After the application of the triple valve had made it automatic, there remained but one further improvement necessary to render the Westinghouse a well-nigh perfect brake. A superabundance of self-acting power had been secured, but no provision was yet made for graduating the use of that power so that it should be applied in the exact degree, neither more nor less, which would soonest stop the train. This for two reasons is mechanically a matter of no little importance. As is well known a too severe application of brakes, no matter of what kind they are, causes the wheels to stand still and slide upon the rails. This is not only very injurious to rolling stock, the wheels of which are flattened at the points which slide, but, as has long been practically well-known to those whose business it is to run locomotives, when once the wheels begin to slide the retarding power of the brakes is seriously diminished. In order, therefore, to secure the maximum of retarding power, the pressure of the brake-blocks on the revolving wheels should be very great when first applied, and just sufficient not to slide them; and should then be diminished, _pari passu_ with the momentum of the train, until it wholly stops. Familiar as all this has long been to engine-drivers and practical railroad mechanics, yet it has not been conceded in the results of many scientific inquiries. In the report of one of the Royal Commissions on Accidents, for instance, it was asserted that the momentum of a train was retarded more by the action of sliding than of slowly revolving wheels; and again, as recently as in May, 1877, in a scientific discussion in London at one of the meetings of the Society of Arts, a gentleman, with the letters C. E. appended to his name, ventured the surprising assertion that "no brake could do more than skid the wheels of a train, and all continuous brakes professed to do this, and he believed did so about equally well." Now, what it is here asserted no brake can do is exactly what the perfect brake will be made to do,--and what Westinghouse's latest improvement, it is claimed, enables his brake to do. It much more than "skids the wheels," by measuring out exactly that degree of power necessary to hold the wheels just short of the skidding point, and in this way always exerts the maximum retarding force. This is brought about by means of a contrivance which allows the air to leak out of the brake cylinders so as to exactly proportion the pressure of the blocks on the wheels to the speed with which the latter are revolving. In other, and more scientific, language the force with which the brake-blocks are pressed upon the wheels is made to adjust itself automatically as the "coefficient of dynamic friction augments with the reduction of train speed." It hardly needs to be said that in this way the power of the brake is enormously increased.