130 On Water-Pressure Engines. 
the statement, and quotes an example to prove it. Tt ap 
pears to me that the only mistake in the paragraph alluded 
to is this: that a relative term is made use of instead of a 
positive one, and that therefore it may be variously applied 
by different persons, according to the standards by which 
they form their comparison of size. Mr. Farey ; may have 
been in the habit of viewing small engines; while, in Corns 
wall and this part of Devon, we are surrounded by large 
ones. 
He savs that the author of that paragraph has made a 
mistake, when, speaking of Water- pressure “Engines, he 
states that ‘* none have yet been successfully made upon a 
Jarge scale,” because, as he says, Mr. Trevithick erected one 
in Derbyshire, which, by a fall of 144 feet, pumped the 
water from a mine 48 feet below the sough. 
Now, if Mr. Farey and myself were severally to describe 
this engine as to its magnitude, judging by the effect as 
he relates it, I apprehend that each of us would appear to 
another person to make mistakes ; for L conceive that he 
would speak of it as being on ‘ a Jarge seste,” and Pshould 
certainly call it one on a small scale. 
As the quantity of water raised by the engine is not 
stated, it is possible that it may be somewhat “larger than 
I conceive it to be by the only means he has afforded 
me of forming any kind of judgement ; namely, the depth 
of 48 feet. But unless the volume discharged by the 
pumps is indeed mueb greater than generally oceurs in 
mines at this depth, 1 sbould not call an engine which 
pumps waler 48 feet, or indeed almecst as many fathoms, 
one-en a targe scale. SH, 
There are pressure engines in Cornwall of a larger size 
probably than the one My, Farey mentions, on Mr. Tre- 
vithick’s plan, I-believe, as well as on other constructions, 
aud with evluiders of a certain diameter they answer ex- 
tremely well on the whole... But pressure engines haye been - 
generally objected to, by those who have seen most of them, 
when an attempt has been niade'to adapt them for copious 
‘streams, on acconnt of the difheulties in the action of the 
pistons, which serve tor valves, where a large bmicon for 
waterway 1s Bettas 
Having purchased one some time since, fora mine deige 
my management. of a size which I conceive must be larger 
than the cue in Derbyshire, and than any other I ever 
‘heard of, | fonnd the inconveniences in its action which 
were apprehended, and it was to remedy these incon — 
veniences that ienceavoured to contrive a new construction 
for 
