M E C H A N I C S. 
Supplying a great and wealthy city with water, and a few 
other purposes where a great expenfe can be borne, they 
are very proper engines; but in a thoufand cafes where 
their unlimited powers might be vaftly ferviceable, the 
enormous expenfe of fuel completely excludes them. We 
cannot doubt but that the attention of engineers was much 
directed to every thing that could promife a diminution 
of this expenfe. Every one had his particular noftrum 
for tlie conftruftion of his furnace ; and fome were un¬ 
doubtedly more fticcefsful than others. But fci.ence was 
not yet fufficiently advanced : it was not till Dr. Black 
had made his beautiful difcovery of latent heat., that we 
could know the intimate relation between the heat ex¬ 
pended in boiling off a quantity of water and the quantity 
of fleam that is produced. 
3. Much about the time of this difcovery, viz. in 1763, 
Mr. James Watt, eltabli/hed in Glafgow in the commer¬ 
cial line, was amufing- himfelf with repairing a working- 
model of the fleam-engine which belonged to the philofo- 
phical apparatus of the univerfity. Mr. Watt was a per¬ 
son of a truly philofophica! mind, eminently converfant in 
all branches of natural knowledge, and the pupil and inti¬ 
mate friend of Dr. Black. In the courfe of the above- 
mentioned amufement, many curious facts in the produc¬ 
tion and condenfation of fleam occurred to him ; and, 
among others, that remarkable faft which is always ap¬ 
pealed to by Dr. Black as a proof of the immenfe quan¬ 
tity of heat which is contained in a very minute quantity 
of water in the form of elaftic fleam : When a quantity 
of water is heated feveral degrees above the boiling point 
in a clofe digeffer, if a hole be opened, the fleam ruffles 
out with prodigious violence, and the heat of the remain¬ 
ing water is reduced, in the courfe of three or four fe- 
conds, to the boiling temperature. The water of the 
{team which has Blued amounts only to a very few drops; 
and yet thefe have carried off with them the whole excefs 
of heat from the water in the digefter. Since then a cer¬ 
tain quantity of fleam contains fo great a quantity of heat, 
it nuift expend a great quantity of fuel ; and no con¬ 
ftruftion of furnace can prevent this. Mr. Watt there¬ 
fore fet his invention to work to difcover methods of huf- 
banding this heat. The cylinder of his little model was 
heated almoft in an inftant, fo that it could not be touched 
by the hand. It could not be otherwife, becaufe it 
condenfed the vapour by abftrafting its heat. But all the 
heat thus communicated to the cylinder, and wafted by it 
on furrounding bodies, contributed nothing to the per¬ 
formance of the engine, and muft be taken away at every 
.injeftion, and again communicated and wafted. Mr. 
Watt quickly underftood the whole procefs which was 
going on within the cylinder, and faw that a very confi¬ 
derable portion of the fleam muft be wafted in warming 
the cylinder. His firft attempts were made to afcertain 
how much was thus wafted ; and he found that it was 
not lefs than three or four times as much as would fill the 
cylinder and work the engine. He attempted to diminifh 
this wafte by ufing wooden cylinders. But, though this 
produced afenfible diminution of the wafte, other reafons 
forced him to give them up. He then cafed his metal cy¬ 
linders in a wooden cafe with light wood-afhes between. 
By this, and ufing no more injection than was abfolutely 
neceft'ary for the condenfation, he reduced the wafte al- 
*ooft one half. But, by ufing i’o fmall a quantity of cold 
water, the infide of the cylinder was hardly brought be¬ 
low the boiling temperature; and there confequently re¬ 
mained in it a fleam of very confiderable elafticity, which 
robbed the engine of a proportional part of the atmo¬ 
spherical prefl'ure. He faw that this was unavoidable as 
long as the condenfation was performed in the cylinder. 
The thought (truck him to attempt the condenfation in 
.another place. His firft experiment was made in the 
flimpleft manner. A globular veflel communicated by 
means of a long pipe of one inch diameter with the bot¬ 
tom of his little cylinder of four inches diameter and 
thirty inches long. This pipe had a ltop-cock, and the 
globe was immeried in a veflel of cold water. When the 
VOL.XIV. No. iojz. 
