[24] If 22 square inches of piston surface be allowed to represent a horse-power, the power of an engine may always be computed by dividing the square of the diameter of the piston expressed in inches by 28. And, on the other hand, to find the diameter of piston which would correspond to any given power, multiply the number of horses' power by 28, and take the square root of the product. These rules, however, cannot be applied if the piston be supposed to move with any other velocity; since, in that case, the same amount of piston surface would cease to represent a horse-power, unless the effective pressure on the piston were at the same time changed. [Pg301] [Illustration: WATT'S CHAPEL IN HANDSWORTH CHURCH.] CHAP. X. NOTICE OF THE LIFE OF MR. WATT. — HIS FRIENDS AND ASSOCIATES AT BIRMINGHAM. — INVENTION OF THE COPYING PRESS. — HEATING BY STEAM. — DRYING LINEN BY STEAM. — THEORY OF THE COMPOSITION OF WATER. — FIRST MARRIAGE OF WATT. — DEATH OF HIS FIRST WIFE. — HIS SECOND MARRIAGE. — DEATH OF HIS YOUNGER SON. — EXTRACTS FROM HIS LETTERS. — CHARACTER OF WATT BY LORD BROUGHAM. — BY SIR WALTER SCOTT. — BY LORD JEFFREY. — OCCUPATION OF HIS OLD AGE. — INVENTION OF MACHINE FOR COPYING SCULPTURE. — HIS LAST DAYS. — MONUMENTS. (179.) Having brought this historical analysis of the invention and application of the steam engine to the date of the decease of the illustrious man, to the powers of whose mind the world stands indebted for the benefits conferred upon [Pg302] mankind by that machine, it will perhaps not be deemed an improper digression in this work, to devote some pages to a notice of the principal labours of the same mind in other departments of art and science, and to circumstances connected with his personal history and the close of his life, which cannot fail to possess general interest.