Page:Popular Science Monthly Volume 39.djvu/151

Rh search for this peculiar alluvial deposit, which is generally free from any admixture of clayey earth, the digger has often to penetrate into the jungle that grows thickly around, and combines the work of clearing with the occupation of gem-digger. No sapphire has yet been extracted of higher value than about fourteen hundred dollars, or ruby of higher price than forty-eight hundred dollars. No artificial or mechanical processes for washing the soil have so far been introduced. Rubies and sapphires are found at all the diggings, often deposited side by side, in the same layer or stratum of sand. The rubies are usually of a dull, light-red hue. The sapphire is of a dark, dull blue, without any of the silken gloss distinctive of the Burmah and Ceylon stones. Stones resembling garnets rather than rubies are found in the dried beds of water-courses at Raheng, two hundred miles north of Bangkok, and there is every reason to believe that rubies, also, at least equal to those discovered in the southeast, exist throughout the Raheng district.

What may be learned from a Spinning Top. —The earth as a spinning top was substantially the subject of an "Operatives' Lecture," by Prof. John Perry, at the British Association. The lecturer's purpose was to exhibit the analogies between the motions of tops and the rotation of the earth, and show how many phenomena can be explained by them. He said that, if more attention was paid to the spinning of tops, much greater advances would be made in mechanical engineering and industrial invention; geologists would not make so many mistakes of millions of years in their calculations; and we should all have a much better knowledge of astronomy, of light, of magnetism, and of electro-magnetic subjects. First the lecturer illustrated the quasi-rigidity that rapid rotation gives to a flexible or fluid body. A thin sheet of paper assumed the stiffness of a board. A chain released from a hub rolled like a solid wheel. A fly-wheel being made to revolve rapidly when inclosed in a brass box, the box did not tumble down, but maintained a vertical position and offered resistance to any attempt to turn it round. If it was tilted, it turned with a precessional motion. Every spinning body, the lecturer said, resists a change of direction of its spinning axis. Rotating machines on board ship offer greater resistance to pitching and rolling. A top thrown up would fall down anyhow; but if it was thrown up spinning, there would be no doubt as to the position it would come down in, because the spinning axis always keeps parallel with itself. The fall of a biscuit or a hat is equally controlled by throwing either with a spinning motion. For this reason the barrels of guns are rifled; and the same principle explains the feats of jugglers with hats, hoops, plates, umbrellas, and knives. All that appears incomprehensible in the curious motions of the gyrostat under various conditions loses its mystery when the motions are regarded as rotation about various axes; and herein is found the key to the phenomenon of precession. The application of these principles to the motion of the earth was illustrated by diagrams and by the wabbling motion communicated by the hand to objects with three axes, which, spun first on the shortest axis, would of themselves rise to spin on the longest axis. A circular chain hanging vertically from a cord, when made to revolve rapidly, first wabbled and at last became a horizontal ring. In dealing with the question whether the earth is a shell filled with fluid, the professor gave a vibratory motion to vessels containing respectively sand, treacle, oil, and water. A boiled egg had a slower oscillation than an unboiled one. When the two eggs were rolled, the unboiled one stopped sooner than the boiled one. The liquid inside went on moving and renewed the motion of the shell after it had been stopped by the finger. It is easy to spin a boiled egg, but not an unboiled one.

Sheep-raising in Algeria. —}Sheep-raising is the principal business of the Algerian Arabs of the high plateaus. Neither the sheep nor the wool are of the first quality, but the sheep can resist a great variety of hardships to which they are subjected in the not very pleasant region they live in. As they require to be watered every two or three days in summer, they can not be taken for pasturage to regions far removed from a supply. They are not at all particular about the quality of what they drink, but water of some kind they must have; and no use can