340. The time during which a machine will continue to perform its work effectually, will depend chiefly upon the perfection with which it was originally constructed upon the care taken to keep it in proper repair, particularly to correct every shake or looseness in the axes--and upon the smallness of the mass and of the velocity of its moving parts. Everything approaching to a blow, all sudden change of direction, is injurious. Engines for producing power, such as windmills, water-mills, and steam-engines, usually last a long time.(1*) 341. Many of the improvements which have taken place in steamengines, have arisen from an improved construction of the boiler or the fireplace. The following table of the work done by steam-engines in Cornwall, whilst it proves the importance of constantly measuring the effects of machinery, shows also the gradual advance which has been made in the art of constructing and managing those engines. A table of the duty performed by steam-engines in Cornwall, shewing the average of the whole for each year, and also the average duty of the best engine in each monthly report Years; Approximate number of engines reported; Average duty of the whole; Average duty of the best engines 1813; 24; 19,456,000; 26,400,000 1814; 29; 20.534,232; 32,000,000 1815; 35; 20.526,160; 28,700,000 1816; 32; 22,907,110; 32,400,000 1817; 31; 26,502,259; 41,600,000 1818; 32; 25,433,783; 39,300,000 1819; 37; 26,252,620; 40,000,000 1820; 37; 28,736,398; 41,300,000 1821; 39; 28,223,382; 42,800,000 1822; 45; 28,887,216; 42,500.000 1823; 45; 28,156,162; 42,122,000 1824; 45; 28,326,140; 43,500,000 1825; 50; 32,000,741; 45,400,000 1826; 48; 30,486,630; 45,200,000 1827; 47; 32,100,000; 59,700,000 1828; 54; 37,100,000; 76,763,000 1829; 52; 41,220,000; 76,234,307 1830; 55; 43,350,000; 75,885,519 1831; 55(2*); 44,700,000; 74,911,365 1832; 60; 44,400,000; 79,294,114 1833; 58; 46,000,000; 83,306,092 342. The advantage arising from registering the duty done by steamengines in Cornwall has been so great that the proprietors of one of the largest mines, on which there are several engines, find it good economy to employ a man to measure the duty they perform every day. This daily report is fixed up at a particular hour, and the enginemen are always in waiting, anxious to know the state of their engines. As the general reports are made monthly, if accident should cause a partial stoppage in the flue of any of the boilers, it might without this daily check continue two or three weeks before it could be discovered by a falling off of the duty of the engine. In several of the mines a certain amount of duty is assigned to each engine; and if it does more, the proprietors give a premium to the engineers according to its amount. This is called million money, and is a great stimulus to economy in working the engine.