THE POPULAR EDUCATOR. 



Trill, however, show us that there are two points at which the 

 crank has no power : these are when the piston is at the top or i 

 the bottom of the cylinder. At thoso times I and K are in one 

 straight line, and the whole pressure of the beam is therefore 

 transferred directly to the axle instead of tending to move the 

 wheel. Thoso positions are called the "dead points "of the 

 stroke, and would cause the motion of the engine to cease 

 altogether were there no means of overcoming the difficulty. 

 After it has passed the dead point the crank acts with increas- 

 ing advantage, until it comes at right angles to I, and then its 

 power diminishes again to the other dead point. This tends to 

 render the motion very irregular, but the contrivance known as 

 a fly-wheel quite corrects this. A largo wheel, v, with a very 

 heavy rim, is fixed on the crank axis. When the crank is in its 

 most favourable position it tends to accelerate the motion of 

 this wheel. On account, however, of its great mass a very 

 minute increase of velocity is produced, the power being 

 absorbed and stored up in the wheel ; the momentum thus 

 acquired is sufficient to carry it over the dead points, and to 

 render the motion of the engine pradtically uniform. The 

 machinery may be driven by a strap passing round the fly- 

 wheel, or in any other convenient way. 



WG must now just explain the remaining parts of the engine. 

 o is the condenser, into which cold water is injected by the 

 pump K, through the pipe T. This water, together with the 

 condensed steam, is removed by the air-pump M, and emptied 

 into the hot well N. Q is the feed force-pump, by which the 

 water is pumped from N into the boiler, through the pipe s. 

 The rarious pumps are worked by the rods, F, a, H, fixed on 

 suitable parts of the beam. In practice the arrangement of the 

 pumps seen in the lower part of the figure is often considerably 

 modified to suit the circumstances of the case ; but the changes 

 are not of much importance. 



Thus far wo have only considered low-pressure, or condensing 

 engines. Those which work with steam at a higher pressure, 

 and do not condense, are, however, very frequently used, as, 

 though less economical in use, they are much simpler and 

 cheaper. In the former there is steam at a low pressure on one 

 side of the piston, and a vacuum on the other ; in the latter, 

 the steam presses on one side of the piston, while the other is 

 open to the air. This pressure therefore opposes the movement 

 of the piston, and causes a considerable loss of power. As will 

 be seen by reference to Fig. 9, much of the mechanism there 

 shown is required for the purpose of condensing the steam, 

 a constant supply of cold water has also to be provided. In 

 many cases these requirements cannot be met, and they nearly 

 always cause inconvenience, and hence non-condensing engines 

 are now very generally employed. The mechanism by which 

 the piston is moved in the cylinder, and its motion transferred 

 to the beam and fly-wheel, is substantially the same in this as in 

 the engine already described. Instead, however, of the waste 

 steam passing into the condenser, it escapes by a suitable pipe 

 into the air. 



Frequently, however, before doing this, it is employed in 

 Some other way for heating purposes, so as to reduce the waste 

 of heat as much as possible. In nearly all cases the beam is 

 altogether dispensed with, the cylinder being so placed that the 

 piston-rod is attached directly to the crank-shaft. The cylinder 

 is very frequently laid on its side, and in this way the engine is 

 rendered very much more compact and convenient. 



In marine engines two or three cylinders are not unfre- 

 qncntly used to drive the same shaft. In this case .the cranks 

 are placed in different positions, so that one shall be exerting 

 its maximum power while another is at the dead point. Some 

 of the cylinders in such cases are frequently mounted on pivots 

 so as to oscillate, and thus impart motion more directly to the 

 crank. In large steam-vessels there are very frequently three 

 cylinders, the middle one being made on this principle. 



When a number of machines are being driven by one engine, 

 ono or other of them is very frequently stopped for a time, and 

 the immediate result of this is to diminish the load of the 

 engine, and therefore to accelerate its speed. In a similar way 

 changes in the pressure of steam alter its rate, and in many 

 instances these fluctuations cause great inconvenience. A 

 throttle-valve was first placed in the steam-pipe to remedy the 

 difficulty. This consisted of a circular disc mounted on an axis 

 passing across its centre, and just fitting the interior of the 

 pipe, When this diao was vertical the steam was completely 



shut off, but when it was horizontal little obstruction was 

 offered to its passage. The valve, however, had to be adjusted 

 by hand, and a self-acting regulator was much needed. This 

 was at length supplied by the invention of the governor-balls, 

 which are represented in Fig. 11. 



