Arizona republican., 1902-11-17

kept under constant pressure for years, their sag being recorded from day to day to determine just how much the timber is dehected during the life of a building in which it is placed-a long continued experiment, which, among other things, looks forward to remedy ing the often uneven floors of a cotton mill.
 If a contractor is building a church a masonry arch large enough for a church door is tested with a weight comparable to that of a church steeple. not loaded. to be sure. with so many pounds of material. but being com pressed to the crushing point by steel beams drawn downward by relentless mechanical power. Steel rods are sub jcCted to torsional or twisting tests in order that the necessary dimensions of shafts for engines steamships and for all sorts of shafting for the transmits sion of power may be definitely settled. Bricks are compressed until they crum ble into dust. but the recorded result of many tests determine the safe height of a chimney or an office building of brick construction. Steel columns are placed under compression until they buckle or break and it is then known for certainty how many pounds they will support without disaster.
 Particularly interesting are the tests of rope, in which the great difficulty is to get it to break in the middle rath- CT than at the knot or fastening. Wire rope can be tested by moulding type metal around the ends and thus furn- ishing a grip for the jaws of the ma chine. In the case of manila rope the sailors faVOrite hitch, the bow line. is found to be the most satisfactory knot of all, while two half hitches develop the largest efficiency. Of course the efficiency of a rope is no greater than that of the knot by means of which it is made fast. but when it is desired to learn the breaking strength of the straight rope, knots are abandoned and a careful eye splice is put in each end. Held by an eye splice, previously dampened, the rope may be broken in the center simply by eXerting sufficient pull. but it has been found that an av- craze rope before breaking will have stretched 20 per cent. beyond it's normal length.
 But not less interesting than the ex- periments is the delicacy of the appar- atus that measures the results. Not long ago, a sheet of steel-or steel plate. as it is called before it is worked into a steam boiler, water tank, ships hull or what not-was put into the huge machine that tests steel to discover how much resistance it will offer to a strain pulling it to pieces. The plate was 20 inches wide and half an inch thick and the opposite ends were firmly gripped by the steel jaws of the testing appar- atus. straining at it with pulling force of 100000 pounds. Measurements of the stretch of the plate were being made by two young men, one measuring from below, and the other measuring from above and for convenience resting his elbows on the tautdrawn steel. It was found that the readings taken by means of a very sensitive micrometer varied slightly and it was suggested that the variation might be due to the weight of the assistants elbows II seemed impossible that a heavy steel under such a heavy strain could be made to sag appreciably by so slight I weight, and yet the demonstration of this fact was even more remarkable Adjusting the micrometer to register an upward bending of the plate the in structor put his hand beneath it and pressed upward with one finger. The bell connected with the micrometer inn mediately stopped ringing, which show ed, that although the plate had been sufficient to make the electric circuit and the sensitive apparatus had record ed the fact.
 The micrometer with electric contact will indeed detect variation of one hundred-thousandth of an inch and re- spOnd to the additional weight of mans hand upon a wooden beam six by eight inches in size already loaded with 6,300 pounds. By means of such appliances, in tests made upon full sized material, the constructing engin- eer has during the last few years been supplied with exact data for his calcu- lations whether he is building a mill or a ship, an engine or an elevator.
 In studying the use Of cement for building purposes, the European, es- pecially the German laboratories, are, so far. well ahead of similar American institutions. This is known to be large ly due to the fact that American build CTS have hitherto found close at hand an abundant supply of wood, stone and brick, while in France and Germany these materials are not so available