Slide Valve. An inspection of Table III shows us the effect of increasing the expansion by the slide-valve in augmenting the loss of power occasioned by the imperfect action of the eduction passages. Referring to the bottom line of the Table, we see that the eduction passage before the piston is closed, and that behind it opened, (thus destroying the whole moving power of the engine.) when the piston is .092 from the end of its stroke, the steam being cut off at from the end. Whereas, if the steam is only cut off at 1/ from the end of the stroke, the moving power is not withdrawn till only .011 of the stroke remains uncompleted. It will also be observed that increasing the cover on the exhausting side has the effect of retaining the motion of the steam longer behind the piston, but it at the same time causes the eduction port before it to be closed sooner. A very cursory examination of the action of the slide-valve is sufficient to show that the cover on the steam side should always be greater than on the exhausting side. If they are equal, the steam would be admitted on one side of the piston at the same time that it was allowed to escape from the other ; but universal experience has shown that when this is the case a very considerable part of the power of the engine is destroyed by the resistance opposed to the piston, by the exhausting steam not getting away to the condenser with sufficient rapidity. Hence we see the necessity of the cover on the exhausting side being always less than the cover on the steam side ; and the difference should be the greater the higher the velocity of the piston is intended to be, because the quicker the piston moves the passage for the waste steam requires to be the larger, so as to admit of its getting away to the condenser with as great rapidity as possible. In locomotive or other engines, where it is not wished to expand the steam in the cylinder at all, the slide-valve is sometimes made with very little cover on the steam side ; and in these circumstances, in order to get a sufficient difference between the cover on the steam and exhausting sides of the valve, it may be necessary not only to take away all the cover on the exhausting side, but to take off still more, so as to make both exhausting passages be, in some degree, open, when the valve is at the middle of its stroke. This, accordingly, is sometimes done in such circumstances as we have described; but, when there is even a small degree of cover on the steam side, this plan of taking more than all the cover off the exhausting side ought never to be resorted to, as it can serve no good purpose, and will materially increase an evil we have already explained ; vis. the opening of the exhausting-port behind the piston before the stroke is nearly completed. The tables apply equally to the common short slide three-ported valves and to the long D valves. We here introduce the boilers of the noted steam vessels Her Majesty and Royal Consort. These boilers supply a pair of engines with steam, the cylinders of which are 65 inches diameter, and the stroke 3 feet 6 inches. There are four boilers in each vessel, fired at opposite ends, 13 inches separate in the direction of the vessel's length, and 2 feet 6 inches asunder athwart-ships. The flues of the four boilers join so as to form the foot of the funnel, which is .S feet 6 inches diameter, and thus make it commence in the boiler instead of making it commence clear of the which is often done by maintaining the flues separate and distinct. Each boiler is 9 feet 1 IJ inches in length, and 9 feet inches in breadth. bright to the top of the steam space is 1 1 feet 9 inches, and the length the tubes is 6 feet 2 inches, the diameter 3 inches, and 4( inches distant ' centre to centre. The number of tabes is 160 in each. The bottom water- spaces are stayed with fifteen two-inch rivet-headed bolts and Boti, five two-inch stays secured by nuts pass through across the boiler, and the furnace and tubes. The water-space at the inner side of each boiler is
Fig 163.
Fig 164.
