The vessel G is to be filled with mercury through the cock H, the pipe I being open to allow of the escape of air. When this vessel has been filled, the cock H should be closed and its cap screwed on; and the pipe I be also closed by a valve, which is to be pressed tight by the cap on the head of the pipe. I is a vent-pipe, open at the top. The space represented by the double lines is a panel of thick plate glass having two horizontal lines described on its surface, whereby the attendant may observe the quantity of mercury within the vessel. The cock F being closed, a quantity of mercury must be allowed to run out of the vessel G, equal to the space 1, 2, 3, 4, which space will become a vacuum. If, therefore, the cock L be then opened, to allow of the discharge of a certain quantity of mercury on the wheel, and the cocks C and L also opened, the mercury will continually rise from the reservoir A into the vessel G, and thence be discharged on the wheel, whence it will again fall into the vessel A, to keep up the supply. The cock F must be so adjusted as to admit into the vessel G a quantity of mercury equal to that which is discharged by the cock L. This can be ascertained and regulated by means of the panel of glass above described. The specific gravity of mercury being 7½ ounces, it is evident that but a small quantity of it is required to turn the wheel, which has no friction but that of the axis on which it turns. Robert Copland's Device In 1819 Robert Copland applied for a British patent on "A New or Improved Method or Methods of Gaining Power by New or Improved Combinations of Apparatus, Applicable to Various Purposes." His specifications describe in great detail his invention in the following language: [Illustration]