The most important application of the instrument to surgical purposes has lately been made by Sir Henry Thompson, aided by Mr. Hughes, for the examination of the bladder in cases of stone. It enables him to ascertain the presence and precise position of calculi, however small they may be. For the purpose of research, he uses a sound, made of a Maillechort rod, a little bent at the end, and placed in communication with a sensitive carbon microphone. When the sound is moved about in the bladder, the rod comes in contact with stony particles, even if they are no larger than a pin’s head, and friction ensues, producing in the telephone vibrations which can be easily distinguished from those caused by the simple friction of the rod on the soft tissues of the sides of the bladder. The arrangement of the instrument is shown in fig. 52. The microphone is placed in the handle which contains the sound, and is the same as that given in fig. 42, but of smaller size, and the two conducting wires _e_ which lead to the telephone, issue from the handle by the end _a_ opposite to that _bb_ to which the sound _dd_ is screwed. As this instrument is not intended to reproduce speech, retort carbons instead of wood carbons may be used. [Illustration: FIG. 52.] Some deaf people, whose sense of hearing is not completely destroyed, have been able to hear by an expedient based upon the principle of the microphone. For this purpose two telephones, connected by a metallic crown, which is placed on the temples, are applied to the ears of the deaf person, and the telephones are placed in communication with a battery microphone, which hangs to the end of a double conducting wire. The deaf man keeps the microphone in his pocket, and presents it as an acoustic tube to the person who wishes to converse with him. Mr. Hughes’s speaker, represented fig. 42, is the one used. _Various Applications._--The microphone may be used in many other ways, some of which are suggested in the ‘English Mechanic’ of June 21, 1878. The article states that by means of this instrument, engineers will be able to estimate the effects of the vibrations caused on old and new buildings by the passage of heavy loads; a soldier will be able to discover the enemy’s approach when he is several miles off, and may even ascertain whether he has to do with artillery or cavalry; the approach of ships to the neighbourhood of torpedoes may be automatically heralded on the coast by this means, so that an explosion may be produced at the right moment.