208 
OF THE PARTS OF 
[sect. VII. 
its friction from the pressure of the steam against the box is considerable ; but in 
order to reduce it, the rubbing surfaces should not be too small, and the harder 
they are the better: for steam boats, gun metal is used; but where salt water is 
not to be employed, the sliding parts which apply together may be made of steel, 
and hardened ; they then act and wear extremely well. 
447. Murdoch’s slides. In slides formed in the preceding manner there 
is a loss of steam, in consequence of the apertures being opened and closed at some 
distance from the places where the steam immediately enters the cylinder. This 
has been avoided in Messrs. Boulton and Watt’s engines, where they have used 
similar slides invented by Murdoch, in which the strong steam is in the place 
assigned by Murray to the weak ; and in engines with a long stroke, they make 
the two sliders separate, and move them by a rod of communication ; because it 
would be more difficult to fit a long slide, so that there would be a certainty of 
its rubbing surfaces being in complete contact, as the least deviation of these 
sliders, whether at the top or bottom of the cylinder, would cause a considerable 
leakage. Maudslay also, in his later boat engines, has adopted the same arrange 
ment of slides as Boulton and Watt. See Fig. 2. Plate iv. 1 
448. Slides are getting into considerable repute for many purposes, and even 
in appearance the intricacy of a double engine is much diminished by using them. 
The contrivance of the slide to shut off the steam at any portion of the stroke, 
is a point of some importance. Mr. Millington justly esteems the want of the 
power to do so a defect, and says it is common to the slide and four-passaged cocks ; 2 
but this objection may be removed in both cases by increasing the quantity of 
motion of the sliding surfaces one-half. For this purpose the slide should be the 
depth of the aperture shorter than will cover both the apertures to the cylinder, 
(see Figs. 1, 2, and 3. Plate v.) and it should be moved twice during the stroke 
by an adjustable tappet: the first motion shuts off* the steam, as in Fig. 2; the 
second opens the passage to the condenser, and admits the steam at the other end. 
In this case let F and D represent the passages to the cylinder, S the place where 
the steam enters, and E the passage to the condenser. Suppose the steam to have 
been admitted to the upper part of the cylinder by the passage F, Fig. 1. and the 
slide to have been moved its first motion in Fig 2. so as to cover F, and still leave 
D open to the condenser; then, at the next movement, Fig. 3. the slide will be 
at the bottom and admit steam at D, and F will be open to the condenser. The 
stefim should encircle the pipe E; it then does not increase the friction materially 
by its pressure. 
1 Messrs. Maudslay and Co., in the first instance, used a four-way cock. 
2 Epitome of Natural Philosophy, p. 313.