Such was the mechanical equipment in 1775; practically what it had been for generations. By 1850 it was substantially that of today. In fact, most of this change came in one generation, from about 1800 to 1840. Since that time there have been many improvements and refinements, but the general principles remain little changed. With so wonderful a transformation in so short a time, several questions arise almost inevitably: Where did this development take place, who brought it about, and why was it so rapid? The first question is fairly simple. England and America produced the modern machine tool. In the period mentioned, England developed most of the general machine tools of the present day; the boring machine, engine lathe, planer, shaper, the steam hammer and standard taps and dies. Somewhat later, but partially coincident with this, America developed the special machine tool, the drop hammer, automatic lathes, the widespread commercial use of limit gauges, and the interchangeable system of manufacture. In a generalization such as this, the broad lines of influence must be given the chief consideration. Some of the most valuable general tools, such as the universal miller and the grinder, and parts of the standard tools, as the apron in the lathe, are of American origin. But, with all allowances, most of the general machine tools were developed in England and spread from there throughout the world either by utilization of their design or by actual sale. On the other hand, the interchangeable system of manufacture, in a well-developed form, was in operation in England in the manufacture of ships’ blocks at Portsmouth shortly after 1800; and yet this block-making machinery had been running for two generations with little or no influence on the general manufacturing of the country, when England, in 1855, imported from America the Enfield gun machinery and adopted what they themselves styled the “American” interchangeable system of gun making.[7] [7] See page 139. The second question as to who brought this change about is not so simple. It is not easy to assign the credit of an invention. Mere priority of suggestion or even of experiment seems hardly sufficient. Nearly every great improvement has been invented independently by a number of men, sometimes almost simultaneously, but often in widely separated times and places. Of these, the man who made it a success is usually found to have united to the element of invention a superior mechanical skill. He is the one who first embodied the invention in such proportions and mechanical design as to make it commercially available, and from him its permanent influence spreads. The chief credit is due to him because he impressed it on the world. Some examples may illustrate this point.