THE STEAM-ENGINE. 137 



fndustrielle for the 24th of October last. I dare say the 

 particulars are known to most of you, but as they may not 

 be to all, I will repeat them. The effect of the solar rays on 

 45 feet of surface when concentrated upon the boiler was 

 sufficient to evaporate 11 Ibs. of water in one hour. The 

 construction was simply this. A conical mirror was employed, 

 the sides of the cone being at an angle of 45 ; the diameter 

 of the cone was 2 -6 metres, and the bottom of it had a flat 

 surface of one metre. In the axis of this cone was a copper 

 boiler about 1 1 inches in diameter, and of the height of the 

 cone. The boiler was blackened, and was surrounded by a 

 glass envelope ; there was also an apparatus to cause the axis 

 of the cone to point always towards the sun, and the sun's 

 rays falling upon the sides of the mirror were reflected 

 radially on to the boiler, and the result was, as I have said, 

 to obtain an evaporation of as much as 1 1 Ibs. of water in 

 one hour from the effect of the sun's rays falling on 45 feet 

 super. 



We frequently hear of very wonderful evaporating per- 

 formances, and when they are very wonderful we should do 

 well to distrust them. We must recollect that one of the 

 defects of a boiler is priming, and we must equally recollect 

 that that abstracts water from the boiler, and, therefore, 

 appears to make the boiler do higher evaporating duty, and 

 a bad boiler is thus credited with a merit for that which 

 is a positive defect. I know no means of" making quite 

 sure that this priming does not take place, except by re- 

 condensing the steam that comes from the boiler and observing 

 the heat given forth by it ; but by this precaution it may 

 soon be ascertained whether the product from the boiler 

 has been dry steam, or partially steam and partially water 

 mechanically carried over. 



Assuming that we have once got from some boiler the 

 steam, we will now consider how we are going to use it in 

 the steam-engine, and I will refer you to these two diagrams 

 one where I show by a vertical line the pressure of the 

 steam from zero up to 20 atmospheres, and by the horizontal 

 line the temperatures ; the other (2), showing the volumes 

 corresponding to the various pressures. 



