Page:Popular Science Monthly Volume 15.djvu/299

 very distant from those of the Zambesi, but which soon takes a north and south direction. Only the upper course of this stream was hitherto known to Europeans. It was supposed that farther down it turned to the west and flowed into the Atlantic as the Owambo or Cunene. Pinto has now probably made the discovery that the Cubango is a tributary of the Zambesi. However this may be, our knowledge of the interior of Africa has been considerably enlarged by Pinto's journey.

On reaching the Transvaal Territory the explorer sent the following dispatch to the King of Portugal: "I am now six days' journey from the Indian Ocean, and on the point of completing my march across Africa from the west coast. I have struggled against hunger and thirst, wild beasts, savages, floods, and drought, and have happily surmounted all these obstacles. My records are safe, and consist of twenty geographical charts, three volumes of important coördinates, meteorological notes, three volumes of sketches, and a voluminous journal. I have lost several men. Complete study of the upper Zambesi, sixty-two cataracts and rapids. Plan of the cataracts. The natives fierce; unceasing wars. The secret of the Cubango.

Strength of Hard and Soft Steel.—It has hitherto been supposed that a soft bar of steel can longer resist the disintegrating action of strains and shocks than a hard one, but experiments made by W. Metcalf, of Pittsburg, appear to prove the contrary to be the fact. His attention was first called to this matter by the constant breaking of steam-hammer piston-rods. Made of ordinary steel, they lasted but six months, an iron rod lasting but half as long. Then lower and lower steels were tried, and broke in about five months. In an emergency, a rod of comparatively high steel was employed, and this, which it was supposed could not serve for more than a week or two, held out for more than two years. This result led Mr. Metcalf to investigate the whole subject systematically, and a lot of small steel connecting-rods were tested in a special machine. The test required was, that a machine should run 4 hours at a rate of 1,200 revolutions per minute, unloaded, before the connecting-rod broke. These rods were unforged in the middle, and consisted of a piece of round bar with a head welded on each end. "The mode of rupture was," says Mr. Metcalf, "as a rule, the same in all cases; the rod heated at the middle, where the vibrations met, as they were imparted by rotary motion at one end, and by reciprocating motion at the other, and by alternating strains of compression and extension. In some cases the rod became slightly red-hot at the middle before rupture. After heating, the next thing observed was the raising or loosening of the surface scale of the middle. Soon after this, rupture began, first at the surface and gradually extending to the center. The breaking was gradual in every case, no piece breaking suddenly, even of the highest steel. The first trial was with ·53 carbon steel: mean time of six trials, 2 hours 9 minutes. Second trial, ·65 carbon steel: mean time of six trials, 2 hours 57 minutes. Third trial, ·85 carbon steel: mean time of three trials, 9 hours 45 minutes, and the trials were stopped." A set of twelve connecting-rods, made from special ingots, was then prepared. These were tested with the following results:

 The ·30 C ran 1 hour 21 minutes, heated and bent before breaking. The ·49 ran 1 hour 28 minutes. The ·53 ran 4 hours 57 minutes, broke without heating. The ·65 ran 3 hours 50 minutes, broke at weld where imperfect. The ·80 ran 5 hours 40 minutes. The ·84 ran 18 hours. ·87 C broke in weld near the end. ·96 C ran 4 hours 55 minutes, and the machine broke down.

The whole twelve were not tested, because the machine was needed for other works, and, when Mr. Metcalf returned to complete the experiment, it was discovered that the foreman of the shop had picked up the unbroken specimens, put them into machines, and sent them off. Enough was done, however, to show that the maximum of strength to resist vibration was not found among the ductile steels. Mr. Metcalf gives some other data concerning the performance of steel suspension-rods in a bridge that corroborate his views, which are practically novel.