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THE POPULAR SCIENCE MONTHLY. 



timber-trees from twelve to sixteen inches 

 in diameter. Dr. F. B. Hough stated that 

 a journal devoted to the interests of for- 

 estry would be begun soon, to be edited by 

 himself, and published by Robert Clarke & 

 Co., of Cincinnati. Committees were ap- 

 pointed to report on the importance of ex- 

 perimental forest stations ; to suggest a plan 

 for the prevention of the destruction of for- 

 ests by fire and cattle ; to report on the 

 importance and necessity of State commis- 

 sions of forestry ; on forest-culture, and on 

 forestry schools. The next meeting was 

 appointed to be held at Montreal, Canada, 

 August 21st and 22d of this year, two days 

 before the annual meeting of the American 

 Association for the Advancement of Science, 

 when reports from these committees are ex- 

 pected. One of the days of the convention 

 was styled " Arbor Day," and was devoted 

 to the planting of trees in the public squares 

 aud Lincoln Park. A large number of the 

 trees were named in honor of public men 

 and distinguished citizens, etc. This, though 

 not of particular practical bearing, tends to 

 awaken interest in a cause that needs such 

 awakening sadly enough. 



Sound-Shadows in Water. Professor 

 John Le Conte has described in the "Amer- 

 ican Journal of Science " a course of inves- 

 tigations, which he has made with the as- 

 sistance of his son, on sound-shadows in 

 water. It is probably within the experience 

 of all that more or less perfect sound-shad- 

 ows are thrown by hills, buildings, piers, 

 and other obstacles, to the transmission of 

 aerial vibrations. Nevertheless, the bound- 

 aries of such shadows are so imperfectly 

 denned that they can hardly be compared, 

 except in a general way, with the shadows 

 of light. Many ordinary obstacles are elas- 

 tic and give passage to a part of the sound, 

 as translucent bodies let a part of the light 

 through ; and waves are liable to a diffrac- 

 tive divergence, which proceeds from the 

 secondary waves that originate at the bound- 

 aries of the obstacle and are propagated 

 within the geometrical shadow. Lord Eay- 

 leigh refers the difference in effect in the 

 case of sound as compared with light to 

 the difference in the proportion of the 

 wave-lengths of the two phenomena to the 

 size of the obstacle. An ordinary obstacle 



bears an immense ratio to the length of a 

 wave of light, but does not bear a very 

 great ratio to the length of a sound-wave. 

 Hence it follows, from the mathematical 

 theory of undulations, that the waves of 

 sound bend around obstacles, and produce 

 more or less effect within the geometrical 

 shadow, while light-shadows have definite 

 boundaries, and are more sharply de- 

 fined. These views appear to be confirmed 

 by experiment; for the shadows cast by 

 acute sounds are more distinct than those 

 produced by grave sounds. It is also a sig- 

 nificant fact that sound-shadows seem to 

 be more perfect or more sharply defined in 

 water than in air. David Colladon found 

 this to be the case in 1826. Professor Le 

 Conte's son, under his direction, made some 

 experiments in this matter during the re- 

 moval of Rincon Rock at San Francisco, by 

 blasting with dynamite, in 1874. The shock 

 produced in the water which was felt at 

 the distance of three hundred feet as a short 

 concussion or click was followed by a sec- 

 ond shock in the air, that was heard. When 

 the observer stood upon the top of a wooden 

 pile, the concussion seemed to come up 

 from the water along the cylinder of wood. 

 A soda-water bottle was let down near a 

 pile about forty feet from the explosive car- 

 tridge. "Whenever it was left within the 

 geometrical shadow of the pile from the 

 cartridge, it was not hurt by the explosion ; 

 whenever it was placed outside of the shad- 

 ow it was shivered to atoms, and this wheth- 

 er it was filled with water or with air. In 

 other experiments stout glass tubes firmly 

 adjusted to a frame-work supporting them, 

 were let down horizontally so as to He across 

 the piles. In every case the shock of the 

 explosion shivered the parts of the tube 

 that projected on either side of the posts, 

 and left the part within the shadow unin- 

 jured. The boundaries between the broken 

 and the protected parts of the glass were 

 sharply defined. The same effects were pro- 

 duced when the tubes were put twelve feet 

 beyond the pile. Professor Le Conte ex- 

 plains these phenomena by endeavoring to 

 show that the sound-shadows are more dis- 

 tinct in water than in air because the sound- 

 waves are shorter in that medium. The 

 character of the explosion also probably has 

 much to do with the nature of the effect : the 



