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.