884

APPENDIX.

“There is another circumstance in this species offraming which demands great attention. The shaftsoften require to be restored to their true situations, fromwhich they may have deviated by the wearing of theparts. Now the framing ought to be so constructed aseasily to admit of this restoration of the shafts, as also ofany other shifting of them which may in practice becomenecessary.

“ But though the framing which supports the parts ofmills and machines should be firm, it is an advantage thatthe part on which any axis rests should have a small de-gree of elastic tremor, when the machine is in motion.Such tremor has considerable power in diminishing thefriction. It may farther be observed, that framing tosupport machinery should be as independent of the build-ing as possible, because the tremor it always communi-cates is exceedingly injurious.”

On Reaction Wheels.

These wheels were slightly noticed at page 176; anda description of Barker’s mill is to be found in nearlyevery work upon hydraulics, together with the im-provement made in it by Rumsey. Within a few yearspast, wheels which operate upon the principle of the ro-tary trunk, in Barker’s mill, have been extensivelybrought into use. We are not informed by whom theywere invented; Mr. Evans alludes to them in the firstedition of this work, published in 1795; but it docs notappear certain that he-had then seen them; it is ma-nifest, at all events, that they were not publicly known.His words are, “ One of these is said to do well wherethere is much back water; it being small, and of a truecircular form, the water does not resist it much. I shallsay but little of these, supposing the proprietors mean totreat of them.”

Their great merit, certainly, is their simplicity; andwhere there is a plentiful supply of water, they may, inmany cases, be preferable to any other. Those interest-