Page:Popular Science Monthly Volume 39.djvu/735

Rh of carbon and baked under great heat. Uniformity in resistance, securing equal consumption of current, of surface, and of incandescence, is also indispensable. To "flash" the filament for resistance, it is lowered into a glass chamber full of a hydrocarbon gas; it is heated by an electric current, and the carbon in the gas is deposited on the heated surface. In mounting the filaments, the important condition is to obtain a perfect electrical contact or joint between the metal and the carbon, and this is a very delicate point. The soldering is effected by electrically heating the joint in a vessel containing a liquid hydrocarbon surrounding, under such condition that the current shall pass through no part of the filament but the joint. Conditions opposite to those of an ordinary light are sought in inserting the filament in the bulb. Not rapid combustion but constant endurance of heating is wanted, and air is carefully removed by exhausting the bulb to one millionth of an atmosphere. In work, during the first two hundred hours of the life of the filament, the electrical resistance decreases slightly, and the brilliancy increases; for the next five hundred hours they are nearly stationary; after that, resistance increases and brilliancy decreases in a progressive ratio. The light is dimmed also by the gradual roughening of the surface of the filament and by the blackening of the glass from the deposition of carbon upon it.

Economical Plants of Australia.—With the exception of timbers, the economic vegetable products of Australia, as presented in Mr. Maiden's book on the Useful Plants of that country, are not of extraordinary importance. The northern regions, where the flora is re-enforced by representatives from the Malayan Archipelago and southern Asia, yield most of the plants possessing medicinal properties. The genus Eucalyptus, comprising more than one hundred and thirty species, yields excellent timber, kinos, and essential oils. Eucalyptus gunnii yields a sweetish sap which is converted by the settlers into excellent cider. This and manna, from two other species, are probably the only food products derived from eucalyptus trees Oil from Eucalyptus amygdalin and Eucalyptus globulus is prepared in Australia and also in Algeria and California. In California it is available as a by-product in the manufacture of anticalcaire preparations for boilers. The acacias of Australia, locally known as wattles, are hardly less useful than the gum trees. Immense numbers of them are destroyed for the sake of the bark used in tanning, and the leaves are greedily eaten by stock. By the operation of these two causes they are becoming scarce in some districts, and systematic attempts are now made to plaut them on a large scale. Gum arabic of good quality is yielded by various species of acacia, but can not be profitably collected in the present condition of the labor market. Water is obtained by the natives for drinking, when springs fail, from the fleshy roots of a tree known botanically as Hakea leucoptera and from the stem of Vitis hypoglauca. Very few native Australian trees yield valuable fibers. The native mode of extracting fibers for their fishing-nets is by chewing with their teeth, and by this the teeth are "worn down to a dead level." The best fodder grass of Australia is the plant commonly known as "kangaroo grass."

Mistakes about Bearings.—Mistakes in orientation—sometimes of the most puzzling character—are usually the result of some incidental and temporary bewilderment, and may under peculiar circumstances overtake any one. Some instances have been cited by Sir Charles Warren in which they are chronic and may afflict even the best informed persons. Erroneous conceptions formed by children as to distances and positions may grow up with them undetected till near maturity. Then, when the discovery is made, it is too late to apply any better remedy than to recognize the error and make allowance for it when possible. Cases are cited of a person whose ideas of certain parts of London were all inverted; of another, who placed Paris north of London; of thirty well-informed young men, "about eighteen were under the impression that, while the sun rises in the east, the stars rise in the west, from having learned that the sun has a proper motion among the stars; and the author believes that there are few educated men who have not grown up with some curious errors with reference to geographical facts, which have bothered them