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Rh due to a non-homogeneous atmosphere produced by inequalities of temperature, and the unequal distribution of vapor. "As I stood upon the deck of the Irene," says Prof. Tyndall, "pondering the question, I became conscious of the exceeding power of the sun beating against my back and heating the objects near me. Beams of equal power were falling on the sea, and must have produced copious evaporation. That the vapor generated should so rise and mingle with the air as to form an absolutely homogeneous mixture, I considered in the highest degree improbable. It would be sure, I thought, to streak and mottle the atmosphere with spaces in which the air would be in different degrees saturated.... At the limiting surfaces of these spaces, though invisible, we should have the conditions necessary to the production of partial echoes and consequent waste of sound." This philosophical explanation Prof. Tyndall was able to verify. On one occasion, when the air was opaque to sound, a cloud arose and threw its shadow over the sea. Some increase in the intensity of the sounds was noticed; but, with decline of the sun, it was more obvious, until at length the signal-sounds were heard at a distance of twelve and three-quarter miles, when at first they were inaudible at two miles. The increase of distance at which the sounds were distinctly heard was gradual with decline of the sun, or, what is quite obvious, with increase of homogeneity of the atmosphere. This was fully shown on another occasion, when, during a violent rain, the transmission of sounds was greatly increased, so that they could be heard more distinctly at seven and one-half miles than at five miles previous to the storm.

The Anderson School at Penlkese.—The Anderson School of Natural History will open this year on Tuesday, July 7th, and close on Saturday, August 29th. During the session. Prof. Mayer, of the Stevens Institute, Hoboken, will deliver a course of lectures on Physiological Physics. Mr. Theodore Lyman, of the Cambridge Museum of Comparative Zoology, will give a few lectures on Pisciculture. Lectures will also be delivered by Dr. W. S. Barnard, of Ithaca, N. Y., on Protozoa; Prof. Jordan, of Appleton, Wis., will take charge of the instruction in Marine Botany. Mr. Alexander Agassiz will have charge of the instruction on Radiates and Embryology; Dr. A. S. Packard, Jr., of Salem, Mass., on Articulates; Prof. B. G. Wilder, of Cornell, on Vertebrates; Prof. E. S. Morse, of Salem, and Prof. C. E. Hamlin, of the Museum of Comparative Zoology, on Mollusca; Mr. T. W. Putnam, Director of the Peabody Academy of Science, on Fishes; Mr. Edwin Bicknell, of the Museum of Comparative Zoology, on Microscopy. Instruction in Drawing will be given by Mr. P. Roetter, of the Museum of Comparative Zoology. Dr. Packard and Mr. S. W. Garmon will take charge of the dredging expedition, and the laboratories will also be under the supervision of Mr. Garmon.

Scientific Apparatus.—While it cannot be questioned that the popular demand in this country for appliances with which to illustrate the first principles of physical science is well met by several makers of philosophical instruments in this and other cities, it is still true that, in the higher branches and grades of scientific illustration, our chief dependence for efficient apparatus is upon English, French, and German makers. In the two great departments of Electrics and Optics this is especially observed, and we take pleasure in commending to the notice of professors and teachers of science the card of Mr. Browning, of London, in our advertising page this month. Mr. Browning is an honored Fellow of the Royal Astronomical Society of England, and, what is more to the present purpose, he is the successful maker to the Royal Society, and to the leading English observatories, of the instruments they employ in their great and varied work. In the construction of spectroscopes of every sort he has, perhaps, no equal, certainly no superior.

Soaring and Sailing of Birds.—Mr. Belt, describing the movement of a pair of black vultures sailing on the wind, says: "Like all birds that soar, both over sea and land, when it is calm the vultures are obliged to flap their wings when they fly; but when a breeze is blowing they are