H I is an axis turning in two bearings. A sheave, A, is placed 

 on its lower end, and motion is imparted to it by a cord or 

 chain passing round this and also round the axle of the fly- 

 wheel, or some other convenient portion of the machinery. 

 Two heavy balls, B, B, are fixed to the end of bent levers, which 

 turn on pivots at c. Small bars, K, K, are hinged to the upper 

 ends of these, and then connected with the loose collar, D. ' 

 When the speed of the engine increases, the spindle rotates 

 more rapidly, and the balls are accordingly thrown wider apart 

 by centrifugal force. By this means the loose collar D is forced 

 up, and carries with it one end of the rod D r, which turns on 

 a pivot at E. The other end of this moves the throttle- valve G, 

 and thus diminishes the influx of the steam, and checks the 

 spaed of the engine. When, on the other hand, the engine 

 moves more slowly, the balls fall, and thus open the valve 

 wider, and in this way the motion of the engine is maintained 

 very nearly uniform. 



The construction of a locomotive is not essentially different 

 from that of a stationary engine. The cylinders are usually 

 horizontal, and the piston-rods are connected with the driving- 

 wheels. A fly-wheel is, of course, unnecessary, as the momen- 

 tum of the engine carries it over the dead points. Ono impor- 

 tant adjunct is known as the link-gearing, and serves to reverse 

 the engine. The direction in which the driving-wheel moves 

 depends upon the part of the stroke at which the steam is 

 allowed to enter either end of the cylinder. Two eccentrics are 

 therefore fixed to the axle, one arranged to drive the engine 

 forwards, the other backwards, and by the link-gearing tho 

 slide-valve is connected with whichever of these we please. 



Before concluding this brief sketch of tho steam-engine, we 

 must just notice the way in which its power is usually 

 described. It is always said to have a certain horse-power. 

 This was at first used very vaguely, but a definite meaning has 

 now been attached to it ; and when wo speak of an engine of 

 one horse-power, wo mean one capable of producing, under 

 ordinary circumstances, an effect equivalent to raising 33,0001bs. 

 a foot high per minuto. One of 5 horse-power would, of courso, 

 accomplish five times this duty. By working with steam of a 

 greater pressure a larger amount of work may be obtained. 



It is found that to produce the effect of ono horse-power, tho 

 boiler should evaporate nearly a cubic foot of water per hour. 

 When, therefore, we speak of a boiler of a given horse-power, 

 we mean that it is capable of evaporating that number of cubio 

 feet of water per hour. 



LESSONS IN LATIN. LVIII. 



COLLOCATION OF WOBDS. 



BY the collocation (con and locus) of words is meant the way or 

 order in which words are placed or stand in relation to each 

 other. That order may be determined, or, if not determined, 

 may bo affected by three causes : first, the sequence of ideas ; 

 secondly, the grammatical construction ; thirdly, the laws of 

 euphony (Greek, tv, eu,well; and fyutvj), fo'-ne, voice, or pleasing 

 sound). To a greater or less degree, and in varying propor- 

 tions, these three causes operate on collocation in all languages ; 

 since a regard to the order of ideas, a regard to the require- 

 ments of grammar, and a regard to the laws of sound, aro 

 universal principles of cultivated speech. In one language, 

 however, construction may predominate over tho sequence of 

 thought ; in another the sequence of thought may prevail more 

 than the usages of construction. This position is exemplified 

 in the English and the Latin ; for in the English, construction 

 has the more force ; in the Latin, the order of the thoughts. 

 Take such a sentence as James gave John a book; hero the con- 

 struction shows that James is the subject and John the remote 

 object to the verb gave. Change the place of the two proper 

 names, John gave James a book, and you have reversed the 

 sense. Hence you learn that in English the sense of words 

 depends very much on their position. 



A great and strict regard to usages relative to position ia 

 necessitated in English, because our nouns are only partially 



