178 Mr Dickson on a new Water-Pressure Engine^ 
to compress the air, and run off by M to dilate it. In order 
that the air, therefore, may not only be taken out at first, but 
that the machine may continue to discharge any that may be 
disengaged, by the water coming in contact with the metal or 
other causes, the under side of the piston is turned, to cause all 
the air below it to be collected at one point, and a small hole be- 
ing made there, by which the air is transferred to the upper 
side of it, and the cover being turned in the same manner, all 
the air is collected at one point, in the cover ; and through the 
underside of the flanch of the cylinder, is introduced a pipe, the 
end of w7iich goes into this hollow of the cover, where the air is 
collected, and in the descent of the piston, the water pressing it 
dowm, also presses out the air by this tube, the other end of 
which is fixed into the pipe M. As tlie piston is alw^ays either 
ascending or descending, however small be the space of time be- 
tween the shutting of one set and the opening of the other set of 
valves, the piston will have nioved. In order, therefore, to al- 
low the piston to travel during the time that all the valves are 
shut, and no waiter either coming in or going out of the cylinder, 
there is a small cylinder D at the back of the large one, having 
a communication with it at top and bottom. Into this cylinder 
tliere is fitted a piston with a screwed rod, which can be length- 
ened or shortened, so that this piston, with its rod, which is 
completely enclosed, may have a range just sufficient to contain 
the water that remains to be displaced from the upper side of 
the large piston when at the top, and the under side of it when 
at the bottom. 
The eccentric wheel V, on the crank shaft, is connected 
wuth the levers R and S, and by alternately depressing them, 
opens the valves, which are shut by the weight T, as in a steam- 
engine. The wheel W on the crank-shaft serves to connect the 
engine with any other machine. When an air-pump is used, it 
may be connected with Z, or at different places along the pipe M. 
The greatest superiority this engine has over a water-wheel 
is, that a fall of any height above, and to the extent of 34 feet 
below, can be concentrated in a single point, whereas the power 
of a water-wheel is confined to the limited extent of its diameter ; 
and as a small quantity of water from a great height, is equal to 
a large quantity with a short fall, it can be used with great power 
iu situation’s where a water-wheel can be of little benefit ; for if 
