AND SURFACE CONDENSER, 



present writer ; it is the only one of Trevithick's 

 numerous letters not written by himself: 



" Mil. GILBERT, " HAYLE, December 30th, 1828. 



" Sir, On the 28th inst. I received your printed report 

 on steam, and have examined Farey's publication on sundry 

 experiments made by Mr. Watt, which are very far from 

 agreeing with the actual performance of the engines at Binner 

 ] )o\vns. Mr. Watt says that steam at one atmosphere pressure 

 expands 1700 times its own bulk as water at 212, and that 

 large engines ought to perform eighteen millions when loaded 

 with 10 Ibs. to the inch of actual work, the amount of condensing 

 water being one-fortieth part of the content of the steam in the 

 cylinder at one atmosphere strength, the cold condensing water 

 at 50, and when heated 100. This would give for the Binner 

 Downs engine, with a 70-inch cylinder, 10-inch stroke, 11 Ibs. 

 effective work on the inch (this load being one-tenth more than in 

 Watt's table, by Farey, for an engine of this size and stroke), 

 57 gallons of injection- water for each stroke, and when working 

 eight strokes per minute, to do eighteen millions would consume 

 11 % bushels of coal per hour. 



" Now the actual fact at Binner Downs, at the rate of working 

 and power above mentioned, is that 3 bushels of coal per hour 

 were burnt, using 13 gallons of inject ion- water at each stroke at 

 70 of heat, which was raised by its use to 104, or an increase 

 of 34, which, multiplied by 13 gallons, gives 442. Mr. Watt's 

 table for this engine and work gives 57 gallons of condensing 

 water at 50, heated by use to 100. This 50 raised, multiplied 

 by the 57 gallons of water, amounts to 2850, or six and a half 

 times the quantity really used in the Binner Downs engine, and 

 nearly four times the coal actually used at present. Mr. Watt 

 further says that steam of 15 Ibs. to the inch, or one atmosphere, 

 from 1 inch of water at 212 occupies 1170 inches, and that 

 steam of four atmospheres, or 60 Ibs. to the inch, gives only 

 471 inches at a heat of 293. Now deducting 50 from 212 

 leaves 162 of heat raised by the fire. Multiply 15 Ibs. to the 

 inch by 1700 inches of steam, and divide it by 162, gives 138, 

 whereas if you deduct 50 from 293, it leaves the increase of 

 heat by the fire 243. Steam of 60 Ibs. to the inch multiplied 



