252 



THE POPULAR SCIENCE MONTHLY, 



scientific institutions maintained by the 

 state are under no less than seven differ- 

 ent governmental departments, all of which 

 have other matters besides science to at- 

 tend to. The writer names six observa- 

 tories, 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 con- 

 sulting the Chancellor of the Exchequer 

 about observing eclipses of the sun, the 

 prime-minister about scientific arctic ex- 

 peditions, 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 in- 

 trusted with the grandest collection of art 

 and natural history in the world." 



The Acoastsc Properties of the Atmos- 

 phere. — The coasts of the British Isles are 

 exceedingly beset with fogs, which make 

 navigation in their vicinity a very danger- 

 ous business. During a period of ten years 

 these fogs were the cause of 2*73 shipwrecks, 

 many of which were attended with serious 

 loss of life. A.S signals in thick weather, 

 lights are almost worthless ; sounds have, 

 accordingly, been substituted, and instru- 

 ments 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 Fore- 

 land Cliff, in the Straits of Dover, as the 

 site of operations, he began a series of ex- 

 periments to test the distances at which 

 various sounds could be heard. The in- 

 struments 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 wide- 

 ly-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 occa- 

 sions, during fog and driving rain, the 

 sounds could be heard at vai'ious 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 preva- 

 lence may be opaque to sound, but not on 

 that account. The movement and arrest 

 of sound in the air depend on other con- 

 ditions 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 re- 

 flection 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 



