This estimate does not appear to us to unite so many conditions of accuracy as to enable us to deduce from it the degree of sensitiveness possessed by a telephone, a sensitiveness which the experiments of Messrs. Warren de la Rue, Brough, and Peirce show to be much greater. Mr. Warren de la Rue, as we have seen, used Thomson’s galvanometer, and compared the deviation produced on the scale of this galvanometer with that caused by a Daniell cell traversing a circle completed by a rheostat: he ascertained that the currents discharged by an ordinary Bell telephone are equivalent to those of a Daniell cell traversing 100 megohms of resistance, that is, 6,200,000 miles of telegraphic wire. Mr. Brough, the Director of Indian Telegraphs, considers that the strongest current which at any given moment causes a Bell telephone to work does not exceed 1/1000000 of the unit of current, that is, one Weber, and the current transmitted to the stations on the Indian telegraphic line is 400,000 times as strong. Finally, Professor Peirce, of Boston, compares the effects of the telephonic current with those which would be produced by an electric source of which the electro-motive force should be 1/200000 part of a volt, or one Daniell cell. Mr. Peirce justly remarks that it is difficult to estimate the real value of these kinds of currents at any precise sum, since it essentially varies according to the intensity of the sounds produced on the transmitting telephone; but it may be affirmed that it is less than the 1/1000000 part of the current usually employed to work the instruments on telegraphic lines. Signor Galileo Ferraris, who has recently published an interesting treatise on this question in the ‘Atti della Reale Accademia delle Scienze di Torino’ (June 13, 1878), states that the intensity of the currents produced by the ordinary Bell telephone varies with the pitch of the sound emitted.