NOTES ON STATIONARY ENGINES. 33 



which the spaces ai'c much smaller than there is any neces- 

 sity for, and consequently scale accuimilates here; it is in 

 such, cases not unconmion to find scale of extromo hardness, 

 half an inch or more in thickness, — but this is a condition 

 of the past rather than of the present. Then as to accessi- 

 bility, there is but little to say, except that the seating 

 of all such boilers should be in tlie form of large fire-clay 

 blocks and that all those under seams or butts should be cut 

 almost through by a saw before firing, so as to be easily 

 snapped by a blow, when tlicse blocks can be easily arranged 

 so as to be withdrawn at any time, and the boiler most 

 thoroughly examined ; it may be here remarked that in good 

 practice boilers are now never set upon a central midfcather 

 wall, which has often set up a lino of ooirosion along each 

 side, and which also acts against uniformity of heating. 



The freedom from priming, which is a happy experience 

 of the! use of these boilers, is caused in a great measui'C by the 

 ample water spaces wliich are so easily provided, and which 

 allow the greatest scope to the currents set up by convec- 

 tion ; the moi'o or less complete absence of this facility in 

 boilers of other types causes trouble of which the source is 

 often unsuspected. The comparatively large steam space 

 also tends to promote the production of dry steam, but in 

 this respect perhaps it would be too mucli to claim special 

 advantages, as many other types of boiler are well provided 

 on this point. It may bo here remai'ked, that if every boiler 

 made were to be provided with a slit or perforated pipe for 

 takinc off dry steam, much ti'ouble would be avoided. 



Little need bo said as to the material of which boilers 

 generally are made, as this is becoming more and moi'e 

 exclusively steel instead of iron. Steel is now made so 

 excellent and reliable in nature, that it may be treated 

 almost anyhow, except at one particular temperature — at and 



Q 



