212 OF THE PARTS OF [SECT. vn. 



457. BRAMAH'S FOUR-WAY COCK. In the common one the pressure being wholly 

 against the side of the conical plug, its wear is unequal and friction considerable : 

 to remove these, the conic frustrum is formed on a cylindric axis, and the steam 

 is admitted upon its larger end, by which the pressure on the seat is nearly 

 equalized ; and by turning in the same direction constantly, the wear is equalized, 

 notwithstanding the inequality of pressure. 



These cocks, with some deviations, have been very much employed by Mr. 

 Maudslay in small engines ; and an example of their application to his portable 

 engine is shown in Plate xv. and the parts to a larger scale in Plate vm. In the 

 plan, C is the cylinder, I the four-way cock, and E the pipe by which the steam 

 enters. The cock is represented with all the apertures shut ; but the figure above 

 the plan is a section through the cock. The steam enters at E, flows over the top 

 of the cock, and by an aperture G in the top it passes either to the top or to 

 the bottom of the cylinder, according as the aperture in the side of the cone is 

 turned to the one or the other of these passages. 



By comparing the effect of turning the cock to the right or left from the 

 position it has on the plan, the manner of opening and closing the passages will 

 be obvious. The higher passage leads to the condenser, (marked F in the two 

 sections,) the middle one to the top, and the lower one A to the bottom of the 

 cylinder. If the cock be turned to the right, so that the opening in the triangular 

 aperture through which the steam descends from the top is opposite the middle 

 passage, then the steam will pass to the top, and the condensed vapour will have 

 a passage open from the bottom to -the condenser, through the body of the cone. 

 If the cock be turned to the left, the centre of the triangular passage will become 

 opposite the passage to the bottom of the cylinder, and the steam will pass in that 

 direction, and a passage from the top to the condenser will be open through the 

 body of the cock. In this cock the motion is back and forwards. 



The escape of steam at the lower part of the cone is prevented by a packing 

 of hemp round the cylindric part, and the cylindric part of the top is pressed by 

 a spiral spring, with an oil cup H, and screw above it to act on the spring, if 

 occasion requires. 



The pressure and friction of this cock will not be greater than that of a slide, 

 if both be equally well executed. The loss of steam in the passages is an objection, 

 and the steam cannot be shut off without closing the passage to the condenser ; 

 this however is in some degree compensated for by the application of Field's 

 valve. See Plate xv. 



458. FOUR-WAY COCK to cut off the steam at any portion of the stroke. The mode 

 by which this may be done, is to make the cock so much larger that there will 

 be the breadth of two apertures between the middle and each adjoining passage. 

 The diameter will be increased only in the ratio of 8 to 10 ; the rubbing surfaces 



