Page:Popular Science Monthly Volume 17.djvu/734

716 that the winds and condition of the northern hemisphere have any perceptible influence at the equator, and ascribes the mildness and uniformity of the equatorial temperature to the prevalence of the rainy season in our winter, the southern summer months. Water acts during this season to reduce the temperature through its great capacity for heat, through the screen of clouds which it interposes between the sun and the surface of the earth, and through evaporation, and this upon the higher as well as upon the lower strata of air. It is admitted that winds have some effect as aids to the cooling, but the insignificance of their influence, as against any active heat-producing force, and in the absence of moisture, is shown by the fact that the Sahara is the hottest region in the world, notwithstanding the winds that blow over it from the cool Mediterranean.

Messages by Heliograph.—The usefulness of the heliograph was recently satisfactorily tested in the transmission of a dispatch from General Stewart, in Afghanistan, announcing the result of an attack on the British troops, which was sent from Camp Ghuzni, April 22d, and was received at the India Office, London, on the following day. The news could hardly have been brought more speedily by electric telegraph. The heliograph, signaling right over the heads of the enemy, if necessary, to stations which may be few and far between, does not require any route to be kept open, and can not be interrupted. A ten-inch mirror, that being the size of the ordinary field-heliograph, is capable of reflecting the sun's rays in the form of a bright spot to a distance of fifty miles, where the signal can be seen without the aid of a glass. The adjustment of the instrument is very simple. If an army corps, having left its base where a heliograph station is established, desires to communicate with the other division from a distance of several miles, a hill is chosen and a sapper goes upon it with his heliograph-stand containing a mirror swung so as to move horizontally and vertically. A little of the quicksilver having been removed from behind the center of the mirror, a clear spot is made through which the sapper can look from behind his instrument toward the station he desires to signal. Having sighted the station by adjusting the mirror, he next proceeds to set up in front of the heliograph a rod on which is a movable stud, manipulated like the foresight of a rifle. The sapper, standing behind his instrument, directs the adjustment of this stud until the clear spot in the mirror, the stud, and the distant station are in a line. The heliograph is then ready to work, and the sapper has only to take care that his mirror reflects the sunshine on the stud just in front of him to be able to flash signals so that they may be seen at a distance.

Ocean Temperatures in the Pacific and Atlantic.—Herr von Boguslawski has been led, from a comparison of the results of recent deep-sea investigations, to the following conclusions respecting the temperatures of the Atlantic and Pacific Oceans: 1. The water of the North Pacific is, in its whole mass, colder than that of the North Atlantic. 2. The water of the South Pacific is, down to 1,300 metres (4,225 feet), somewhat warmer than that of the South Atlantic, but below this depth colder. 3. The bottom temperatures are generally lower in the Pacific than in the Atlantic at the same depths and in the same degree of latitude; but nowhere in the Pacific are found such low bottom temperatures as in the Antarctic portion of the South Atlantic, between 36° and 38° south and 48° and 33° west longitude, in which bottom temperatures of –0·3° C. to –6° C. have been measured. 4. In the western parts of the Pacific, and the adjoining parts of the East Indian Archipelago, the temperature of the water reaches its minimum at depths beweenbetween [sic] 550 and 2,750 metres (1,787 and 8,937 feet), remaining the same from this depth to the bottom. In the whole of the Atlantic the temperature from 2,750 metres (8,937 feet) to the bottom gradually though very slowly decreases.

The Harvard Medical Course.—Professor James C. White, of the Medical School of Harvard University, states that the enlargement of the course of instruction which was adopted by the department ten years ago has been followed by a general elevation of the standard of medical education