all cases to form an opinion about matters occurring in our own time, for we ourselves are subject to the influence of the time, and must judge it while we form part of it. The immense number of cases existing, on the other hand, and the exactness of our knowledge of them, here help us very greatly. An examination of the way in which the gradual perfecting of machines is to-day going on teaches us, however, one thing,—as we shall presently see,—namely, that the process of the replacement of force-closure by pair- and chain-closure goes on quietly extending itself further and further to this hour. We may therefore consider this process as showing the essential general tendency of the whole machine-development up to our time; we may even go further, and say that we must consider it as an essential characteristic of future machine-development.
Fig. 175.
In Newcomen's steam-engine, Fig. 174, force-closure still predominated, and it remained thus through the whole eighteenth century. The machine was force-closed in its pit-work, in its beam-chains, in its steam-piston and in its valve-gear, although in the latter Potter's invention had substituted a machinal arrangement
