident at the 

 Montreal meeting of the British association 

 last summer : and the metamorphic origin of 

 certain alpine rocks is not yet generally aban- 

 doned. As to 'regional metamorphism,' the 

 revelations of recent detailed and minute 

 studies in the field are not always such as to 

 disprove it, but rather to attribute the meta- 

 morphic changes, where they occur, to me- 

 chanical instead of simply to chemical action ; 

 and, when disturbed and distorted rocks are 



found in altered mineralogical conditions over 

 considerable areas, 'regional metamorphism' 

 does not seem to be a misleading or confusing 

 term to apply to them. Finally, the impli- 

 cation that Mr. Geikie makes insufficient ref- 

 erence to the results of his predecessors is 

 certainly unwarranted. He states sufficiently 

 that other observers have preceded him in the 

 views he has now come to hold, and promises 

 that they shall be duly mentioned in the de- 

 tailed report which is to follow the present 

 brief and preliminary publication. His out- 

 spoken frankness in admitting his previous 

 error leaves nothing to be desired, and sets an 

 example worth} 7 of imitation. 



LETTERS TO THE EDITOR. 



*** Correspondents are requested to be as brief as possible. The 

 icriter's name is in all cases required as proof of good faith. 



Trowbridge's Physics. 



In a recent number of Science will be found a 

 criticism of Professor Trowbridge's 'New physics.' 

 Those who have carefully read the work alluded to 

 have doubtless found errors here and there, which 

 would not appear in a second edition; but no one 

 can fail to recognize a master mind in the organiza- 

 tion of this new method of teaching natural science. 

 The allegation that Professor Trowbridge has mis- 

 stated some of the fundamental laws of mechanics 

 is not sustained by a closer examination. 



It is hardly necessary to point out that the for- 

 mulae for the ballistic pendulum become perfectly 

 intelligible if we understand by the first h, not the 

 maximum height attained, but the observed distance 

 through which the pendulum is acted upon by the 

 force F, whose average value is thus determined 

 'without involving the element of time;' that the 

 laws for the lever, which caused the critic even more 

 surprise, are perfectly correct, when, as in the case 

 in point, angular acceleration is considered, since 

 the work spent upon equal masses, like their moment 

 of inertia, is in this case proportional, as stated, to 

 the square of the distance from the fulcrum ; that it 

 is indifferent, in the experiment, whether we find the 

 length, or the radius of gyration, of the equivalent 

 simple pendulum, since the two are identical; and 

 that force is constant over the concentric spherical 

 equipotential surfaces in question. The last two 

 statements, therefore, as made by the author, need 

 only to be restricted. 



Such oversights as the critic is easily able to detect 

 are not insidious, like some of those which have 

 crept into many modern text-books. The underlying 

 principles are brought out by the course of experi- 

 ments in their clearest light; and therefore the work 

 must be regarded by teachers as a safe and trust- 

 worthy guide. 



It may be added that the experiments considered 

 so difficult by the critic have already been employed 

 with success in an elementary course, and are un- 

 doubtedly in place in any work whose object is to 



