280 



POPULAR SCIENCE MONTHLY. 



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 i9 

 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 millionth s or less of an 

 atmosphere. 



Early Traction Cables. The first use of 

 cables for transmitting power to a distance 

 telodynamic transmission was made, ac- 

 cording 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 facto- 

 ries 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 dis- 

 tance of seven hundred and fifty feet was 

 next arranged, with cables running on pul- 

 leys 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 mate- 

 rial 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 "Cactns" 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 cir- 

 cle 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 sin- 

 gle 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 pro- 

 duce and the most uncertain. These some- 

 what 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 cata- 

 logued 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 18*73, when the 

 first u 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 



