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Letters by Benjamin Franklin
Benjamin Franklin on the conduction of heat:
The following is taken from a letter Benjamin Franklin wrote to Cadwallader Colden on December 6, 1753.
"Damp winds, though not colder by the thermometer, give a more uneasy sensation of cold than dry ones. Because, to speak like an electrician, they conduct better: that is, are better fitted to convey the heat away from our bodies. The body cannot feel without itself; our sensation of cold is not in the air without the body, but in those parts of the body that have been deprived of their heat by the air.
My desk and its lock are, I suppose, of the same temperature when they have been long exposed to the same air; but now, if I lay my hand on the wood, it does not seem so cold to me as the lock; because I imagine, wood is not so good a conductor, to receive and convey away the heat from my skin and the adjacent flesh, as metal is. Take a piece of wood of the size and shape of a dollar between the thumb and fingers of one hand, and a dollar in like manner with the other hand; place the edges of both at the same time in the flame of a candle; and though the edge of the wooden piece takes flame, and the metal piece does not, yet you will be obliged to drop the latter before the former, it conducting the heat more suddenly to your fingers.
Thus we can without pain handle glass and china cups filled with hot liquors, as tea, etc., but not silver ones. A silver teapot must have wooden handle. Perhaps it is for the same reason that woolen garments keep the body warmer than linen ones equally thick: woolen keeping the natural heat in, or, in other words, not conducting it out to the air."
Benjamin Franklin on bifocal spectacles:
This is an excerpt of a letter written to George Whatley by Benjamin Franklin on May 23, 1785.
...By Mr. Dollond's saying, that my double spectacles can only serve particular eyes, I doubt he has not been rightly informed of their construction. I imagine it will be found pretty generally true, that the same convexity of glass, through which a man sees clearest and best at the proper distance for reading, is not the best for greater distances. I therefore had formerly two pairs 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.
Sources:
Editors. Science Milestones. Windsor Press: NY 1954.
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