Mr. Henwood also gave indicator diagrams of the expansion of the steam, on one of which the writer has marked ten horizontal lines, indicating the position of the piston at each foot of its stroke, and ten longitudinal lines dividing the diameter of the cylinder into tenths. The steam pressure in the boiler was 46·8 lbs. on the square inch above the atmosphere, or 4·68 lbs. for each of the ten longitudinal line divisions. _x_ to _c_ represents the top of the steam-cylinder 80 inches, diameter; _x_ to F the length of the cylinder for a 10-feet stroke of the piston. By the time the piston had moved through one-twentieth of its course, reaching _c_, the expansive working had commenced; and when one-tenth of the stroke had been run, half of a division was cut off, showing by the curved indicator line the decrease in pressure of steam to 44.46 lbs. The comparatively small passage through the steam-valve not giving room for sufficient steam to follow up the increasing speed of the piston, led to its continued expansion in the cylinder, and by the time the piston had moved 2 feet, reaching D, the steam pressure was reduced by two divisions or 9.36 lbs., or a pressure of 37.44 lbs. on the piston; at this point the steam-valve was closed, and the remaining four-fifths of the stroke was performed by expansion; at the fifth horizontal line, or middle of the stroke, only three divisions of steam are left, giving a pressure of 14.04 lbs. to the inch; at the finish of the stroke there is only half a division, from E to F, or 2.34 lbs. of steam to the inch above the pressure of the atmosphere. On the return up-stroke of the piston, when it had reached within a foot of the finish of its course at C, the equilibrium valve closed, causing the enclosed steam of 2.34 lbs. to the inch to be compressed at the finish of the up-stroke shown by the curve G A to 9.36 lbs. on the inch, equal to its pressure about the middle of the down-stroke at N. Trevithick's expansive engine therefore, commencing its work with steam of 46.8 lbs. on the inch above the atmosphere, only took a full supply from the boiler during one-tenth of its stroke, and none after one-fifth had been performed, while at the finish of the stroke it had about the same pressure as Watt began with. The _power_ of the Watt low-pressure steam vacuum pumping engine was increased by Trevithick from two to three fold, and its economical duty in about the same proportion; in other words, he increased the effective power of the steam-engine two or three fold without additional consumption of coal.