Through the industry and ability of Dr. Forbes of Chichester, who translated the work of Laennec in 1821, and who has also lately published an original volume of great merit on the subject; and through the medium of our medical journals, more especially the excellent quarterly works published at Edinburgh by Dr. Duncan, jun., and in London by Dr. James Johnson, the professional public has been made acquainted with the nature and uses of the stethoscope; but up to the present time, it is so much a novelty in medical practice, as well as with the public at large, that I trust no apology is necessary for presenting an abridgment of the opinions of M. Laennec, with a few reflections on the subject derived from my own observation and practice. As with percussion, the use of the stethoscope is founded upon a simple philosophical principle; that of ascertaining whether the cells of the lungs are healthy, and fitted for the free reception of air in the function of respiration; or whether the membranes of the chest, or the vessels, valves, and cavities of the heart, are obstructed by any disordered action, or by any permanent disease. The instrument which has received the name of _stethoscope_ (derived from στῆθος, pectus, and σκοπέω, speculor or exploro), was invented by M. Laennec; and the idea of it occurred to him from reflecting on the well-known fact in acoustics, that the impression of sound is augmented, when conveyed through certain solid bodies, as, when we hear the scratch of a pin at one end of a piece of wood, on applying our ear to the other. In examining a disease of the heart, he first made use of a quire of paper rolled into a cylindrical form, and, satisfied with the result, he soon proceeded to investigate the various phenomena afforded by the vibrations of air within the chest, through the medium of the stethoscope; which he constructed from the suggestion already mentioned. After making various experiments as to the form of the instrument, and the kind of material, he found most success in using a wood of medium density, as cedar. This proves very favourable for conveying the delicate vibrations caused by respiration, from the walls of the chest to the ear. For the information of those who have never seen the instrument, I may observe, that it is of cylindrical form, shorter in length, but larger in diameter, than the common flute. It has a cylindrical perforation throughout its whole length, and is divided into two parts for the convenience of using the whole or half length, according to the situation of the patient in bed. The end of each part terminates in a funnel-shaped cavity, the one to receive the separate half of the instrument, and the other to receive the part acting as a stopper, which latter has a short pipe, made of brass or silver to enter the bore of the cylinder.