Page:Popular Science Monthly Volume 19.djvu/198

186 has been by the Bussey Institute, in connection with the Arnold Arboretum, at Brookline, Massachusetts. Under the able and judicious management of Professor Sargent, it has already fine plantations of forest-trees, has diffused much valuable information in regard to the growth and importance of trees, and has secured the planting of a large number in various parts of the country. On the foundation of such institutions will naturally be built up in due time schools of instruction in forestry like those of Europe, which will have a recognized and permanent place among us. The European schools of forestry will form the subject of another article.

 

N a paper read before the American Association for the Advancement of Science, last August, I described certain experiments made by Mr. Sumner Tainter and myself which had resulted in the construction of a "Photophone," or apparatus for the production of sound by light; and it will be my object to-day to describe the progress we have made in the investigation of photophonic phenomena since the date of this communication.

In my Boston paper the discovery was announced that thin disks of very many different substances emitted sounds when exposed to the action of a rapidly-interrupted beam of sunlight. The great variety of material used in these experiments led me to believe that sonorousness under such circumstances would be found to be a general property of all matter.

At that time we had failed to obtain audible effects from masses of the various substances which became sonorous in the condition of thin diaphragms, but this failure was explained upon the supposition that the molecular disturbance produced by the light was chiefly a surface action, and that under the circumstances of the experiments the vibration had to be transmitted through the mass of the substance in order to affect the ear. It was therefore supposed that, if we could lead to the ear air that was directly in contact with the illuminated surface, louder sounds might be obtained, and solid masses be found to be as sonorous as thin diaphragms. The first experiments made to verify 