PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 417 



The iron covering 1, wliich is applied upon the masonry 2, around the 

 base of the turret M, is very strong, and is held down by bolts not repre- 

 sented. A heavy flange m, extends outward from the base of the turret M, 

 and stands under the inner edge of the iron covering 1. This guards the 

 turret from being overturned by any chance, or by any severe recoil of its 

 guns. 



The blower 3, is operated vigorously by the crank 4, during an action, 

 and by drawing in air through the pipe*5, induces a plus pressure in the 

 entire structure. This pressure induces an upward flow of air through the 

 cylinder P F, and turret M, the air finally escaping through apertures in 

 the top of the turret. 



Therens an intermediate floor 6, in the cylinder P, and a ladder for mov- 

 ing up and down therein, may be arranged in any obvious manner. 



Sight holes J J, are provided as represented, which may be furnished 

 with mirrors, and other approved appliances, if preferred. 



Figure 42 is a plain view, showing one of the rollers L, on which the 

 turret 31, turns, and the rings L^, L"^, which connect the rollers, and compel 

 them to maintain uniform distances, each from the other. 



When it becomes necessary, from any cause, to take out or replace one 

 of the rollers L, I have provided means of doing so, without lifting the tur- 

 ret M. Figure 43, is a side view of one part of the circular track 3P, 

 where this is accomplished. A slight shaft, or guide spindle I, is inserted 

 through each roller L, and the outer end is provided with a T-shaped pai't 

 1 1, which is bolted to the outer ring L^, as represented. 



To take out a roller, I turn the turret until the defective roller comes to 

 the position shown in dotted outline, in Figure 43. I then remove all the 

 bolts which hold the several parts Z^, to the outer ring L'^, and raise or 

 lower L^ out of the way. I then remove the nut which holds the defective 

 roller to the inner ring L^, remove the guide spindle I, and the defective 

 roller is then free, except that it is still supporting its share of the turret, 

 and consequently cannot be drawn out, being prevented by the flange on 

 its inner end, as also b}^ the friction due to the great load resting on it. 



I next remove the small portion 3f^, from the circular track M^, it being 

 made in a piece separate from the other parts, to allow its removal for this 

 purpose. I then turn the turret until the defective roller is brought over 

 the place where this piece il/^ stood, as shown by the dark lines. The 

 previous removal of the piece M^, now allows the defective roller to be 

 readily withdrawn, because it readily falls down by gravity out of contact 

 with the turret M, leaving all of the weight to be supported by the remain- 

 ing rollers; and the space made by the removal of 31^, is sufficiently wide 

 and deep to allow the roller to be removed and a new one substituted; 

 after which a reversal of the process, above described, makes all again 

 complete. Figure 43" shows a front elevation of spindle 7, showing slight 

 projections P, which hold l^ in horizontal position, l^ being removed. 



There must, of course, be a suitable passage, at the point required, 

 through the masonry, to allow access to the place in the circular track, 

 where this operation is performed. 



The fort above described was designed for New York harbor, to be 

 erected in the center of the river at the Narrows, and for other like places. 



TAm. Ins.1 • 21 



