the same purpose as Ainslee's polar planimeter, and though not so handy,
is free from the defect due to the sliding of the integrating wheel on
the paper.

The rules connected with maxima and minima and points of inflexion are
illustrated by the machine, for the cart cannot be made to describe a
maximum or a minimum unless the pointer, A, _crosses_ the axis of x, or
a point of inflexion unless A passes a maximum or minimum.

* * * * *




UPON A MODIFICATION OF WHEATSTONE'S MICROPHONE AND ITS APPLICABILITY TO
RADIOPHONIC RESEARCHES.

[Footnote: A paper read before the Philosophical Society of Washington.
D. C., June 11, 1881.]

By ALEXANDER GRAHAM BELL.


In August, 1880, I directed attention to the fact that thin disks or
diaphragms of various materials become sonorous when exposed to the
action of an intermittent beam of sunlight, and I stated my belief that
the sounds were due to molecular disturbances produced in the substance
composing the diaphragm.[1] Shortly afterwards Lord Raleigh undertook
a mathematical investigation of the subject and came to the conclusion
that the audible effects were caused by the bending of the plates