[260] The engine was of 40-horse power. It was erected at the “Black Works,” Etruria, where it continues working with the sun and planet motion,--one of the very few engines of the old construction still remaining in existence. [261] Watt to Boulton, 22nd June, 1784. One reason why Watt was more than usually economical of his time was, that he was then in the throes of the inventions patented by him in the course of this year. Though racked by headaches which, he complained, completely “dumfounded” him and perplexed his mind, he could not restrain his irrepressible instinct to invent; and the result was the series of inventions embodied in his patent of 1784, including, among other things, the application of the steam-engine to the working of a tilt-hammer for forging iron and steel, to driving wheel-carriages for carrying persons and goods, and for other purposes. The specification also included the beautiful invention of the parallel motion, of which Watt himself said, “Though I am not over anxious after fame, yet I am more proud of the parallel motion than of any other mechanical invention I have ever made.” Watt was led to meditate this contrivance by the practical inconvenience which he experienced in communicating the direct vertical motion of the piston-rod by means of racks and sectors, to the angular motion of the working beam. He was gradually led to entertain the opinion that some means might be contrived for accomplishing this object by motions turning upon centres; and, working upon this idea, he gradually elaborated his invention. So soon as he caught sight of the possible means of overcoming the difficulty, he wrote to Boulton in Cornwall,-- “I have started a new hare. I have got a glimpse of a method of causing a piston-rod to move up and down perpendicularly by only fixing it to a piece of iron upon the beam, without chains or perpendicular guides or untowardly friction, arch heads, or other pieces of clumsiness; by which contrivance it answers fully to expectation. About 5 feet in the height of her house may be saved in 8-feet strokes, which I look upon as a capital saving, and it will answer for double engines as well as for single ones. I have only tried it in a slight model yet, so cannot build upon it, though I think it a very probable thing to succeed. It is one of the most ingenious, simple pieces of mechanism I have ever contrived, but I beg nothing may be said on it till I specify.”[262] [262] Watt to Boulton, 30th June, 1784. Boulton MSS. [Illustration: THE PARALLEL MOTION.] He immediately set to work to put his idea to the practical proof, and only eleven days later he wrote,--