148

ON PRIME-MOVERS.

CHAP. IV.

ON THE UNDERSHOT WATER WHEEL .

Before the introduction of iron, undershot water wheels werefrequently employed, and were in almost every instance con-structed with straight radial floats, as in the annexed sketch,

Fig. 123.

the water being discharged against the float-boards, as it rushedwith considerable velocity underneath the shuttle. This wasthe invariable practice down to Smeaton ’s time even, the prin-ciple being to employ the impulse of the fluid stream, and notits gravity or weight. Indeed, there appeared to be an im-pression that this was the more effective and economical modeof application, and probably arose out of the circumstancesof the original employment of water as a moving power. Theearliest wheels of which we read are undershot wheels placed