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or left hand, the steam pressure does not vary; hut, in the first case, the piston is descending; in the second it is ascending.
3. If the line proceeds obliquely to the right upwards, or to the right downwards; in the first case the steam pressure is increasing and the piston is descending; in the other case, the pressure is decreasing and the piston descends.
4. If the line proceeds obliquely to the left downwards or to the left upwards; in the first case the pressure is decreasing and the piston ascends; in the other case, the pressure is increasing and the piston ascends.
45. The indicator may also be used for ascertaining particulars of some of the internal parts of the engine without actual inspection; for example, it may serve for the formation of the "slide diagram," in which the string that turns the barrel of the indicator is fixed to the cross-head of the slide, instead of that of the piston, and thus the index shows on paper the various positions of the slide. It will indicate if the slides are properly set, or leaky, if the steam ports are of the proper size, &c. &c. &c, for which the reader is referred to Mr. Main's interesting work on that subject."
The scale of the indicator for high pressure (atmospheric) engines should be made to extend considerably above the atmospheric line, but it need not of course go below it.
When the Russian war steamer 'Wladimir' was tried in the Thames in 1848 the average pressure of steam on the piston by the indicator was 20-275 (— 1 for friction) = 19 • 2 75 in each of the two cylinders. Hence the surface of each piston being 4214 square inches, the length of stroke 6 feet, and the number of revolutions l9 per minute, the indicated horse power was for both engines ll22-38: thus—
a The reader will find a detailed account, illustrated by a good engraving, of this beautiful instrument in Mr. Main's treatise on the Marine Steam Engine already noticed, and which instrument, that distinguished author has well said, is, in the hands of a skilful engineer, to the steam engineer as the stethoscope of the physician; disclosing at any instant, and under any circumstances, the actual power of the engine, revealing the secret workings of the whole inner system, and detecting minute derangements in parts obscurely situated.
42l4 x l9-275 x 6 x 38 ..
= 56l . l9. One engine;
56l-l9 X 2 = ll22-38 for the two engines."
46. The second instrument above alluded to is called a Dynamometer: when employed to measure the pressure on a screw-shaft, it shows the power of the engine to propel the ship; and this object is obtained by means of a lever, simple or compound.
In the revolution of the screw the reaction of the water against its surface, supposed to be resolved in the direction of the shaft or axle, produces a pressure in that direction and, consequently, propels the ship; and its intensity is a measure of the power of the steam engine. The end of the screw-shaft presses, through the intervention of a pin, against a knife edge, on a lever, which it may be sufficient to consider as simple; this lever is in a vertical position, with its lower extremity fixed by a joint, as a fulcrum, to an immoveable object (the plomer block) in the ship. The opposite end of the lever is connected, by a joint, with one end of a rod in a horizontal position, the opposite end being attached to a spring balance; this rod is also attached at the same end to another rod which is parallel to it, and carries a pencil. A cylindrical barrel, whose axis is parallel to this rod, is made to revolve, with the screw-propeller, by means of straps going over pulleys, so that the revolution of the barrel may be made quicker or slower at pleasure; on the convex surface of this barrel the pencil traces a line, straight or curved, as described in the account of the indicator.
* The following empirical formula is given by Mr. Roughton for calculating, with a near approximation to the truth, the speed of paddle-wheel steamers, one of the data being obtained from the Indicator :—
l -8d*fsn\ . ,. ,
1 > : in which
\ w -B -D )
v expresses the required speed in knots per hour,
47. The spring balance is provided with an index and a graduated scale; and the point at which the index stands when the dynamometer is disconnected from the screw-shaft is the zero of the scale, and a circle described about the barrel in a plane perpendicular to its axis, and through this point, is called the zero line. The instrument being connected with the screw-shaft, the revolutions of the barrel cause the pencil to describe an undulating curve on one side of the zero line; and the ordinates of the curve, being measured on a scale of pounds formed on the strength of the spring balance, give the number of pounds which denote the pressure of the lever on the spring of the balance; a mean of all these is to be taken, and this being multiplied by the number expressing the ratio of the whole length of the lever to the distance of the fulcrum from the end of the screw-shaft, the product will be the pressure in pounds exerted by the screw-shaft on the dynamometer, and consequently, in the same (horizontal) direction, on the vessel. The pressure, in pounds, thus exerted, being multiplied by the velocity of the ship in feet per minute, the product will be the number of dynamical units in the effective power of the engine; and this, divided by 33,000, will express the horse power.
