The siren in its simplest form is an enlarged edition of the “Deviline” toy whistle. There is a Daboll trumpet with a small throat, in which is placed horizontally, not a reed, but a metal disc, so as to fill the whole circular space of the throat. The sheet of metal is pierced with a number of radial slits. Behind this disc is a second plate of a similar character, and likewise pierced with radial slits of the same size, shape and number; but whereas the first disc is fixed, the second is mounted on a spindle. The free disc rotates at high speed, so that the twelve jets of air which are driven through the throat are interrupted intermittently by the blanks of the revolving disc coming over the openings in the fixed disc, while when the two slits are in line the air has a free passage. If the revolving disc completes 3,000 revolutions per minute, and there are twelve slits in the discs, then a total of 36,000 vibrations per minute is produced while the instrument is in operation. The speed of the revolving disc, as well as the number and size of the openings, varies according to the size and class of the siren; but in any case an intensely powerful, dense and penetrating musical tone is emitted, which can be heard a considerable distance away. The blast of a high-powered large siren has been heard at a distance of twenty to thirty miles in clear weather, though of course in thick weather its range is reduced. While Professor Holmes was experimenting with this device, another investigator, Mr. Slight, of Trinity House, was wrestling with the same problem. Indeed, he may be described as the inventor of the modern siren. Although he effected only an apparently slight modification, it was the touch which rendered the instrument perfect, while it also removed the possibility of a breakdown at a critical moment, as he rendered the moving part freer in its working and eliminated the severe strains to which it was subjected. The improvement was appreciated by Professor Holmes, who adopted it immediately.