Hr 
1 1 7 
pifton was at the top, and fire cylinder filled with flrong 
fleam, he turned the cock. It was fcarcely turned, nay 
he did not think it completely turned, when the fides of 
his cylinder (only flrong tin-plate) were crufhed together 
likean empty bladder. This furprifed aud delighted him. 
[It furprifed Dr. Gregory alfo to read the account; he 
lays—it cannot he.] A new cylinder was immediately 
made of brafs fufficiently thick, and nicely bored. When 
the experiment was repeated with this cylinder, the con¬ 
denfation was fo rapid, that he could not fay any time was 
expended in it. But the mod valuable difcovery was, 
that the vacuum in the cylinder was, as he hoped, almoft 
perfeft. Mr. Watt found, that, when he ufed water ia 
the boiler purged of air by long boiling, nothing that was 
very fenfibly inferior to the prefl'ure 01 the atmofphere on 
the pifton could hinder it from coming quite down to the 
bottom of the cylinder. This alone was gaining a great 
deal; for in molt engines the remaining elafticity of the 
fteatn was not lefs than one-eighth of the atmospherical 
prefl'ure, and therefore took away one-eighth of the power 
of the engine. 
Having gained this capital point, Mr, Watt found many 
difficulties to ftruggle with before he could get the ma¬ 
chine to continue its motion. The water produced from 
the condenfed fleam, and theair which was extricated from 
it, or which penetrated through unavoidable leaks, be¬ 
hoved to accumulate in the condenling veflel, and could 
net be voided in any way fimilar to that adopted in New¬ 
comen’s engine.. He took another method : he applied 
pumps to extraft both, which were worked by the great 
beam. The contrivance is eafy to any good mechanic ; 
only we muft obferve, that the pifton of the water-pump 
mull he under the furface of the water in the condenfer, 
that the water may enter the pump by its own weight, be¬ 
caufe there is no atmofpherical prefl'ure there to force it in. 
We muft alfo obferve, that a confiderable force is necefl'a- 
rily expended here, becaufe, as there is but one ftroke 
for rarefying the air, and this rarefaftion muft be nearly 
complete, the air-pump muft be of large dimenfions, and 
its pifton muft aft againft the whole prefl'ure of the atmo¬ 
fphere. Mr. Watt, however, found that this force could 
be eafily fpared from his machine, already fo much im¬ 
proved in refpeft of power. 
Thus has the fleam-engine received a very confiderable 
improvement. The cylinder may he allowed to remain 
very hot; nay, boiling-hot, and yet the condenfation be 
completely performed. The only elaftic fleam that novy 
remains is the fmall quantity in the pipe of communication. 
Even this fmall quantity Mr. Watt at laft got rid of, by- 
admitting a fmall jet of cold water up this pipe to meet 
the fleam in its paffage to the condenfer. This both 
cooled this part of the apparatus in a fituation where it 
was not neceffary to warm it again, and it quickened the 
condenfation. He found at laft that the l'mall pipe of 
communication was of itfelf fufficiently large for the con¬ 
denfation, and that no feparate veil'd, under the name of 
condenfer, was neceflary. This circutnftance {hows the 
prodigious rapidity of the condenfation. We may add, 
that, unlefs this had been the cafe, his improvement would 
have been vallly diminifhed ; for a large condenfor would 
have required a much larger air-pump, which would have 
expended much of the power of the engine. By thefe 
means the vacuum below the pifton is greatly improved ; 
for it will appear clear to any perfon w ho underftands the 
fubjeft, that, as long as any part of the condenfor is kept 
of a low temperature, it will abdraft and condenfe the va¬ 
pour from the warmer parts, till the whole acquires the 
elafticity correfponding. to the coldeft part. By the fame 
means much of the walk is prevented, becaufe the cylin¬ 
der is never cooled much below the boiling temperature. 
Many engines have been erefted by Mr. Watt in this form, 
and their performance gave univerfal fatisfaftion. 
We have contented ourfelves with giving a very flight 
defeription, without a figure, of this improved engine, 
becaufe we imagine it to be of a very eafy eomprehenfion, 
and becaufeit is only a.preparation for Hill greater improve- 
9 M ments 
