Firth. — On Felting Steam Boilers. 33 



boiler working at these high, pressures should be so protected that heat and 

 steam may not be wasted by radiation or condensation. 



It is true that common felt cased with bricks will prevent much conden- 

 sation and radiation, but with the serious disadvantage that a leakage from a 

 rivet, or otherwise, causes rapid corrosion when running unobserved under the 

 brick casing, and with the further disadvantage that the felt is destroyed in a 

 very short time. 



Various materials, siich as asbestos, cloth, or fabrics saturated with 

 chemical preparations, have been tried, but, so far as my investigations have 

 gone, much the best material yet discovered is common felt. 



The proper application of felt has been, and I believe still is, the real 

 diflB.culty. Applied in contact with the surface of a steam boiler at even 

 50Ibs. pressure, felt will need replacing about once in six months. About 

 two years ago, to lessen this destruction of felt, I made a species of hurdles or 

 gridirons of common hoop-iron, with wooden battens of 1 in. thick rivetted to 

 the hoop-iron. These were placed upon the boiler, and the sheets of felt laid 

 upon them, the upper surface of the felt being protected by canvas. This 

 plan secured a small space between the boiler and the felt, but, though a great 

 improvement upon the old plan, I found that in the course of about fifteen 

 months the wooden battens had become charred and the felt a stratum of dust, 

 slightly adhering, indeed, to the canvas back if undisturbed, but practically 

 useless. Both substances had simply been destroyed, as before, by too close a 

 contact with the boiler. 



A very simple contrivance now presented itself to my mind, which I 

 immediately put in operation. I constructed an iron grid as before, but with 

 one important difierence. I placed pieces of hoop-iron aa (Figs. 1 and 2) at 

 10 in. distance, to lay on the circumference of the exposed portion of the 

 boiler. I then prepared transverse pieces of hoop-iron ebb (Figs. 1 and 2), 

 putting two double cranks in each cc (Fig. 1) 2^ in. high x 2|^ in, wide. 

 I placed these transverse pieces at 10 in. distance, and rivetted each of the 

 cranks at d (Fig. 1) to the pieces of hoop-iron intended to lay on the circum- 

 ference of the boiler. When cranked, the transverse pieces were 21 in. long, 

 about the width of an ordinary sheet of felt. Upon these cranked pieces I 

 placed wooden battens 2 in. broad by l-|in. thick ee (Figs. 1 and 2), screwing 

 them together at fff (Fig. 1). This completed the hurdle, or grid, 21 in. 

 wide, and of sufficient length to lay across the boiler from side to side. 

 I next provided sheets of felt long enough to cover each grid, sewing each 

 sheet to strong canvas 24 in. wide, thus leaving at one side a margin of canvas 

 to lay over the sheet of felt on the adjoining gi'id. The 4 in. air space (which 

 may be increased at pleasure by increasing size of cranks) between the boiler 

 and felt, besides preventing all cliarring of wood or felt, is an excellent 



