G. W. Hawes—Eruptive Rocks in New Hampshire. 147 
The fourth series of results are those before and after heat- 
ing the bars to redness for three or four hours. 
4th Series. 
Before heating. After heating. 
66-4 66-2 67-7 68-4 68°5 9 68°9 
79°9 80°5 TT5 18-9 78°3 TTT 5 
163 6 79°] T8°T 78-2 78°6 78-6 
72°1 72-2 0 TOL 70°1 70 
13% 13-7 13°6 14 13-8 73°8 73°15 
different parts. The views are of the front and back of the 
divided circle. The iron bars are shown in the position to the 
north and above the line of dip, the initial position of the above 
description. They are on a frame which turns about the cen- 
ter of the circle, and from which they can be easily detached 
and reversed. The other plan shows the testing magnet in the 
casing at the middle of the disk. 
No particular care was taken to have the plane of the circle 
in the plane of the magnetic meridian. As a self-registering 
apparatus, the instrument seemed faulty ; as at no time during 
the experiment did the magnetic state of the iron remain con- 
stant. 
Physical Laboratory, Harvard College, Dec. 6, 1878. 
Art. XVIII. — On a Group of dissimilar Eruptive Rocks in 
Campton, New Hampshire ; by GrorcE W. Hawes. 
AMONG other results of the sperenpies studies made by 
me under the direction of the New Hampshire State Survey, 
* Geology of New Hampshire, Hitchcock; Part IV; Mineralogy and Lithology. 
