The hydraulic engine now in use, seems to have been introduced into the mining operations of England about the year 1680, and into S_otland about theyear 1712 ; it was an improvement upon the chain and bucket engine, moved by a water-wheel. Several of the present hy draulic engines have wheels 30 feet diameter, and four feet wide in the water-buckets ; the cranks are generally from two feet and a half to three feet and a half in length, and are fixed upon each end of the axle or journal ; there arc two beams of wood placed upon a pillar-head or frame of wood ; each beam is about forty feet in length, three feet thick, and two feet Moad at the cen tre, tapering towards the end, and firmly bound toge ther with iron glands, and, to prevent them bending, they have each a king-post and martingale stays ; one of the beams is attached to the crank on one side of the axle by a connecting rod, and the other beam is connect ed in the same manner to the crank on the other side of the axle, to the other ends of the beams which project over the pit mouth, the spears or pump-rods arc attach ed, which work in the pumps. IF the pit is from 20 to 30 fathoms in depth, then two piles or sets of pumps reach front the bottom to the mouth of the pit, where they both deliver their water ; if the pit is from 30 to 60 fathoms in depth, then the depth is divided. One set of pumps reaches from the bottom to half way up the pit, where the water is delivered into a cistern, from which cistern the upper set or pile of pumps reaches to the top of the pit, where they deliver the water. This ma chine is of very simple construction ; the working parts are few, and requires no attendance—it places a coal field drained by it nearly upon as moderate a footing in point of expellee as a coat-field which is level free, and in some instances even upon a superior footing. Many attempts have been made to improve this engine, and to render its powers more efficient, but without success.
Newcomen's atmospheric steam-engine being of very simple consu tenon, is still generally used as a pumping engine in collieries, when the kind of coal used in work ing them is of little or no value, and when the depth does not exceed 120 yards; for a greater depth, and where pumps are used above ten inches diameter, the improved engine of Watt is preferred.
The steam-engine invented by Watt, known by the name of Watt and Boulton's engine, applied in draining collieries, is of two kinds, named single and double pow er engines.
In the single power engine, the cylinder is close at the top, and has the steam operating against the piston, as it descends, by means of the vacuum formed below it. This engine, like Newcomen's, only draws water by the descending stroke of the piston ; but with much greater power, that is, a much smaller cylinder is re quired to produce the same effect.
In the double-power pumping engine of Watt and Boulton, the piston acts with equal force, whether as cending or descending, so that an arrangement altoge ther different is required for working the pumps, be cause one half of the pumps in the pit is worked by the descending, and the other half by the ascending of the piston in the steam cylinder.
Both the single and double engine of Watt have been applied in a different manner from that before stated ; it had occurred to colliery engineers, that it would be a great improvement in the pumping steam-engine, to throw aside the great and massive lever beam, which in some instances exceeded ten tons in weight. This plan was accomplished by placing the cylinder in a perpendi cular direction, directly over the pumps above the pit mouth ; but this plan has not succeeded. It appears that nothing has been gained by laying aside the great lever beam, weighty as it is, because, when well con structed, very little power is required to move it.
Trevethick's engine is of unlimited power, with re gard to the pressure upon each square inch of the pis ton. As it operates by means of steam raised to a great elastic power, it is employed either as a single or double engine, in the same manlier as the engines of Watt. They wet e first applied to collieries in a very simple form, by allowing the steam, which had acted by its pressure on one side of the piston, to escape into the air, while the elastic steam acted on the opposite side of the piston, and so alternately. Mr. Woolf has applied the highly elastic steam principle ol Trevethick to the prin ciples of Watt's engine, and these engines of combined principles are now much used in draining the very deep and heavily watered mines in Cornwall. Some of these engines work with a pressure of more than four atmos pheres on the square inch of the piston, that is, more than 60 pounds upon the square inch ; hut they are not in ge neral use, as a considerable degree of danger attends them.
With respect to the general range of the power of the different kind of engines before described, milling engineers arc in the practice of calculating their powers according to the weight of the column of water from the bottom to the top of the pit, independently of friction, and the vis inertiec which has to be overcome at each incoming and returning stroke, at which points the whole power and moltment of the pumping engine must be brought to rest, and the direction of the power altered.
The winnings or fittings of collieries are of various depths, from a few to two hundred fathoms, which is the deepest winning in Great Britain, that is, the deepest at which coals are wrought; but the deepest coal-pit is only one hundred and fifty fathoms. The depth of any winning or fitting is made to col respond with the capi tal to be employed, and the vend or sale of coals which may be calculated upon, according to the state of the demand for coal ; it being very evident, that a very li mited vend will not, at the ordinary rate at which coals are sold, admit ol a gi eat capital being invested in mak ing a winning, either by a day-level or by machinery.