sensitive to yellow, but on the other hand the atmosphere favors the passage of red more than yellow. Hence it is that finally the yellow, too, is lost when the sun has gone down so far beyond the horizon that the path at last becomes so long that red only can compass it.

Meanwhile in the east a cold rounded shadow cast by the earth itself has crept up and up. At first the outline of the mountain on which we stand can be distinguished, projecting from the edge of the earth's shadow, but soon it is lost as the sun goes lower beyond the horizon, and at last all is twilight. But now a second sunset glow, of colors much more subdued, yet richer, is to be seen in the west, as all the sky grows dark and stars appear. But this, too, fades, and all the heavenly host glitters above us as diamonds from a velvet field.

But the daylight sky-why is it blue? We have said that the air does not readily transmit the blue and violet sun rays, but we have not explained why. It is because the molecules of oxygen and nitrogen which compose the air scatter these rays profusely in all directions. So then the sunbeams which strike all parts of the sky that we can see are scattered, and the rays which they lose are mainly of blue and velvet. These fly off in all directions and reach our eyes, whichever way we look, unless clouds cut them off. This is why the sky is blue. Its rays were sun rays once, but were lost and strayed away in the atmosphere.

Sometimes one sees some little old, old, window

panes in an ancient house that have a beautiful tinge of violet. This, too, is caused by the sun rays, but in quite a different way. The glass contains some chemical, which, like the photographic plate, is acted upon by sun rays, and in such a way as to turn violetcolored instead of gray or black like a photoplate.

CHAPTER XVIII

COOKING BY SUN RAYS

Polly put the kettle on

And we'll all take tea.

OLD SONG.

A HOT Oven without a fire, a kitchen as cool as the parlor, these are the luxuries that come with a solar cooker. There are two ways of making one. The simpler kind is like that which Mr. W. Adams of Bombay made nearly a half century ago and described in the Scientific American of June 5, 1878. He built of wood an eight-sided cone which he lined inside with mirror glass and hinged upon a board so it could face the sun. The dish to be cooked was enclosed in a blackened, tightly covered pail hung at the center. A glass cover enclosed the pail to keep away wind. If, now, the cone was kept pointed towards the sun, a lot of its rays would be roughly focused upon the blackened pail, and its contents to be cooked could quickly be brought to boiling, or, if no water was present, even hotter. Mr. Adams states that the rations of seven soldiers, consisting of meat and vegetables, were thoroughly cooked by it in a couple of hours, in January, the coldest month of the year in

Bombay; and that the men declared the food to be cooked much better than in the ordinary manner. The dish is stewed or baked according as the steam is retained or allowed to escape.

But suppose the sun goes behind a cloud just as one wants to get dinner, and the cloud hangs provokingly in front of the sun for an hour though the rest of the sky remains clear. Such a catastrophe a few times repeated would be apt to make the men of the house kick the sun cooker out into the brush.

To avoid this unreliability in solar cooking, a new scheme designed to keep the food cooking during nights and cloudy weather has been tried on Mount Wilson, California, at the solar observatory of the Smithsonian Institution. Instead of hanging the dish to be cooked directly in focus of the sun rays, they provided a hot reservoir of oil above the mirror in which were ovens for the cooking. In order to carry heat from the focus of the mirror to the ovens, there was a circuit of oil pipe running up from the mirror focus to the top of the reservoir and back from the bottom of the reservoir underneath the mirror to complete the circuit. This plan is exactly like the gravity water circulation used with cookstoves and furnaces to heat a reservoir of water for the house, only that gas engine oil instead of water had to be used for the solar cooker so that baking temperatures much hotter than boiling water would be possible.

In the contrivance as shown in the illustration, the sun rays are focused by a great hollow cylindrical

sheet of shiny aluminum metal onto a blackened brass tube containing the oil. The tube must lie so as to point at the pole star, and the metal mirror must rotate around it to follow the sun from east towards west every day.

This motion is arranged in a very cheap but effective way. A large grooved wheel attached to the mirror has hung from its outer edge a wire and heavy weight which tends to turn the mirror westward. But on a groove in a second smaller part of the wheel is another wire wound around the wheel in the opposite way which restrains the turning of the mirror. This second wire ends in a clockwork, so that as the mirror turns westward the clockwork hums along pretty fast. But soon a hand like a clock hand, which it carries, hits a pin, and the motion is stopped. Then the mirror must wait still until a little common alarm clock with a 12-pin wheel on its back has ticked off about two minutes and released the catch which held back the mirror. This happens once each five minutes. So the mirror is always within a very small angle of exactly right in its position to focus the sun rays on the hot tube of oil. A single setting of the mirror to face the east in the morning is all the attention it needs. The clockwork does the rest.

In order that the solar heat may not be wasted, or the mirror covered by dust, the whole top of it, about 10 feet by 7 feet in size, is covered by 10 sheets of flat window glass, laid on a steel framework. This glass may be easily cleaned occasionally, so that