t will
remain at rest fixed upon a lath of cast iron or stout steel." Watt is
very particular in all his details: "I am sorry," he says in one note,
"to trouble you with so many things; but the alterations on this
spindle and socket [he annexes a drawing] may wait your convenience."
In a further note, Watt says. "The drawing for the parallel lathe is
ready; but I have been sadly puzzled about the application of the
leading screws to the cranes in the other. I think, however, I have now
got the better of the difficulties, and made it more certain, as well
as more simple, than it was. I have done an excellent head of John
Hunter in hard white in shorter time than usual. I want to show it you
before I repair it."

At last Watt seems to have become satisfied: "The lathe," he says, "is
very much improved, and you seem to have given the finishing blow to
the roofed frame, which appears perfectly stiff. I had some hours'
intense thinking upon the machine last night, and have made up my mind
on it at last. The great difficulty was about the application of the
band, but I have settled it to be much as at present."

Watt's letters to Murdock are most particular in details, especially as
to screws, nuts, and tubes, with strengths and dimensions, always
illustrated with pen-and-ink drawings. And yet all this was done
merely for mechanical amusement, and not for any personal pecuniary
advantage. While Watt was making experiments as to the proper
substances to be carved and drilled, he also desired Murdock to make
similar experiments. "The nitre," he said in one note, "seems to do
harm; the fluor composition seems the best and hardest. Query, what
would some calcined pipe-clay do? If you will calcine some fire-clay
by a red heat and pound it,--about a pound,--and send it to me, I shall
try to make you a mould or two in Henning's manner to cast this and the
sulphur acid iron in. I have made a screwing tool for wood that seems
to answer; also one of a one-tenth diameter for marble, which does very
well." In another note, Watt says: "I find my drill readily makes 2400
turns per minute, even with the large drill you sent last; if I bear
lightly, a three-quarter ferril would run about 3000, and by an engine
that might be doubled."

The materials to be drilled into medallions also required much
consideration. "I am much obliged to you," said Watt, "for the balls,
etc., which answer as well as can be expected. They make gr