Report on the Progress and State of Applied Mechanics. 227 



The efficiency of overshot and breast-wheels is now from seven to eight- 

 tenths. 



78. Undershot-Wheels have had their efficiency doubled — (it 

 having been increased fi'om three-tenths to six-tenths), — by Poncelet's 

 invention of curved float-boards, which throw the water backwards 

 relatively to the wheel, and cause it to drop into the tail-race without 

 horizontal motion, so as to make it communicate the greatest possible 

 proportion of its energy to the wheel. 



79. Turbines, or horizontal wheels, acted upon by a vortex or 

 whirlpool of water, have long been used in a rude and imperfect form . 

 but the bringing of them into an efficient state has been the work of 

 recent inventors, who have made them realize an efficiency equal, or 

 nearly equal, to that of overshot- wheels. Whitelaw and Stirrats' Tur- 

 bine is a modification of Barker's mill or Eeaction-wheel. In Fourney- 

 rou's Turbine, a vortex moving spirally outwards, drives a vane-wheel 

 surrounding the case from which the water is supplied. In Cadiat's 

 Turbine, the vortex moves vertically, and the vane-wheel is below the 

 casing. In Professor James Thomson's Turbine, or Vortex- Wheel, the 

 vortex moves spirally inwards, and drives a vane-wheel, surrounded by 

 the casing that supplies it with water. This wheel possesses over the 

 others the advantage of being easily regulated, and of requiring a less 

 speed for the production of its maximum efficiency. Its invention was the 

 result of an investigation, by Mr. Thomson, of the theory of the motion 

 of fluids in whirlpools. 



80. Windmills have undergone no alteration in principle since 

 Smeaton investigated their best forms and proportions. The details of 

 their construction have been improved, along with those of other ma- 

 chinery. 



81. The energy of Heat may be made to drive a prime mover, by its 

 effects on the volume and pressure of any elastic substance ; but the 

 only elastic substances which have been used in practice for that pur- 

 pose are water, air, alcohol, and ether ; and of these the last three have 

 been used to so limited an extent, that water, operating in the steam 

 ENGINE, may be said to be the only vehicle which is extensively used 

 in practice for the mechanical action of heat. 



82. In a steam engine have to be considered. — first, the efficiency 

 of the furnace and boiler, being the proportion of the heat made avail- 

 able in the evaporation of water to the whole heat produced by the 

 combustion of the fuel ; secondly, the efficiency of the steam, being the 

 proportion of the heat which disappears in performing mechanical work 

 upon the piston to the whole heat expended in producing the steam ; 

 and, thirdly, tlic efficiency of tlie mcclianism, which is the proportion 



