270 A. M. Mayer—Device for projecting on a Screen 
conversion of sediments into plutonic rocks like granite, he 
conceives to be “ mechanical compression, with the heat and chem- 
cal action which proceed therefrom,” and adds in a note, allud- 
ing to the view which explains their conversion by the action 
of heat from beneath, “we should prefer to get the heat 
needed by the compression which accompanies the disturbance 
of the strata where metamorphism occurs.”. [Orographie Geo- 
logy, pp. 129, 180.] This view of Mr. Vose isconfirmed by the 
late researches of Robert Mallet, who concludes that, “as the 
solid crust sinks together to follow down the shrinking nucleus 
the work expended in mutual crushing and dislocation of its 
parts is transformed into heat, by which at the places where the 
crushing sufficiently takes place, the material of the rock so 
crushed and that adjacent to it are heated even to fusion. The 
access of water at such points determines volcanic eruption” 
{this Jour., III, iv, 411]. To this it may be added that, mas- 
much as the crushing process takes place in strata which from 
their depth are already at an elevated temperature, the heat 
developed by the mechanical process comes in to supplement 
that derived by conduction from the igneous center. Moreover, 
these strata include besides water, in many cases the com- 
Institute of Technology, Boston, Jan., 1873. 
Arr. XXIX.—On a simple device for projecting on a screen the 
deflections of the needles of a Galvanomeler, and thus obtaining an 
instrument convenient in research, and suitable for lecture expert 
ments ; by ALFRED M. Mayer, Ph.D. 
THE instrumental problem of obtaining on a screen the 
deflections of a galvanometer needle in magnified proportions 
has occupied the thoughts of several physicists. The subject 
is evidently one of considerable importance. In “delicate 
researches it is often necessary that the body of the observer 
