In a paper published in the ‘American Scientific Journal,’ June 28, 1878, Mr. Edison gives some interesting details on the application of his system of a telephonic sender to measuring pressures, expansions, and other forces capable of varying the resistance of the carbon disk by means of greater or less compression. Since his experiments on this subject date from December 1877, he again claims priority in the invention of using the microphone as a thermoscope; but we must observe that according to Mr. Hughes’s arrangement of his instrument, the effect produced by heat is precisely the reverse of the effect described by Mr. Edison. In fact, in the arrangement adopted by the latter, heat acts by increasing the conductivity acquired by the carbon under the increased pressure produced by the expansion of a body sensitive to heat: in Mr. Hughes’s system, the effect produced by heat is precisely the contrary, since it then acts only on the contacts, and not by means of pressure. Therefore the resistance of the microphone-thermoscope is increased under the influence of heat, instead of being diminished. This contrary effect is due to the division of some substance which is only a moderate conductor, and I have shown that under such conditions these bodies, when only slightly heated, always diminish the intensity of the current which they transmit. I believe that Mr. Edison’s arrangement is the best for the thermoscopic instrument, and makes it possible to measure much less intense sources of heat. Indeed he asserts that by its aid the heat of the luminous rays of the stars, moon, and sun may be measured, and also the variations of moisture in the air, and barometric pressure. This instrument, which we give fig. 50, with its several details, and with the rheostatic arrangement employed for measuring, consists of a metallic piece A fixed on a small board C, and on one of its sides there is the system of platinum disks and carbons shown in fig. 28. A rigid piece G, furnished with a socket, serves as the external support of the system, and into this socket is introduced the tapering end of some substance which is readily affected by heat, moisture, or barometric pressure. The other extremity is supported by another socket I, fitted to a screw-nut H, which may be more or less tightened by a regulating screw. If this system is introduced into a galvanometric circuit _a, b, c, i, g_, provided with all the instruments of the electric scale of measure, the variations in length of the substance inserted are translated by greater or less deviations of the galvanometric needle, which follow from the differences of pressure resulting from the lengthening or shortening of the surface capable of expansion which is inserted in the circuit. [Illustration: FIG. 50.]