About seven years ago a committee was appointed for the express purpose of ascertaining the fact by the teat of unexceptionable experiments ; and these were of such a nature, and so conducted, as to determine the question with cuff. cient precision for all practicable purposes. The committee resolved to estimate the force of the steam by the column of mercury sustained, and for this pur pose a glass tube, consisting of 13 pieces, each piece two metres (78.74 inches) in length, 5 millimetres (0.2 of an inch) in diameter, and the same in thickness, was prepared and erected in the tower of the dilapidated church of St. Genevieve. Lest the safety of the apparatus might be endangered by the sudden agitation of the mercury under the action of the steam from the boiler, a kind of mano meter was constructed, in which the compression of a given volume of air should be ascertained by the column of mercury, and afterwards be employed to mea sure the elasticity of vapour at various temperatures. In this way the estimates would be as accurate as if made directly by the column of mercury. Great pre cautions were used to ascertain the temperature correctly. The first was to take account of the cooling effect of the air on that part of the thermometer which was exterior to the boiler; this was done by retaining it constantly at the same temperature. The next was to prevent alteration in the capacity of the bulb, by allowinf thevapour to press upon it : this was effected by putting the ther mometers into gun barrels made than, closed at one extremity, and filled with mercury; these, when fitted to the boiler, were made to descend, one to the bot tom of the boiler nearly, to give the temperature of the water; the other to with in a few inches of the water, to give the temperature of the vapour.
The temperature and pressure were then experimentally ascertained up to 24 atmospheres ; after which a formula was sought for, to determine higher pres sures, and the accuracy of the formula ultimately adopted was evidenced by its near correspondence with the results of the experiments actually made by means of the ap= : the greatest error was found atmospheres, and was then 0.9 of a It was more accurate for the higher pressures, being calcu lated from them ; and the commissioners entertained no doubt that at 50 atmo spheres the error is not more than 0.1 of a degree. The subjoined Table exhibits the estimates thus derived : the first, third, and fifth columns representing the elasticity of the vapour (taking the pressure of the atmosphere as unity,) and the second, fourth, and sixth, representing the temperature according to the thermometer of Fahrenheit.
This is an exceedingly neat and effective contrivance, and the engines erected by Mr. Maudaley on this modification, fully realized the expectatiora of the inventor, and, by the superior workmanship exhibited in all their parts, gained for him a well-deserved popularity, as one of the best engine builders of his day.
As a conclusion to this account, the two following modifications of the steam engine are introduced, which, together with the description before given of the atmospheric engine, under Smeatan's improvements, may be taken as fair illus trations of the three varieties of this power, in the most simple and effective con dition. They have been selected as affording general representations of the state to
which the high-pressure and low-pressure engines have been now brought. From among the many excellent engines which proceed from the establishments of our manufacturers at the present day, it would be no easy task to particularize any one exclusively as being the best in contrivance or in workmanship ; it is enough to instance such as have been found, by practice and experience, to be among the best.
The first represents a portable high-pressure engine, of great simplicity in its arrangements, and effective in its operations. That from which the drawings were made is calculated at two horse-power, and is employed at Sheffield in working three pairs of large bellows, and in turning an enormous grindstone used for grinding the faces of anvils. The cylinder is seven inches in diameter. The engine is worked sixty-six hours per week, and consumes one ton of small coal or culm, and 1,200 gallons of water, in that time.
Fig. 1 is a side, and Fig. 2 a front elevation ; the boiler to which, drawn to the same scale, is given under the article /3ottra, at page 199, Vol. I. We shall therefore briefly state in this place, that it is of the form of a long cylinder, fixed horizontally over the furnace and main flue, the flame and heated gases being returned through a tube, running longitudinally throughout the interior of the boiler, in their way to the chimney. A stone float and counter-weight is em ployed to regulate the height of the water, in the usual manner.
A, the steam-pipe leading from the boiler, in which is the throttle-valve 1 ; the side-pipe, in which work the slide-valves k k, moved by the rod 1, attached to the eccentric m, in the shaft of the fly-wheel m 000 are brass stuffing-boxes; p, the upper steam entrance to the cylinder ; q r, the piston-rod working through the bridge s, and communicating with the crank t by the side rods u u,—forming a very simple parallel motion ; e e, pedestals support ing the main shaft, the revolution of which gives motion to a parr of bevel wheels, and thereby to the governor se, the expanding or contract ing of the arms of which raises or depresses the collar s, and acts on the valve i, through the medium of the lever 1 and handle 2; 4 is the pump for supplying the boiler through a feed-pipe (not shown) worked by the rod 5, and eccentric 8; 7 7 are the metal cheeks of the frame ; 8, the metal foundation plate, under which is a small cistern (not shown,) containing a day's consumption for the boiler. At the bottom of the side-pipe is an eductlon-pipe, (not shown,) from which the steam is discharged into the cistern, to heat the water for sup plying the boiler after the steam has performed its office in the cylinder. The periphery of the fly-wheel is round in its transverse section, and of cast iron, the arms or radii are of wrought iron, and are inserted into the former while casting. From this explanation of the wood-cuts the operation of the engine will be easily understood.