The difference, if any, between this last and the horse power obtained by the indicator, expresses (in horse power) the loss of power in consequence of friction, resistance, &c.
From the results of trials with these instruments it has been found that the , speed of a steam vessel varies directly with the square root of the pressure on the piston and with the cube root of the horse power of the engine. The useful effect of the engine, meaning that which remains after deducting the power spent in overcoming friction, &c, is supposed to bear a constant ratio to the power developed in the cylinder commonlycalled the indicated horse power. It is further estimated that the speed of a vessel varies directly with the cube root of the horse power, and inversely with the area of the mid-ship section; or, directly with the cube root of the horse power, and inversely with the cube root of the square of the vessel's displacement.
48. The term horse-power, where used to designate the registered horse-power, gives but a remote idea of what the capabilities of the engine really are, and differs from the term horse-power as originally used, which showed the actual power exerted by the engine ; it now merely serves to estimate approximatively the money to be paid for the engine, and by no means shows the actual amount of working horse-power. Mr. Atherton, in his work 'Steam-ship Capabilities,' shows this to be the case, and proves from a comparison of the nominal horse-power with the power actually produced in ten packets, that a marine horse-power may be represented by a pressure on the piston equivalent to l32,000 lbs. moving at the rate of one foot per minute. He therefore proposes to make that the unit of power expressed by the word horsepower in all cases, whether nominal or effective, as shown by a dynamometer.
49. Captain Ryder also, in his valuable work 'On the Economy of Fuel in Steam Engines,' shows that the known results of working steam expansively at a higher pressure than 7 lbs. on each square inch, the mechanism of the engine being at the same time in the best state of efficiency, should deter engineers from using the vague term nominal horse-power. The value of that power was, at the time of its introduction, obtained from the mean effective pressure in the engines of that day; but it is far inferior to the value deduced from the greater and more efficient engines at present in use.
50. The circumstances which immediately led to the introduction of the screw in the steam-vessels of the Royal Navy were the competitive trials made in 1840 between the 'Archimedes' screw-ship, which had been built in 1838, and the wheel-steamer 'Widgeon.' In the first of these trials, four runs of 19 miles, from Dover to Calais, and as many in return, during calm weather, the 'Widgeon' accomplished the distance on an average, in 5' 50" less time than the 'Archimedes;' but in a run to Calais and the return, with a fresh breeze and all sails set, the 'Archimedes' accomplished the distance on an average, in 7' 30" less time than the 'Widgeon.' These trials were, however, far from being decisive, and the ships were not well matched, as the steam-power of the 'Archimedes' was less than that of the 'Widgeon,' and her burthen greater. In order, therefore, to test the relative values of the screw and paddle, the 'Rattler' screw-ship was put in competition with the 'Alecto' paddle-ship, both of which had been built on the same lines, and, in the relation of tonnage to horse-power, were considered as nearly equal to one another.
51. The trials were made in the North Sea, in the year 1845. In five of them the two vessels were impelled by steam only, and it was found that, whether moving in a calm or on a wind, the advantage in speed was on the side of the screw-ship in every trial except one. The distances run varied from 34 to 80 miles, and the mean excess of the 'Rattler's' speed over that of the 'Alecto' was half a knot per hour. In the exceptional case the 'Alecto' gained half a mile on the 'Rattler' in a run of about 30 miles. In three other trials the two vessels were under sail only, the 'Alecto' unshipping her paddles, and the 'Rattler' fixing the blades of her screw right up and down; and, in all of these, the speed of the 'Rattler' exceeded that of the 'Alecto,' as she accomplished a run greater in extent by four miles in less time than the other ship by 40' 20", a circumstance which militates against the supposition that the two ships were equal in qualities