Page:Popular Science Monthly Volume 15.djvu/877

 protected by the branches and foliage of the plant, the more solid earth beneath is also protected from the wash of rain by its massive roots (the author refers to the roots of Simmondsia Californica), while all around erosion goes slowly on, facilitated by the peculiar susceptibility of the soil to wash. In the course of time the plant dies—is smothered by the drift which nearly covers it, or is destroyed by the annual fires. Thus deprived of its protection, the winds in turn and the rains which fall upon it wear down the top of the loose deposit, and to some extent widen its base. While this is going on the surrounding earth is being continually lowered by the action of water. The wash being greater at the base than at the summit, its effect is to perpetually maintain or increase the prominences. Such is the explanation of these hummocks offered by Dr. Barnes.

Steadiness of the Electric Light.—In employing the electric light for projection on a screen, two chief points are to be considered, viz., brilliancy of illumination and steadiness of the light. When the source of electricity is sufficient, the first of these ends is easily obtained. The second is not so easy of accomplishment. The difficulty here met with is pointed out in the "American Journal of Science," by Mr. H. W. Wiley, who also proposes a method of obviating it. The carbons burn away so rapidly that when no mechanism is present to produce alternating currents the electric arc is constantly passing out of the focus. Often, too, Mr. Wiley finds that when the current is quite strong the arc will extend itself momentarily between points as far as a centimetre from the end of the carbons. To prevent this too rapid combustion of the carbons, he coats them with a thin film of copper—a plan well known in France, though seldom tried here. With this coating of copper the carbons work satisfactorily for a short time; but soon the film is oxidized, and the combustion is as rapid as before. Mr. Wiley therefore protects the copper from oxidation by covering the carbons (after copper plating) with a film of plaster of Paris. After the plaster has set, the edge of the carbon which is to be turned toward the condenser is carefully denuded of its plaster covering, which is also cut away till quite thin on the adjacent surfaces. These precautions are taken so that the plaster may not interfere with the radiation of light in the direction of the condenser. The copper surface at the end of the carbon being uncovered, it is fastened in the holder in the usual way. The light now produced is steady and the combustion of the carbon slow. The film of plaster melts gradually as the point is consumed, and thus never interferes with the illumination. The points of both the negative and positive carbons remain blunt, and there is no wasting away of the stem. A carbon prepared in this way will last at least ten times as long as one used in the ordinary way. The chief advantage, however, is found in the comparative steadiness of the light.

The Shape of the Earth.—There is in England a man named Hampden who believes the earth is flat, and is sorely tried because he can not win all his fellows to this opinion. He is fond of conducting controversies on this subject in the public press, and evidently derives great satisfaction from every contest, being a member of that fraternity who are "of the same opinion still," however convincing may be the facts and arguments which are adduced against their peculiar ideas. Mr. R. A. Proctor has lately found time to engage in a published correspondence with this interesting person, and now proposes to settle the matter by an experiment. It appears that, some years ago, Hampden agreed to forfeit a certain sum of money if the result of a similar experiment should prove to be adverse to his opinion. He lost the money. To this experiment Mr. Proctor alludes in the opening sentence of his challenge, which is as follows: "In the Bedford Canal experiment, the result of which cost Mr. Hampden so much loss and annoyance, he distrusted the evidence of the referee's eyes, and considered also that there should have been three boats in line, one at each end and one in the middle of the long distance. Now, as nothing would be easier than to photograph three boats so arranged on a clear, quiet day, and as the collodion-film can neither be deceived nor lie, I can not understand why he should not try that simple