SECT, vii.] STEAM ENGINES. 225 



full force to open the valve, but as soon as it begins to move, it is retarded by the 

 water, till it be finally stopped. During the ascent, a valve opens inwardly at 

 the bottom of the vessel, and therefore the engine has not more than the weight to 

 raise again. 



In engines for raising water this mode of opening valves has always been fol- 

 lowed. The difficulty of opening large valves was probably the cause of its 

 introduction, and the ingenuity of its mechanism has preserved it in use ; but I 

 think there will be an advantage both in simplicity and effect, to let the motion 

 of the plug tree act directly on the valves, as shown in art. 482 : the tappet by 

 which the steam is shut off should be capable of considerable range, whether for 

 adjusting by hand or by a self-acting apparatus. (See art. 554.) 



479. In an engine having a fly, it is esteemed better to apply an eccentric 

 wheel within a hoop upon the fly-wheel shaft, and this by its evolution alternately 

 pushes and draws a rod connected to the hoop, and thus gives motion to the 

 valves, cocks, or slides. Such an apparatus is shown in Plate xv. Fig. 2. in 

 which N is a cross section of the fly-wheel shaft, and k the eccentric wheel fixed 

 upon and revolving with it ; a circular hoop of metal encompasses the eccentric 

 wheel in such a manner as to permit its turning round, and from this hoop the 

 arm i projects, and it is braced to increase its strength. It terminates in an arm 

 upon a centre, which by a second arm gives motion to the rod /, and causes 

 another axis to move, which, by a pair of bevelled wheels, moves the cock of the 

 engine partly round upon its axis n, and back again. The advantage of an 

 eccentric wheel is the easy changes of motion it makes ; for being constantly 

 moving, it gives no stroke at the times of change ; and in large engines part of 

 the weight of the eccentric apparatus is balanced by a weight, so that there is 

 only a slight pressure on the shaft. (See Plate xix.) 



Let r be the radius of the eccentric circle, and a the distance of its centre from 

 the centre of motion ; then (r + a) (r a) will be the extent of the movement, 

 = 2 a, or twice the eccentricity ; and in any other position the place counted from 

 the centre will be a cos. a where a is the angle between the centres, whose cosine 

 is equal to the horizontal distance. When they are in a vertical line, = 90, 

 and cos. = 0, the distance is 0, and this corresponds to the termination of the 

 stroke. Now we know from the nature of the circle that the cosines increase 

 rapidly at first in departing from the angle of 90 ; but at one-sixth of the stroke 

 counted from either end of it, a valve, slide, or cock, can be only half way 

 opened, and unless its motion be greater than that required to open it, the time 

 it will be about fully open, will be only one-ninth part of the stroke. 



480. Eccentric rollers to raise the valve rods have the same defect ; but the 

 application is ingenious. Conceive the shaft Y, Fig. 1. Plate vm. to be kept in 



2F 



