Page:Popular Science Monthly Volume 5.djvu/264

252 scientific institutions maintained by the state are under no less than seven different governmental departments, all of which have other matters besides science to attend to. The writer names six observatories, of which one, Greenwich, is under the Admiralty; another, Edinburgh, under the office of Works; a third, at the Cape of Good Hope, under the Colonial office; and the rest under the India office. The other departments of state which assume to direct scientific work are the Privy Council and Board of Works, and Board of Trade. Colonel Strange favors the creation of a Science Minister, under whose control all these scientific institutions shall be placed. "Let this be done," says he, "and we should cease to witness the farce of consulting the Chancellor of the Exchequer about observing eclipses of the sun, the prime-minister about scientific arctic expeditions, and the Treasury about tidal reductions. We should perhaps, too, then perceive that overworked law-officers are not the best managers of a great, or what should be a great, technical museum, and that fifty irresponsible gentlemen, however eminent individually, ought not to be intrusted with the grandest collection of art and natural history in the world."

The Acoustic Properties of the Atmosphere.—The coasts of the British Isles are exceedingly beset with fogs, which make navigation in their vicinity a very dangerous business. During a period of ten years these fogs were the cause of 273 shipwrecks, many of which were attended with serious loss of life. As signals in thick weather, lights are almost worthless; sounds have, accordingly, been substituted, and instruments for producing them, such as bells, fog-horns, steam-whistles, etc., have been set up and employed at numerous stations. But it has been observed that the distances at which these sounds could be heard were extremely variable; that while at one time they would give warning seven or eight miles away, at another they were inaudible at half the distance, and perhaps totally useless for the purpose intended.

The authorities last spring requested Prof Tyndall to look into the matter, which he did, and, as is usual with him, when he undertakes an inquiry, with interesting and valuable results. Selecting the South Foreland Cliff, in the Straits of Dover, as the site of operations, he began a series of experiments to test the distances at which various sounds could be heard. The instruments used for producing the sounds were, trumpets sounded by air, whistles sounded by steam, the steam-siren, and cannon. The observations were continued at intervals from the 20th of May last to the 25th of November. At different times sounds of like intensity were heard at widely-varying distances. For example, a sound that at one time could be heard only two miles, could at another time be heard twelve miles. On one occasion the sound of the steam-siren was heard fifteen miles. On the morning of June 3d, the sky being of a stainless blue, and the sea calm, the sound of a cannon could not be heard beyond two miles, and a "mortar fired with a three-pound charge yielded only a faint thud; it was mere dumb-show on the Foreland." The air was optically clear, but opaque and impenetrable to sounds. On other occasions, during fog, and driving rain, the sounds could be heard at various distances, as four, five, seven, and nine miles; and once, when the air was hazy, they were heard twelve and three-quarter miles. The inference is, that the transmission of sound through the atmosphere is not affected by fogs and rain; the air during their prevalence may be opaque to sound, but not on that account. The movement and arrest of sound in the air depend on other conditions than the mere presence of fog or rain, and these conditions may exist when the atmosphere is wonderfully clear to the eye. What, then, becomes of the enormous volumes of sound produced by cannon and the steam-siren, seeing that they are neither transmitted nor annihilated? In order to determine this question, Prof. Tyndall and his companions took a position on the shore overlooking the sea, and there for the first time demonstrated by experiment "the reflection of sound from aerial surfaces. From a perfectly clear air the sounds came back in echoes. They reached us as if by magic from absolutely invisible walls." Now, what are the conditions which thus intercept the sound-waves? The phenomenon is clearly