220 Report on the Progress and /State of Applied Mechanics. 



improvement in macliines is to diminish the lost work, so as to make 

 the efficiency approximate to unity. The introduction into the theory 

 of machines of correct ideas respecting those general principles, has 

 been of great pi'actical utility. It is due chiefly to Carnot, Coriolis, and 

 Poncelet, in France, and to Smeaton, Mr. Moseley, and other writers, in 

 Britain. 



72. Delusive machines, from which their projectors expect to realize 

 an efficiency greater than unity, form the class commonly called pee- 

 PETtTAL MOTIONS. It is Satisfactory to observe that the patent hsts 

 show a slight diminution in the proportionate number of such projects. 



73. The ordinary sources of the power of Pbime Movers are, Ani- 

 mal strength, the weight and motion of water, the motion of wind, and 

 heat ; to which may be added electricity and magnetism, as sources of 

 power, sometimes, though rarely, employed. Dr. William Thomson 

 has proved (what, indeed, George Stephenson is said to have main- 

 tained before him), that the original source of the energy obtained by 

 all these means, is the sunshine ; and from Dr. Thomson's researches 

 also, founded on an idea first put forth by Mr. Waterston, there is 

 reason to believe that the light of the sun is produced by the friction 

 of showers of matter faUing towai-ds him under the force of gravitation. 



74. Little has been done of late towards the improvement of the 

 modes of using animal strength. It is worth noting, however, that 

 Dr. Scoresby and Dr. Joule, a few years ago, pointed out, that animals 

 are the most efficient of all prime movers. They are not the most 

 economical, because of then- food being more costly than the materials 

 consumed by inanimate engines. 



75. The WEIGHT ai^d motion op Watee act in prime movers, 

 which may be classed as Water-pressure Engines, Oversltot and Breast- 

 Wlieels, Undershot- Wheels, and Turbines. 



76. Wateb-peessttee Engines, in which the water drives a piston, 

 have been brought to high perfection by various inventors, especially 

 Mr. Armstrong, so as to work with great efficiency. They are apphed 

 to hoists and cranes, to pumping, and to the driving of mechanism in 



77. The greatest improvement in Oveeshot and Beeast- Wheels, 

 is that invented by Mr. Fairbairn, which consists in the simple expe- 

 dient of providing an outlet at the back of each bucket, to permit the 

 escape of air while the water enters in fi'ont, and thus to prevent that 

 loss of power from the agitation and waste of water, which was formerly 

 an obstacle to the use of over shot- wheels at all except veiy low speeds, 

 and which even at low speeds increased their cost, by making it neces- 

 sarv to use a wheel considerablv broader than the stream of water 



