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, a = 90°, 
and cos. a = 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 
2 F