﻿Notices respecting New Books. ;>29 



times its volume, and one with three cylinders for successive ex- 

 pansion to ten times the volume. 



We read the history of the steam-engine with much humility. 

 It is a history of the work of theorists without practical knowledge 

 and of practitioners without theoretical knowledge, entirely out of 

 sympathy with one another because each side of them could Bee 

 that the other side was wrong. This is even now only slowly 

 being replaced by a better state of things, a number of young 

 practitioners being fairly well equipped with scientific knowledge; 

 but the prejudices of the " rule of thumb "engineer are not by any 

 means dead ; and many of his sneers at the so-called theorists are 

 not altogether undeserved. 



The solution of the problem which is now generally adopted is 

 that of expanding the steam successively in a series of cylinders 

 so that the range of temperature in each shall be small, and to 

 shorten the periodic time to take advantage of a well-known fact 

 in heat-conduction — that the amount of heat entering the metal 

 when its skin is higher in temperature than the portions inside or 

 leaving the metal when the skin is lower in temperature, say, 

 during a half period, is inversely proportional to the square root of 

 this periodic time. The phenomena are very complicated, and we 

 are only gradually getting to understand how to make experiments. 

 There can be no doubt that the film of water which adheres to the 

 metal surfaces performs a most important function ; for no ordi- 

 nary emissivity of surface will account for the great rapidity with 

 which the entering steam gives up its heat ; but it can be explained 

 by t\e evaporation or condensation of water and steam at their 

 surface of separation. If we take it that the whole of this water is 

 always at the temperature of the steam and consider the metal to be 

 nonconducting, the work per stroke will now be proportional to the 

 amount of water present, and will be independent of the periodic 

 time. Such experimental results as we have seem to indicate that 

 the loss per stroke is not inversely proportional to the square 

 root of the periodic time, but it is not independent of the periodic 

 time. 



If the water alone were operative, it is an interesting inquiry, 

 Ought not the amount of water present to go on increasing ? The 

 calculation is more difficult than it seems at first sight. How* 

 much steam will condense when it enters the cylinder ? How much 

 will conderse in the adiabatic expansion ? These are easy enough, 

 although the first is an irreversible process ; but in the irreversible 

 phenomenon of release to the condenser there is a step-by-s^ep 

 approximate integration to be performed, which is very tedious ; 

 we know from actual experiment that the average mathematical 

 physicist needs ten days to find out the difficulty of this problem. 

 It is difficult to see why, as a rule, in all engines there is so little 

 condensation or vaporization during the expansion when there is 

 such enormous condensation during the admission, and yet the 

 piston is continually exposing fresh cold surfaces. Condensation 

 is probably going on at these cold surfaces, and nearly equal 



Phil Maq. IS. 5. Vol. 38. Xo. 232. Sept 1894. Z 



