172 HiMory of Mechanical Inventio7is and 



giving a more equable power than that which is produced by the singlci 

 engine, whether high or low pressure, since it is well known that at two 

 points of the revolution of the crank the power ceases during at least one* 

 twelfth of the time, which is the reason that so large a fly-wheel is neces- 

 sary. It is particularly applicable to steam-boats, and may be used to 

 great advantage in the double pump, as well as the balance-bob lifting 

 pump, used in Cornwall for mining purposes, by the use of proper gear- 

 ing. The present single stroke expansive engines used in Cornwall for 

 pumping are preferred to all others on account of their economy, although 

 they are very limited as to the extent of the expansive principle, for want 

 of compensation, as nearly the same power is wanted to finish the stroke 

 of the pump as to begin it. 



The variation of the velocity of the piston occasioned by the compound 

 motion of the crank and connecting rod is not taken into view in this dia- 

 gram. As the connecting rod is intended to be four diameters of the path 

 of the crank, the variation will make no practical objection, being, at its 

 greatest value, but one-thirty-second part of its range. If the engine 

 should be worked by a connecting rod, as is sometimes the case in steam- 

 boats, say only one diameter of the path of the crank, the variation at each 

 end of the stroke would amount to a practical defect, since the piston would 

 move with nearly three times the velocity at the lowest quarter of the 

 stroke that it would at the first quarter. Thus circumstanced, the crank 

 must be above the cylinder. 



As the law of expansion seems not yet to be settled, an arithmetical 

 expansion has been used for this diagram which, from its approximation 

 to the real law, will be quite near enough for practical purposes. Many 

 who are of the school of Tilloch and Wolf believe that the expansive power 

 of steam depends upon heat only ; while the Soho experiments are said to 

 prove that elasticity depends simply on density, without regarding tem- 

 perature, viz. that if a cubic foot of steam at atmospheric pressure weighs 

 one ounce, 50 atmospheres of steam would weigh 50 ounces ; but Dalton, 

 who is undoubtedly much nearer the true law, would make 50 atmo- 

 spheres weigh but about 34) ounces. 



I have no doubt that the nearer the atoms of water are made to ap- 

 proach each other by compression, the greater will be the repulsive action 

 of caloric, and that in a more rapid ratio than has hitherto been allowed, 

 especially in highly compressed steam. Its comparative density, with the 

 increase of power, dirainishesfaster than has been supposed even by Dalton. 



S. Method qfmakittg Transparent Soap. 



Tallow is the basis of all soaps for the toilette, known under the name 

 of Windsor, because olive oil forms a paste too difiicult to melt, and hav- 

 ing an odour too powerful for mixing with perfumes. 



Tallow-soap dissolved with heat in alcohol returns to its solid state on 

 cooling. It is this fact which has led to the discovery of transparent soap. 

 When well prepared, this soap should have the appearance of fine white su- 

 gar candy. It may also be coloured, and vegetable colours are for this pur- 

 pose preferable to minerals. Any person can make the soap by putting 



