148 THE POPULAR SCIENCE MONTHLY. 



come completely filled and their valves closed, and then sets it in mo- 

 tion on the return-stroke. Thus the pistons move alternately. These 

 engines have given a very high duty. The condenser is seen at C, and 

 the air-pump is at D, the latter being worked from the bell-crank lever 

 H by means of links, Z, K. The steam-valves, Q, JR, are balanced. 

 V ~P"are the water-induction valves, and T T those on the eduction- 

 side. 



Here we leave the steam-engine as applied to raising water. We 

 have invariably noticed, in the forms of engines so used, that a con- 

 denser forms a part of the apparatus. 



We will next briefly trace the history of that now familiar form 

 of engine in which the steam, having done its work, is discharged 

 directly into the atmosphere. 



-*-- 



STAR OK STAR-MIST. 



By EICIIARD A. PKOCTOR. 



A REMARKABLE discovery has been made by the astronomers 

 of Lord Lindsay's observatory at Dunecht a discovery the true 

 meaning of which is not as yet fully perceived. It may be remem- 

 bered that some nine months ago a new star, as it was called, made 

 its appearance in the constellation Cygnus. 1 This object shone out 

 where before no star had been known to astronomers not merely, be 

 it noticed, where there was no visible star, but where none was re- 

 corded even in lists like Argelander's " Durchmusterung," containing 

 hundreds of thousands of telescopic stars. It was not, however, alto- 

 gether impossible that some small star within moderate telescopic 

 range had existed in the spot where the new star shone out, and that 

 in some way this small star had escaped observation. This seemed 

 the more likely because the new star had appeared in a part of the 

 heavens very rich indeed in telescopic stars ; at any rate, astronomers 

 had reason to believe that they would be readily able to determine 

 the question with a high degree of probability by watching the star 

 as it gradually faded out of view. For a " new star " which had 

 shone out in the constellation of the Northern Crown in May, 1866, 

 and had been identified with a tenth-magnitude star in Argelander's 

 list, had gradually faded out of view, and, growing yet fainter, had 

 sunk through one telescopic magnitude after another until it shone 

 again as a tenth-magnitude star only. Since that star had resumed 

 its former lustre, or rather its former faintness, it seemed not unrea- 

 sonable to conclude that so also would the star in Cygnus. We shall 

 presently see how far this expectation was from being fulfilled. 



1 See Popular Science Monthly, vol. xi., p. 59. 



