NATURAL PHILOSOPHY. 173 



direction of their motion in passing round a curve. It precludes 

 the emploj-ment of Avery's engine for driving locomotives, and 

 suggests that, if his engine should be used for this purpose, it 

 should be run on a vertical, instead of horizontal, axis." 



WORKING STEAM EXPANSIVELY. 



The following is a letter from Prof. Rankine, of Glasgow, to 

 the editor of the " Scientific American," in 1866 : — 



" The circumstances under which steam undergoes expansion 

 inay be classed under five heads : 1. When the steam expands 

 without performing work. 2. When it expands and performs 

 work, the temperature being maintained constant by a supply of 

 heat from without. 3. When it expands and performs work, 

 being supplied from without with just enough of heat to prevent 

 any liquefaction of the steam, so tluit it is Jjept exactly at the sat- 

 uration point. 4. When it expands and performs work in a non- 

 conducting cylinder. 5. When it expands and performs work in 

 a conducting cylinder, not supplied with heat from without. 



" 1. Wlien steam expands witliout performing work (as in rush- 

 ing out of a safety-valve, or through a throttle-valve), it becomes 

 superheated, as is well known ; the temperature falling very 

 slightly in comparison with the boiling-point corresponding to 

 the diminished pressure. The precise rate at which the temper- 

 ature falls is not yet known ; but it will probaljly be soon ascer- 

 tained through some experiments by Prof. Thomson and Mr. 

 Joule. 



"2. When steam expands and performs work, tlie temj^erature 

 being maintained constant by supplying heat through the cylin- 

 der, the law of expansion at first deviates from Marriotte's law by 

 the pressure falling less rapidly than the density ; but, as the ex- 

 pansion goes on, the law ai^proaches more nearly to that of Mar- 

 riotte, as recent experiments by Messrs. Fairbairn and Tate have 

 shown. 



" 3. When the steam expands and performs work, being main- 

 tained exactly at the temperature of saturation, the law of expan- 

 sion, as you observe, is perfectly definite. In the treatise to whicii 

 you have referred, I have shown what it is ; and also that it is ex- 

 pressed, nearly enough for practical pur230ses, hy taking the press- 

 m-e as l3eing proportional to the seventeenth power of the six- 

 teenth root of the density ; a function very easily calculated by 

 means of a table of squares and square roots. In many actual 

 steam-engines, the circumstances of this case are practically 

 realized, as is shown by the agreement of their performance with 

 the results of calculation. 



" 4. When steam expands and performs work in a non-con- 

 ducting cylinder, it was shown by Prof. Clausius and myself, in 

 1850, that the lowering of the temperature, through the disappear- 

 ance of heat in pei'forming work, goes on more ra|)idly than the 

 fall of the boiling-point corresponding to the pressure, so that 

 part of the steam is liquefied. This result was experimentally 

 verified by Mr. G. A. Ilirn, of Mulhouse, a few years afterward 

 IG* 



