145 
revolving mirror to minute measurements. In applying this prin- 
ciple to the adjustment of the measuring rods of the Coast Survey, 
as well as for certain other minute measurements, he had made use 
of a graduated scale instead of a beam of light, the reflected image 
of which, considerably magnified, was observed by means of a 
telescope. With this improvement, an elongation which does not 
exceed the one hundred thousandth part of an inch becomes a very 
distinct and measurable magnitude. The same apparatus was ap- 
plied, at the request of General Meigs, to determine the expansion 
of different specimens of marble cut into prisms of the same length 
and cross section. The principle involved is evidently applicable 
in all cases where changes in length, in angle or in position are to 
be determined. Saxton himself applied it in the Magnetic Observ- 
atory of Girard College to indicate changes in the magnetic dip 
and also to magnify the motion of the axis of an aneroid barome- 
ter. For this latter purpose, the case of the instrument is removed 
and a mirror about one-half an inch square is attached to the first 
axis of motion. The aneroid is then fastened to a bracket on the 
wall, the axis carrying the mirror being placed horizontally. At a 
distance of about fifteen feet from the mirror a telescope is perma- 
nently adjusted, so that the image of a divided scale placed imme- 
diately below the object glass can be seen in the mirror. With this 
arrangement, the slightest change in the pressure of the air becomes 
apparent. The opening or closing of a door, or a gust of wind over 
the house, produces marked disturbances in the pressure of the at- 
mospheric column, the extent of which can be readily measured.* 
Besides the representative investigations of Franklin in Electro- 
statics, of Hare in Electrokinetics, of Henry in Electromagnetics, 
and of Saxton in Magneto-electrics, which have now been con- 
sidered, members of the American Philosophical Society have 
made important researches in Magnetism, and especially in the 
magnetism of the earth. 
David Rittenhouse became a member of the Philosophical 
Society on the roth of January, 1768. On the 6th of February, 
1781, he read a paper before the Society, entitled ‘‘ An Account of 
Some Experiments on Magnetism,’’f in which he set forth his 
theory of the magnetism of iron. In this paper he says: ‘‘Isup- 
* Henry’s Biographical Memoir of Saxton, loc. cit., p. 312. 
+ Trans. Amer. Philos. Soc., 1, ii, 178, 1781. 
PROC. AMER. PHILOS. SOC, XxxtI. 148. 8. PRINTED DEC. 28, 1893. 
