“Now, on February 1, the wind at places to the west of Spithead was generally light, and from the west or nearly so, though near Lyndhurst there was a fresh breeze from about W.N.W. or N.W. At Portsmouth, again, the wind is described as from the shore. On the other hand, many of my correspondents at great distances from Spithead state that the wind, when sensible, was southerly in direction. Thus the sound-rays were first of all refracted by contrary winds over the heads of observers between 10 and 45 miles, and were afterwards brought down again by favourable upper currents, so that the reports were clearly audible beyond 50 miles and up to 140 miles from Spithead, and were so loud at a distance of 84 miles that labourers in the fields put down their spades and listened.” The same explanation has been given of the extraordinary differences that are found at various times in the distance at which lighthouse fog-horns are heard by ships at sea. There is in this case, however, another possible explanation, due to what is called interference of sound waves, the explanation of which will be given presently. The late Professor Tyndall, who was an authority on this subject, was of opinion that in some states of the atmosphere there existed what he called “acoustic opacity,” the air being non-uniform in temperature and moisture; and through this very irregular medium, sound waves, when passing, lost a great deal of their intensity by internal reflection, or eclipses, just as light is stopped when passing through a non-homogeneous medium like crushed ice or glass. At each surface a little of the light is wasted by irregular reflection, and so the medium, though composed of fragments of a transparent substance, is more or less opaque in the mass. On the subject of sound-signals as coast-warnings, some exceedingly interesting information has recently been supplied by Mr. E. Price-Edwards (see _Journal of the Society of Arts_, vol. 50, p. 315, 1902). The Lighthouse Boards of different countries provide the means for making loud warning sounds at various lighthouses, as a substitute for the light when fog comes on. The distance at which these sounds can be heard, and the distance-traversing power of various kinds of sounds, have been the subject of elaborate investigations.