HERSCHEL AND HIS WORK
duty was, under cover of a roof, to watch the clock, and to enter the measurements or remarks of the observer, as well as with the workman in attendance. A "speaking-pipe," as it was then called, of variable length to suit changes in his position, but 115 feet long at the most, was devised and fitted up. Usually his sister Caroline was the recorder who did the work, all-night work at times.
The mechanical skill shown in the construction of the telescope was proved sixty years after by Herschel's distinguished son. Sir John, in a letter already referred to, dated March 13, 1847: "The woodwork of the telescope being so far decayed as to be dangerous, in the year 1839, 1 pulled it down (the operation commenced on December 5), and having cleared away the framework, etc., piers were erected on which the tube was placed, that being of iron, and so well preserved, that although not more than one-twentieth of an inch thick, when in the horizontal position it sustained within it all my family, and continues to sustain enclosed within it to this day, not only the heavier of the two reflectors, but also all the more important portions of the machinery, such as being of iron and brass stood in no fear of decay, as well as all such portions of the polishing apparatus as would go into it, to the amount, I presume, of a great many tons, which had, when I last saw it, produced no sign of weakness or sagging down. This great strength and resistance to decay is to be attributed to the peculiar principle of its internal structure, which is, in effect, very similar to that for which, in later times, a patent has been taken out under the name of Corrugated Iron Roofing, etc., but of which the idea was, I