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��SCIENCE.
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��incredible transgreaaion of an older mass upon a newer one, now re|>orted, hns few parallels, unless one may be found in the faraoiis over- taming of the Windgalteii Alps, studied out by Escher voii der Liulb. and coDfiimcd by Helm. In the face of such an example, so Utterly beyond explanation without the aid of irresistible lateral tompresaion, we feel that the Gonti'actional hypothesis gains new support; and against the English school of physi- cal geologists, who claim to show its insuf- ficiency, the conclusion of Ileira may be now qnoled with new force: more may be learned of the earth's structure fram 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 eonclu- Bions from his researches in Germany during the past few years. Mechanical metamorphism thus gains the support of a series of facts that chemical metamorphism can ill afford to lose.
��The article on this suhjcct, contributed to the present number of Science by 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 metamorpbic 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 lo attribute the meta- morpbic 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 lo them. Finally, the impli- cation that Mr. Geikie makes insufficient ref- erence K» the results of his predecessors ia cei'tainly unwarranted. He states suflSciently that other observers have preceded him in the vievfa 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 fVankness in admitting his previous error leaves nothing to be desired, and sets an example worthy of imitation.
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��LETTERS TO THE EDITOR.
��Ik a
��Trowbiidee'a Phyaios.
- number of -Sctmce wilJ be found a
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��crilieUm of Professor Trowbridge's ' New physics,' Those who have carefully read Uie work alluded to have doubtless found errors hefe ajid there, which would nut appear in a second edition; but iio one con fail to recogni/e a master mind in llie organiza- tion of this new melliod of tenching natural science. The allcgalioii that Professor Trowbridge has mis- stated some of thv fundamental laws of mecliaiilcs is not sustained by a closer examination.
It is hardly necessary to point out that the foi^ mulae fur the ballistic pendulum become perfectly intelligible if wc understand by the first A, not the uiBxinmm heielit attained, but the observed distance through which the pendulum is acted upon hy the force F, whose average value is thus determined 'without involving the eletaent of time;' that the laws for the lever, which cause<] 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 (lie fulcrum; that It is iudlllerent, In the experiment, wbether we find the length, or the radius of gyration, of the equivalent simple pendulum, since Uie two are Identical; and that force is constant over the concentric spherical equlputentlat surfaces in question. The Inst two statements, Uierefore, as made by the author, need only to be restricted.
Sucb oversights as the critic is easily able to detect arc not insidious, like some of those which have crept into many modern teit-books. The underlying principles are brought out by the coarse of experi- ments in Ibelr clearest light; and therefore the work must be regarded by teachers as a safe and trust- worthy guide.
It may be ailded that the experiments considered so difficult by the critic have already been employed with success in an elementary conrse, and are un- doubtedly in place In any work whoae object la to
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