imp LECTURE XXVIII. 



lumn of water, or of water mixed with air, in the ascending pipe. The air 

 nearest the joint is compressed into a space much smaller than that which it 

 occupied at its entrance, so that where the height is consider; ble, it be- 

 comes advisable to admit a larger portion of air than would naturally fill 

 half the coil, and this lessens the quantity of water raised, but it lessens also 

 the force required to turn the machine. The joint ought to be conical, in 

 order that it may be tightened when it becomes loose, and the pressure ought 

 to be removed from it as much as possible. 1 he loss of power, suppobmg 

 the machine well constructed, arises only from the friction of the water on ' 

 the pipe, and the friction of the wheel on its axis ; and where a large quantity 

 of water is to be raised to a moderate height, both of these resistances may 

 be rendered inconsiderable. But when the height is very great, tl*e 

 length of the spiral must be much increased, so that the weight of the pipe 

 becomes extremely cumbersome, and causes a great friction on the 

 axis, as well as a strain on the machinery : thus, for a height of 40 feet, I 

 found tliat the wheel required above 100 feet of a pipe which was three 

 quarters of an inch in diameter ; and more than one half of ihe pipe being 

 always fiiU of water, we have to overcome the friction of about 80 feet of 

 such a pipe, which will require 24 times as much excess of pressure to pro- 

 duce a given velocity, as if there were no friction. The centrifugal force of 

 the water in the wheel would also materially impede its ascent if the velocity 

 were considerable, since it would be always possible to turn it so rapidly as 

 to throw the whole water back into the spoon. The machine which I had 

 erected being out of repair, I thought it more eligible to substitute for it a 

 common forcing pump, than to attempt to make any further improvement in 

 it, under circumstances so unfavourable. But if the wheel with its pipes were 

 entirely made of wood, it might in many cases succeed better : or the pipes 

 ^might be made of tinned copper, or even of earthenware, which might be 

 cheaper and lighter than lead. (Plate XXII. Fig. 303.) 



The centrifugal force, which is an impediment to the operation ofWirtz's 

 machines, has sometimes been employed, together wirh the pressure of the at- 

 mosphere, as an immediate agent in raising water, by means of the rotatory 

 pump. This machine consists of a vertical pipe, caused to revolve round its 

 axis, and connected above with a horizontal pipe, which is open at one or at 

 both enxis, the whole being furnished with proper valves to prevent the 



