THK \VATT AM) THK TREVJTH1CK ENGIXKS. 137 



or where coal is plentiful, the engine would be best 

 without it." 



Those words accurately describe the practice of the 

 present day, though written sixty-six years ago, and 

 were followed by others equally true in principle, 

 though varied in form to suit special requirement. 

 " They say at the Dale about putting two cylinders, 

 but I think one cylinder partly filled with steam would 

 do equally as well as two cylinders." 



These sagacious views required the untiring labour 

 of the following twelve years to perfect and make prac- 

 tical, when applied to the largest engines of the time ; 

 which we shall now trace in the construction of a strong 

 and economical boiler, supplying high-pressure steam to 

 the cylinder during only a comparatively small portion 

 of the stroke, completing it by expansion, so that at 

 its finish the steam had become of low pressure when 

 passed to the condenser. The moving parts and expan- 

 sive gear were so simplified as to be applicable to the 

 then existing low-pressure steam vacuum engines with- 

 out the complication of the double cylinders of Horn- 

 blower and Woolf. 



" DEAR SIR, " PKNYDARUAN PLACE, December 26th, 1804. 



"I have been favoured with your letter, and in answer, 

 respecting Mr. Mitchell, I am at a loss to know from your letter 

 what kind of iron he may likely want. If you will direct him 

 to write to me, and explain himself, I will immediately reply to 

 him and do what I can to assist and serve him. I believe there 

 are vessels going over frequently from Cardiff to Cornwall with 

 coals, that he might have part in cargo and the remainder in 

 coals. I am happy to give you the most satisfactory account of 

 our ' Trevithick's engine ' going on well. It has now been at 

 work many months, and is by far the best engine we have. 

 \\V have for weeks weighed the coal, and knowing the work it 



