20 ~=©Mr. Henwood on the Steam-Engines of Cornwall. 
a similar white powder, which, perhaps, may be another iodide 
of lead. If metallic tin be boiled with the iodide of lead, no 
reaction takes place; but if the dry iodide be mixed with gra- 
nulated tin, and exposed to heat, combination takes place, and 
a double iodine of tin and lead results, of a brown colour and 
differing from either iodide separately. By boiling this double 
iodide in water, very beautiful crystals of the yellow iodide of 
lead are obtained. [To be continued.] 
V. Observations on the Steam Engines of Cornwall; in Reply 
to John Taylor, Esq., F.R.S., Treas. G.S., Sc. By 
W. J. Henwoop, F.G.S. Lond. & Paris, Hon. M.Y.P.S., 
Curator of the Royal Geological Society of Cornwall. 
To the Editors of the Philosophical Magazine and Journal. 
GENTLEMEN, 
R. TAYLOR’s communication in your present month’s 
Number* appears to imply that a rotatory single engine 
working expansively is something of a novelty. 
Now, Mr. Watt’s first expansion engine was erected in 
1778+; the patent for his rotatory engine was taken out in 
1781, October 25th ¢; and that for his double engine in 1782, 
March 12th §. 
Thus the rotatory single engine working expansively pre- 
ceded the invention of the double engine; and some of the 
former construction were erected by Messrs. Boulton and 
Watt on the Cornish mines. 
At Binner Downs mine in this county Messrs. Gregor and 
Thomas have erected five rotatory single engines working ex- 
pansively ; the first of them in 1828, the last in 1833. Captain 
Gregor also set up a similar one, for driving a common grist 
mill, for Messrs. Harvey and Co. of Hayle Foundry. 
All these have performed their work extremely well; are 
quite as manageable as double engines ; and, where they have 
taken their place, have worked with much less coal. The 
duty of those at Binner Downs, which are used as winding 
(whim) engines, Captain Gregor estimates at about 15 millions 
of pounds raised one foot, high by the use of each bushel of 
coal consumed ||. 
Mr. Taylor speaks of “the method of working high pres- 
sure steam expansively, which we owe to Mr. Woolf.” 
* Vol. vii. p. 369. + Farey on the Steam-Engine, p. 341. 
t Ibid., p. 346. § Ibid., p. 350. 
Captain Lean reports the duty of Mr. Sims’s engine at Charlestown 
44, and not 60, millions, as stated by Mr. Taylor. 
§ Lond. and Edinb. Phil. Mag., vol. vil. p. 369. 
