Franklin also possessed to a striking degree the inventive capacity which is such a valuable qualification for experimental philosophy. We have already seen how ready his mechanical skill was in supplying printing deficiencies. Speaking of the pulse glasses, made by Nairne, in which water could be brought to the boiling point with the heat of the hand, he tells us: I plac'd one of his glasses, with the elevated end against this hole (a hole that he had opened through the wainscot in the seat of his window for the access of outside air); and the bubbles from the other end, which was in a warmer situation, were continually passing day and night, to the no small surprize of even philosophical spectators. As he sat in his library at Philadelphia, in his last years, he was surrounded by various objects conceived by his own ingenuity. The seat of his chair became a step-ladder, when reversed, and to its arm was fastened a fan that he could work with a slight motion of his foot. Against his bookcase rested "the long arm" with which he lifted down the books on its upper shelves. The hours, minutes and seconds were told for him by a clock, of his own invention, with only three wheels and two pinions, in which even James Ferguson, mathematician as he was, had to confess that he experienced difficulty in making improvements. The very bifocal glasses, now in such general use, that he wore were a triumph of his own quick wit. Describing this invention of his in a letter to George Whatley, he said: I therefore had formerly two Pair of Spectacles, which I shifted occasionally, as in travelling I sometimes read, and often wanted to regard the Prospects. Finding this Change troublesome, and not always sufficiently ready, I had the Glasses cut, and half of each kind associated in the same Circle.... By this means, as I wear my Spectacles constantly, I have only to move my Eyes up or down, as I want to see distinctly far or near, the proper Glasses being always ready. This I find more particularly convenient since my being in France, the Glasses that serve me best at Table to see what I eat, not being the best to see the Faces of those on the other Side of the Table who speak to me; and when one's Ears are not well accustomed to the Sounds of a Language, a Sight of the Movements in the Features of him that speaks helps to explain; so that I understand French better by the help of my Spectacles. The shrinking that a mahogany box, given to him in England, underwent, when subjected to the atmospheric conditions of America, suggested a hygrometer to him which Nairne afterwards constructed in accordance with his plans.[55]