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280 numerator and one, followed by eight ciphers, for its denominator could still affect the ear. To make a comparison with the limits of microscopic vision, the vibration that is just perceptible to the ear would have to be multiplied by one hundred before it could be seen by any possible microscope; or, put in another way, the sensitiveness of the ear is such that it could distinguish differences of pressure one hundred times less than the residual pressure of the best vacuum, which is to be measured in millionths or less of an atmosphere.

Early Traction Cables.—The first use of cables for transmitting power to a distance—telodynamic transmission—was made, according to Prof. W. Cauthorne Unwin, in 1850, at Lozelbach, Alsace, when some large factories which had been idle for nine years were started up again. The buildings were scattered at considerable distances apart, and there was only one steam engine. A steel band, working like an ordinary machine belt, was introduced for driving one of the factories about two hundred and fifty feet from the engine. It was mounted on pulleys a little more than six feet in diameter and making one hundred and twenty revolutions per minute, and was used for eighteen months, transmitting twelve horse power. On the suggestion of an English engineer, a wire-rope cable, a quarter of an inch in diameter, was then substituted for the band, while the same pulleys were used, with grooves turned in the rim to hold the cable—till after a few years they were replaced by pulleys of iron. A transmission to a distance of seven hundred and fifty feet was next arranged, with cables running on pulleys ten feet in diameter, at a speed of about fifty feet per second, and transmitting forty horse power, which, with pulleys at mid-distances, are still in use. The amount of work transmitted by a cable is proportionate to the amount of effectual tension in the cable and its speed. The strongest material should be used for the cables, and they should be run at the highest practicable speed. The largest cables which it appears practicable to use are about one inch in diameter. In order that the bending stress may not be excessive, the pulleys are of large diameter, usually from twelve to fifteen feet. Gutta-percha, soft wood, and leather have been used for the throat of the pulley, on which the rope runs. The greatest speed at which it is practicable to carry the rope depends upon the centrifugal tension of the pulley, and is usually about one hundred feet per second. With pulleys from three hundred to five hundred feet apart, a one-inch rope will transmit about three hundred and thirty horse power.

Dahlias and "Cactus" Dahlias.—The first dahlias seen in Europe grew in the Botanical Gardens at Madrid, in 1789, from seeds sent from Mexico. The flowers were "single" and had eight rays disposed in a circle around the yellow disk. The first double forms were produced in Louvain, Holland, in 1814, after three years' work. All members of the composite family that have been through the process of doubling and have enough flexibility to entitle them to extended cultivation exhibit, Mr. Wilhelm Miller says in the Bulletin of the Cornell University Experiment Station, at least three strongly marked tendencies—to reproduce single forms; to develop large globular flowers that are completely double; and a tendency toward what are called pompons. The single varieties are the most natural and the easiest to produce and fix, while the large-flowering and pompon varieties are to a greater extent products of art. The large-flowering varieties are the hardest to produce and the most uncertain. These somewhat conventional and artificial forms are still supposed to be essential to the nature of the dahlia; but they are not. In the evolution of the dahlia too much attention has been paid to color and not enough to form. The twelve hundred varieties catalogued in 1841 "were too much like twelve hundred variously painted balls of two sizes. No new or original idea found place in the evolution of the dahlia till 1873, when the first "cactus" dahlia, Juarezii, was produced. Instead of short, stiff, artificially formed rays, it has loose, flat rays with pointed or twisted ends, and the peculiar red that is associated with the cactus. Other colors have since been developed, which are not that of the cactus, and that part of the name of the class is no longer appropriate. Only the rays have been cultivated, while