AGRICULTURAL ENGINES. The late Mrs. Trevithick said "that during the difficulties in London in 1808 and 1810, when Trevithick was overwhelming himself with new experiments and the cost of patents, and law expenses, lawyers and bailiffs took everything worth having from her house, including account-books, drawings, papers, and models, which she never saw again." His earlier account-books left in safety in his Cornish home, though very disconnected, give trustworthy traces of his work up to 1803. From that time only detached accounts or papers are found until 1812, when the unused pages in two old mine account-books of his father served as his letter (rough-draft) books; and judging from their number and style, his correspondence was most extensive and varied. [Rough draft.] "HAYLE FOUNDRY, _February 13th, 1812_. "TO SIR CHRISTOPHER HAWKINS, Baronet. "Sir,--I now send you, agreeable to your request, a plan and description of my patent steam-engine, which I lately erected on your farm for working a thrashing mill. The steam-engine is equal in power to four horses, having a cylinder of 9 inches in diameter. The cylinder, with a moderate heat in the boiler, makes thirty strokes in a minute, and as many revolutions of the fly-wheel, to every one of which the drum of the thrashing mill (which is 3 feet in diameter) is turned twelve times. The boiler evaporates 9 gallons of water in an hour, and works six hours without being replenished. The engine requires very little attention--a common labouring man easily regulates it. [Illustration: TREVITHICK'S HIGH-PRESSURE STEAM-PUFFER THRASHING ENGINE, 1812.] "The expense of your engine of 4-horse power, compared with the expense of four horses, is as follows:-- £ _s._ Original cost of the steam-engine 80 0 Building material and rope 10 0 ------ £90 0 ------ Interest on the above 90_l._ at 5 per cent. 4 10 Wear and tear at 5 per cent. 4 10 ------ 9 0 ------ Original cost of horse machinery for four horses £60 0 Interest on the above at 5 per cent. 3 0 ------ Wear and tear at 15 per cent. 9 0 ------ 12 0 "Two bushels or 164 lbs. of coal will do the work of four horses, costing 2_s._ 6_d._