Reflection being thus considered to take place at the surfaces of the suspended particles, it followed that the greater the number of particles, or, in other words, the denser the fog, the more injurious would be its action upon sound. Hence optic transparency came to be considered a measure of acoustic transparency. On this point Dr. Robinson, in the letter referred to, expresses himself thus: “At the outset, it is obvious that, to make experiments _comparable_, we must have some measure of the fog’s power of stopping sound, without attending to which the most anomalous results may be expected. It seems probable that this will bear some simple relation to its opacity to light, and that the distance at which a given object, as a flag or pole, disappears may be taken as the measure.” “Still, clear air” was regarded in this letter as the best vehicle of sound, the alleged action of fogs, rain, and snow being ascribed to their rendering the atmosphere “a discontinuous medium.” Prior to the investigation now to be described, the views here enunciated were those universally entertained. That sound is unable to penetrate fogs was taken to be “a matter of common observation.” The bells and horns of ships were affirmed “not to be heard so far in fogs as in clear weather.” In the fogs of London the noise of the carriage-wheels was reported to be so much diminished that “they seem to be at a distance where really close by.” My knowledge does not inform me of the existence of any other source for these opinions regarding the deadening power of fog than the paper of Derham, published one hundred and sixty-seven years ago. In consequence of their _à priori_ probability, his conclusions seem to have been transmitted unquestioned from generation to generation of scientific men. § 2. _Instruments and Observations_ On the 19th of May, 1873, this inquiry began. The South Foreland, near Dover, was chosen as the signal-station, steam-power having been already established there to work two powerful magneto-electro lights. The observations for the most part were made afloat, one of the yachts of the Trinity Corporation being usually employed for this purpose. Two stations had been established, the one at the top, the other at the bottom, of the South Foreland Cliff; and at each of them trumpets, air-whistles, and steam-whistles of great size were mounted. The whistles first employed were of English manufacture. To these was afterward added a large United States whistle, and also a Canadian whistle, of great reputed power.