272 LECTURE XXIX. 



[protruding above deck. To connect it with the piston rod, cross pieces are 

 attached to the top of the latter, which extend beyond the cylinder, aiid 

 are, on each side of it, united by parallel motions to connecting rods 4 which 

 communicate with the beam. 



In this country the steam is applied to marine engines at a temperature 

 not greatly exceeding the boiling point ; but in America the case is other- 

 wise, the steam being often applied at such a temperature as to produce 

 double the pressure it does under ordinary circumstances. 



Where lightness is an object, the condensing apparatus is altogether 

 done away with, steam of a high temperature being employed, which after 

 it has done its work, is allowed to escape into the air. An engine on this 

 principle is designated a high pressure engine. The pressure of steam in- 

 creases very rapidly with its temperature, because its density increases at 

 the same time. The law which connects the two is given empirically by 

 Dr. Young (vol. ii. p. 398), as 



d(l + -0029/) 7 , 



d being the depth of mercury in atmospheres of 30 in. each, which would 

 press as much as steam at a temperature f of Fahrenheit above 212. 

 Thus, at 212, d = 1 atmosphere = weight of 30 in. of mercury = about 

 15lbs. per square inch; at 250, /= 38, d= (1-1102) 7 = a little more 

 than 2, or the pressure is more than doubled. Many analogous formulae 

 have been proposed at different times. That of the Franklin Institute is 



For the purpose of inland transport, the condensing engine is inappli- 

 cable, on account of the weight of the condensing apparatus. As early as 

 1802, Mr. Trevi thick constructed a high-pressure engine, in which the 

 boiler and apparatus formed one machine ; but it was soon found that the 

 roughness of common roads prevented the use of such an engine, and 

 finally destroyed it. Mr. Trevithick consequently turned his attention 

 to railroads ; but a difficulty arose, which gave much unnecessary trouble, 

 from the fact of its being almost imaginary. The adhesion of the wheels 

 was not supposed to be sufficient to prevent their slipping. To obviate 

 tthis, various devices were put in requisition,* such as rack work, pro- 

 jecting moveable feet, &c. Experience finally taught that the friction of 

 the driving wheels .is more than sufficient, in ordinary cases, to prevent 

 slipping, provided a considerable portion of the weight be made to press 

 on them. This being established, the construction of the working engine 

 presents no insuperable difficulties. The following is a brief description 

 of one of the most approved forms of the locomotive. 



The first thing to be attained is the supply of a large quantity of 

 steam at a high temperature, and from a small apparatus. To effect this, 

 as large a surface as possible must be exposed to the action of the fire, 

 and the fuel itself must be kept in a vigorous state of combustion by a 

 great draught. These objects are attained by perforating the boiler, which 

 is cylindrical, from end to end, by upwards of 100 hollow tubes of about 

 two inches diameter. Through these the flame and heated air find their 

 way from the grate to the chimney ; thus imparting heat to a vast surface] 

 * See Gordon's Treatise on Elemental Locomotion, 1832. 



