370 MARINE STEAM-ENGINES. 



and requires 72 bushels of coal in twenty-four hours ; therefore 

 the cylinder-case must in condensing high-pressure steam use 

 24 bushels of coal in twenty-four hours. Boulton and Watt's 

 case for a 63-inch cylinder, working with low-pressure steam, 

 condensed only 4 J bushels of coal in equal time, the proportions 

 of surface being as 190 to 240 in Wheal To wan. Nearly five 

 times the quantity was condensed of high steam than of low 

 steam, proving that there is a theory yet unaccounted for." 



Trevithick's portable high-pressure steam-puffer en- 

 gine, when it discharged the first cargo of coal from a 

 vessel at Hayle, was worked by the writer ; it stood on 

 the wharf near the ship, and on a signal from the hold, 

 steam was turned on, raising rapidly the basket of coal 

 the required height. In trying how quickly the work 

 could be done the hook missed the basket-rope, and 

 caught the man under the chin, swinging him high in 

 the air, much to the engineman's discomfiture. Fortu- 

 nately the suspended man had the good sense to lay 

 hold of the rope above his head, and so supporting his 

 weight, no great harm was done. 



The object and the means were the revival of the 

 nautical labourer of twenty years before. 1 The boiler 

 was a wrought-iron barrel on its end, on small wheels, 

 with internal fire-tube, in shape like the boiler of the 

 recoil engine of 1815; 2 but less high in proportion to 

 its diameter. The cylinder was let down into the top 

 of .the boiler, and like Newcomen's atmospheric engine 

 had no cylinder cover. The piston-rod was a rack 

 giving motion to a small pinion fixed on a shaft on the 

 top of the boiler, and to a large grooved wheel, around 

 which was wound the whip-rope from the vessel's hold ; 

 a brake-lever enabled the engineman either to stop or 



1 Sec chapters xiv. and xv. 



2 See Trevithick's letter, 7th May, 1815, vol. i., }>. o(J-L 



