H, a case which entirely covers the wheel (being of the same metal, and of a piece with the pipe), through which the axis of the wheel passes to set another wheel agoing; so becom [ing] the principal mover in the clock or engine to be contrived. _The Manner of Setting It to Work_ Stop the cock at C and fill mercury into the cup A, higher than the line B; then stop the cock at D and turn in mercury at the cock F, till K and E are full; stop the cock at F, very close, open C, first, and then D, out of which the mercury will fall upon the buckets of the wheel G, down the funnel I, L, into the cup A, and be pressed up K, by the weight of the air, as in the barometer. Why the Devices Described in this Chapter Failed to Work The devices explained in the preceding chapter are of such complicated and ridiculous structure that it is impossible to explain anything from them. It is better to abandon them all and to discuss in a general way why Perpetual Motion has not been, and cannot be, attained by devices constructed on similar plans. An examination of the preceding devices in this chapter shows that they depended ultimately upon the fact: 1. That air or some other gas is to be compressed by work done upon it and that upon expanding it will do a greater amount of work than was required for the compression, or 2. That a bag empty, or partially filled with air, or other gas, can be easily immersed, and that if blown full of gas while immersed it will, in its tendency to float, do more work than was required to immerse it, or 3. That the weight of the atmosphere and its consequent pressure upon vacua can be utilized to drive a piston, or compress a bag and by some sort of means at the same time produce another vacua ready for a similar operation, the loss of the driven piston, or the compressed bag being utilized to drive machinery, if desired.