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Page:Popular Science Monthly Volume 21.djvu/432

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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 forestry would be begun soon, to be edited by himself, and published by Robert Clarke & Co., of Cincinnati. Committees were appointed to report on the importance of experimental forest stations; to suggest a plan for the prevention of the destruction of forests by fire and cattle; to report on the importance and necessity of State commissions 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 expected. One of the days of the convention was styled "Arbor Day," and was devoted to the planting of trees in the public squares and 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 "American Journal of Science" a course of investigations, which he has made with the assistance of his son, on sound-shadows in water. It is probably within the experience of all that more or less perfect sound-shadows are thrown by hills, buildings, piers, and other obstacles, to the transmission of aerial vibrations. Nevertheless, the boundaries of such shadows are so imperfectly defined that they can hardly be compared, except in a general way, with the shadows of light. Many ordinary obstacles are elastic 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 diffractive divergence, which proceeds from the secondary waves that originate at the boundaries of the obstacle and are propagated within the geometrical shadow. Lord Rayleigh 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 defined. 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 significant 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 removal 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 second 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 cartridge. 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 shadow it was shivered to atoms, and this whether 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 lie 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 uninjured. The boundaries between the broken and the protected parts of the glass were sharply defined. The same effects were produced when the tubes were put twelve feet beyond the pile. Professor Le Conte explains these phenomena by endeavoring to show that the sound-shadows are more distinct 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