208 OF THE PARTS OF [SECT. vn. 



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. l 



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 ;* 

 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 

 steam 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. 

 Epitome of Natural Philosophy, p. 313. 



