Following the milling machine, came the grinder, in which a revolving wheel of an abrasive material served to wear away the surface of a piece of work, and with this form of machine steels of great hardness could be finished with accuracy and a high polish. THE INTERCHANGEABLE SYSTEM The most notable advance in machine work came early in the nineteenth century, when what was known as the “American System” of manufacture, or the interchangeable system, was introduced. As long as mechanics were obliged to perform their operations largely by hand, it was impossible to attain great accuracy. Each workman put his own individuality into the work. As a consequence, no two pieces were of exactly the same size or shape. This was true even with the early power-driven machine tools. The parts might be very close to the same size, but careful measurements showed that they varied by a minute fraction of an inch. Hence, when a machine was assembled the unyielding metal parts had to be filed and trimmed and hammered to fit them together. If any accident occurred to a machine, the damaged part could not be replaced by another taken from stock. The entire machine had to go back to the shop where an experienced mechanic would make a new part to replace the damaged one. In those days a machine was not manufactured but was built as an individual mechanism, just as a house or a boat is built to-day. With the advent of accurate machine tools came the idea of standardizing the parts so that hundreds and thousands of pieces could be made of exactly the same dimensions, and in assembling a machine the parts could be picked at random from the stock and put together without the use of special tools and without requiring any special fitting. This was of special importance in the tools of warfare, because armies need quantity production, i. e., rifles, cannon, etc. More machines of the same kind were required for an army than for any other organization or line of work. At the close of the Napoleonic War the British Government had 200,000 parts of muskets either partly finished or waiting repairs. Their muskets were made after the old system. Each one was built separately with its parts individually fitted together, so that whenever any part was injured, the musket had to be laid aside and sent back to the workshop for repairs.