472 Scientific Intelligence. [Junx, 
3:3, while that of our mineral is only 2°238. Dioptase is foliated, 
but our mineral is compact and conchoidal in its fracture. Dioptase 
is emerald-greea, while our mineral is verdigris-green, with a tint 
of blue. Perhaps they may be only varieties or sub-species. But a 
greater number of specimens than i had an opportunity of seeing 
would be necessary to decide the point.—T. 
VII. On the Boiler of the Steam-Engine. 
(To Dr. Thomson.) 
SIR, Tradeston, Glasgow, April 15, 1816. 
’ From living in the neighbourhood of Mr. James Cook’s extensive 
steam-engine manufactory, I have occasionally examined the new 
boilers and the old, which they have often to repair. I can easily 
see the propriety of the length and width of a steam-engine boiler, 
but I am at a loss to account for the depth. Does it not naturally 
suggest itself to you that two or three feet in depth, when the water 
presents the same surface, would serve the purpose as well as 20? 
I have no doubt that Mr. Watt, when he first erected his engines, 
and before the boiler apparatus was so complete as he afterwards 
planned it, and is now in use, found it necessary to have them of a 
considerable depth, so as that the carelessness or neglect of the 
fireman for an hour or two might not injure its bottom. But now 
that the feed-pipe, or supply of water, is regularly wrought by the 
engine itself, and that supply from the hot cistern, it is certainly 
obvious that a considerable saving of expense in making the boiler, 
and a much greater in fuel, might be attained by lessening the 
depth. 
LT have likewise observed that almost the whole of the boilers: 
which are repaired have been injured round the seat of the bottom. 
May not this be ascribed in a great measure to the turning of the 
plates, as the separating of the metal is visible in that operation, 
which admits the smallest particle of water to lodge, for water will 
sometimes escape from the most perfect made boiler, and run down 
its sides? Being so nigh the heat, the water is decomposed. Of 
course the metal becomes oxidated. This, I think, might be-pre- 
vented, by bringing the almost separated particles of metal again in 
contact by hammering, as is practised in making the bottoms of 
copper boilers for use of brewers, &c. &c. 
I have submitted the preceding hints to your observation that 
you may insert them in the Annals of Philosophy, if you think they 
are of sufficient consequence. 
Iam, Sir, your most obedient servant, 
Joun THOMSON. 
{ happened to be engaged to dinner with Mr. Watt on the day 
that I received the precediag letter; and as I knew that at first the 
boilers of steam-engines had been much shallower than they are 
now, I asked Mr. Watt the reasons which induced him to adopt the 
