SCIENTIFIC TESTING AND MEASURING Science, whether it be of the pure variety, that which is pursued for its own sake--for the mere greed for knowledge--or applied science, the purpose of which is to assist manufacture, is based entirely upon accurate testing and measuring. It is only by discovering and investigating small differences in size, weight or strength that some of the most important facts can be brought to light. There are some problems, too, that defy theory, since they are too complicated; they involve too many theories all at once, and such can only be solved by accurate tests. And all these necessitate the use of very ingenious and often costly devices. Electrical measuring instruments were of sufficient importance and interest to warrant a chapter of their own, but there are many others of great value, and not without interest to the general reader. For example, some years ago there was a collision in the Solent, just off Cowes, between the cruiser _Hawke_ and the giant liner _Olympic_. The cause of this was a subject of dispute and of litigation; the theorists theorised; some reached the conclusion that the _Hawke_ was to blame, and others the _Olympic_; and where doctors disagree who shall decide? It was wisely decreed that tests should be made to settle the question. The main point was this. The officers of the _Hawke_, by far the smaller vessel, averred that they were drawn out of their course by suction caused by the movement of so large a ship as the _Olympic_ in the comparatively narrow and shallow waters of the Solent; in other words, that the _Olympic_ in moving through the water caused a swirling, eddying motion in the water, tending to draw a lighter vessel towards itself. And that is just one of those problems with which theory is unable to deal. So it was transferred to the National Physical Laboratory at Teddington, near London, for investigation by experiment. At this institution, which is a semi-national one, there is a tank constructed for purposes such as this. The word tank leads us to underestimate its size somewhat, for it is 494 feet long and 30 feet wide. It is solidly constructed of concrete, with a miniature set of docks at one end, and a sloping beach at the other. On either side are rails upon which run trollys which support the ends of a bridge which spans the whole. This bridge can be propelled along, by means of electric motors operating the wheels of the trollys, from one end of the tank to the other, at any desired speed, within, of course, reasonable limits, and from it may be towed any model which it is desired to test.