Materials, too, require the most careful testing, in order that they may be put to the best possible use in modern machinery and structures. For example, anyone can measure the strength of a spring, but what do we know as to its lasting power? Springs often have to form part of a machine in which they are stretched and compressed millions of times, and the question arises as to what is the best shape and material for the purpose. It may be that the spring which works best a few times will be the first to become "weary," for with repeated strain such things as steel get tired, just as the human frame does. Now that is a matter which will yield to no calculation, the only way to determine it is actual test. So a mechanism has to be employed which will extend and compress the spring over and over again, just as it will be in actual use, with a counter of the nature of a cyclometer to count how many times it has been subjected to this distortion. Then the apparatus is set going and left to itself for hours, or even for days, during which time it may work the spring millions of times. This may go on until it breaks, or else it may be done a prearranged number of times, and then the spring taken out and tested by other means to see how its strength has been affected. Metal bars are often subjected to sudden blows, light in themselves but oft repeated. The point to be determined then is how many times the blow may fall before permanent injury is done to the bar. To investigate such matters we have the "repeated-impact" machine. The bar is held in a suitable holder, under a hammer which gives it a blow, the force of which can be easily regulated, at regular intervals, the number of blows being counted by a suitable recording mechanism. Ultimately the bar breaks, under a blow the like of which it can endure singly without any apparent strain at all. The machine, by the way, can be caused to turn the bar round to some degree after each blow, so that it is struck from all directions in succession. The microscope, too, has established its place in the testing laboratory. It is a very valuable adjunct to chemical and mechanical tests.