THE SERAI, OR CARAVANSARY, AT LANDI KOTAL, INDIA In this walled inclosure travelers and their beasts of burden find water, food, shelter, and protection from the bandits that frequent the hills of Baluchistan, Afghanistan, and the Northwest Frontier Province. "Where do we locate solar observatories, and why? On desert mountains; far separated; at high altitudes; in regions easy of access" (see text, page 113). A CARAVAN ON THE MARCH IN THE ALGERIAN SAHARA, WHERE AMERICA'S THIRD SOLAR-RADIATION OBSERVATORY MAY BE ESTAB LISHED (SEE TEXT, PAGE 113) A MOVING SAND DUNE NEAR SWARTOOG, IN SOUTHWEST AFRICA Because of the conditions required, only a few parts of the world remain available for the location of a solar-radiation observatory. One of these is THE BEACON ON THE SITE OF DIAZ PILLAR, LÜDERITZ BAY, SOUTHWEST AFRICA In 1487 Bartholomeu Diaz, the Portuguese explorer who first rounded the Cape of Good Hope, erected a pillar on this point of land in Lüderitz Bay. At present a fog-signal station is located there, and the picture shows a steam boiler being hauled up to it. It is possible that somewhere in this region of Southwest Africa the new solar-radiation observatory sponsored by the National Geographic Society will be established. sparks from stellite as it does from hard steel, but only very slowly makes any impression on it. Within the observatory, the sunbeam falls into a spectroscope, which breaks it up into the beautiful band of color which we call the spectrum-violet at one end, red at the other. The eye is not able to see that for some distance beyond the red there are still solar rays, and beyond the violet others also. X rays lie still farther beyond the violet, but with these we do not have to deal. Since neither the eye nor photography can observe the whole gamut of solar spectrum rays, we are obliged to absorb them on a lampblack surface, so that they produce their equivalent as heat. We then measure the solar-ray power in terms of the heat it produces. A very delicate electrical thermometer for this purpose was invented by Dr. Langley, a pioneer in solar-radiation measurements, as he was later in aviation. He called his instrument a bolometer, which means "a ray-measurer." It consists of two blackened, hairlike ribbons of platinum, each about one-half inch long, 1/250 inch wide, and 1/2000 inch thick. These are joined by two coils of wire to form a "Wheatstone's bridge," and connected with an electric battery and a highly sensitive galvanometer. Houses are lighted by electric lamps which use about one-fourth ampere of current. The apparatus we are describing detects current changes ten billion times smaller. It also detects changes of temperature of one of the little platinum ribbons of less than a millionth of one degree. The solar spectrum, being moved along from beyond the violet to beyond the red, passing over one of the sensitive ribbons of the bolometer, produces its tiny heating effects, and causes thereby swings of the sensitive galvanometer index. A record of these is kept on a moving photographic plate. Thus we produce automatically, in seven minutes of time, a A THIRD-CLASS RAILWAY COACH FOR WOMEN IN BRITISH BALUCHISTAN So many languages and dialects are used in India and there are so many people who cannot read any language that pictures are used in labeling the compartments of the railway coaches. measurement of the heat found in all these solar spectrum rays, from far beyond the visible violet to far beyond the visible red. HEAT RAYS ON A WORLD WITHOUT ATMOSPHERE ARE COMPUTED Why is this necessary? Because our atmosphere attenuates differently the several rays of the spectrum. We have to measure its effect upon each of them. By measuring intensities first at low sun, when the atmospheric path is oblique and very long, and again, later, at high sun, when the path is nearly direct and much shorter, we come into possession of the facts which enable us to compute what energy each solar-spectrum ray contained outside the atmosphere altogether. Summing up these computed energies for all the spectrum rays, we find at last what the total of solar heat would have been if we had taken our stand on the moon, with no troublesome atmosphere to interfere. Reducing our result to what it would be at the average solar distance, for the earth is 3,000,000 miles farther away in July than in January, we reach at last our goal. We thus find the total intensity of energy of the sun rays for the day. Comparison with similar measurements of other days reveals the variation of the sun. This is the simplest picture one can draw of the subject. There are various complexities not necessary to rehearse here. They demand still other kinds of apparatus, much of which we have invented and also constructed at the Smithsonian Institution. Altogether, the outfit for an observing station for this work. comprises about 20 principal pieces of physical apparatus, with many little auxiliaries, weighs when boxed for transportation some two tons, and costs nearly $8,000. Having given an inkling of what we are after and how it is attained, let us now turn to the actual results and their applications to weather forecasting. We express our measurements of the intensity of solar radiation in calories, that familiar word to those who study food values. There are, however, two kinds of calories. |