ON THE STEAM-ENGINE. 

 LECTURE II. 



REVERTING to the subject of boilers, I did not yesterday 

 mention the kind of boiler which is in most general use 

 now-a-days in England for manufacturing purposes I 

 mean the Lancashire boiler, the internally fired two-flued boiler. 

 It probably is one of the very best that we have from a 

 practical point of view, as it gives in a comparatively small 

 diameter a large area of fire grate, and it gives also the con- 

 siderable heating surface of the internal tubes. Moreover 

 it affords ample opportunity for getting in below the tubes 

 for the purposes of cleaning or of repairs when necessary. 

 The marine boiler of the present day is, as we know, a boiler 

 from 11 to 12 or 13 feet in diameter, made of plates up 

 to an inch in thickness, and competent to stand a pressure 

 of steam of 70 Ibs. above atmosphere. It contains generally 

 three cylindrical fire-tubes, the flame-box, the return-tubec, 

 and a dry uptake. The marine boiler of old days was, in 

 truth, nothing but a tank, the form of which was determined 

 by consideration as to its adaptability to the shape of the 

 vessel more than by any other rule. The flues were rect- 

 angular and flat-sided, and it was wholly unfit for use except 

 with low pressure. To show you how low the pressure was, 

 I may state that there was more pressure on the bottom of 

 such a boiler (considered merely as a tank) from the weight of 

 water in it (when the steam was not up) than was pressing 

 against the under side of the top of the boiler when steam 

 was up. 



The cylindrical cases of boilers resist the tendency of the 

 internal pressures to burst them by means of the tensile 

 strength of the plates and of the riveted joints ; and flat 

 surfaces, where they must exist, such as at the ends of 

 cylindrical boilers, are stayed either by stay-bars or by 

 gussets ; or, in the case of the fire-boxes of locomotives, 

 by screw stays 4 inches apart and upwards ; but there is 



