APPLIED MECHANICS. 



[OOMBIXED EXGI.VKS. 



imp- 

 il j is 



rd *, which pane* through a stuff. 

 cover ; ami the lower end of 



this trunk. The water discharged 



:<:<* by a pipe n into a <; nee a 



- drawn by the feed-pump for the supply "f the 



boiler, the rest escai>in^ by the discharge-pipe <>, which 



U carried through the side of tho vessel above the level 



water : ;> is the hand-lever, by wlii' 



engiue-drivi-r *'<]<, starts, or reverses; it is connected 

 by a rod q and bell-crank lever, with tho link motion 



When the lever p is held 



in its middle position by a catch dropping into a notch 



on an arc against which it sli> ;ines remain at 



rest ; when it is raited upwards, or depressed to either 



he engines work at full speed, 



r ahead or astern ; and \vhrn it U set at either of 



the intermediate notcln s, they work at half speed, or are 



toted in either ditvction. 



Many other form* of engine for screw-propulsion 

 might be described ; but as they all, more or less, dis- 

 play the same general features, we do not think it neces- 

 sary, in our limited space, to cuter on a full description 

 of their d. 



ro.Mr.INKl> KXtJIXES are those which embody the 

 principles of both tho condensing and non-condensing 

 engines. In these, a small cylinder and large cylinder are 

 fd side by side, and their piston-rods are connected so 

 as to work together. High-pressure steam admitted to the 

 small cylinder, works its piston as in a non-conde; 

 engine ; but, instead of being permitted to escape into 

 th atmosphere, is conveyed to the larger cylinder, in 

 which it expands to greater volume but lower pressure, 

 and acts on a greater area of piston. Thence the ex- 

 panded steam Hows into a condenser, as in the ordinary 

 condensing engine. 



This arrangement gives the opportunity of securing a 

 large amount of useful effect from the steam, and th 

 economising fuel in the boiler. If we M .rex- 



ample, that steam with an absolute pressure of 7."' Ibs. 

 is admitted into the smaller cylinder during Jth of the 

 stroke, and allowed to expand during the remainder of 

 the stroke, its tinal pressure, allowing for little loss by 

 cooling, will be about 18 Ibs., and the mean piv 

 throughout tho stroke will be about 42 Ibs. The steam 

 at 18 Ibs. being now admitted into the larger cylinder 

 during A the stroke, will have a final pressure of about 

 8 Ibs., and a mean pressure of about 13 Ibs. throughout 

 the stroke. The back pressure of rarefied air and vapour 

 in tho condenser may average 2 Ibs. throughout the 

 stroke, and the mean effective pressure on the larger 

 n would then be 13 2=11 Ibs., while that on tlie 

 smaller is 42 13=29 Ibs. per square inch. Now, if the 

 area of the small piston be to that of the large in the 

 proportion of ll to l".i, the total mean pressure oil each 

 will be alike ; and the power of the engine will bedouMe 

 that of either, with an expenditure of steam at 75 Ibs. 

 pressure, or CO Ibs. above that of the atmosphere, only 

 sufficient to fill Jth of the capacity of tho smaller cylinder. 

 titling off at an earlier period of the stroke, and 

 using a still larger second cylinder, still greater economy 

 of steam may be attained. 



In d. it miming the power of a condensing engine, it 

 it necessary to know tho pressure of the steam employed, 

 the amount of vacuum produced in tho condenser, the 

 size ami velocity of the piston, and tho various losses of 

 force occasioned by the friction of the machinery, and 

 the resistance of tho air-pump, fird-pump, ami cold- 

 water-pump. Assuiniiigth.it, Keneially, the speed of tho 

 piston U about -i*i f. ' per minute, and that the pres- 

 sure in tlie boiler doi'i not exceed 5 or 6 Ibs. per *\\ 

 inch alK.ve that of the atmosphere, we may take the 

 ing rule as to power : 



diameter of the cylinder to find the power. 

 -From the diameter (in inches) subtract 6, 



r, and divide by 20 for the power. 

 ,./,-. \\li.-tt in the jKiwer of a condensing engine 

 having a cyhnd. -i diameter I 



64-8-48, 





-115 Lome-power. 



; io power to find the diameter of tho cylinder. 



: add 



the diameter in inches. 



'///!. Required the diameter of cylinder for 115 

 horse-power. 



20 x 115=2300, square-root -48 nearly, and 48 + 

 =54 inches. 



These, however, merely furnish rough guesses at the 

 power of any engine. The proper method of oscertai 

 the real power is to apply the indicator, when i 

 is only moving itself, so as to ascertain the power 

 necessary to overcome the friction and resistance of its 

 working parts ; and to deduct this quantity, with some 

 allowance for additional strain, from tho power indi< 

 when the engine is in full work. No definite nil 

 be offered for estimating these allowances, as varieti< 

 construction and workmanship introduce great differences 

 among experimental results. In general, it is safe to 

 reckon not more than jths of the indicated power as 

 really effective to move machinery, the remaining Jth 

 being absorbed in friction and moving the pumps. 



ROTARY KNGIXKS. In all steam-engines, except 

 those that are single acting, a good deal of mechanism 

 is necessary in order to convert the reciprocating 

 tilineal motion of the piston into the continuous circular 

 motion of the crank. It has, therefore, bei 

 object with many mechanics to devise a rotary engine, 

 or one in which the steam pressure shall at once give 

 the required rotary motion, without the in 

 machinery of connecting-rod, beam, parallel motion, and 

 crank. Many of the arrangements devised for this pur- 

 present great ingenuity, and it is not improbable 

 that some may prove ultimately successful ; but hitherto 

 no rotary engine has proved so far satisfactory as to 

 warrant its adoption in the place of those already in use. 



A singular error regarding rotary steam-engines has 

 crept into some of tho best books on the subject ; ami 

 as it is calculated to discourage inventors who may apply 

 themselves to devising engines of this kind, we will 

 endeavour to point it out. It is asserted that st 

 applied to give direct rotary motion to a piston is less 

 etlective, or loses part of the power which it would pro- 

 duce if applied to move a piston in a straight line. If 

 we suppose ED (Fig. 210) to be a piston iitted to an 

 annular cylindrical vessel B G A F, so that it may be 

 Fit. 2101 

 F 



moved ronnd the centre C by the pressure of steam ad- 

 mitted behind it, we have to ascertain whether a cci taiii 

 quantity of steam applied to a piston thus arr.ii 

 will generate as much power as it would produce when, 

 applied to a piston in tho ordinary way. Miors 



(among them Tredgold, in his largo work on the steam- 

 engine) say that "the quantities of steam being equal, 

 the power of rotary action will bo lexs than that of rec- 

 tilineal action ;" and this fallacy is but too generally 

 admitted among engineers. That it is a : y be 



very easily shown by a practical example. Let us sup- 



