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the string or sinew tight, which will cause the thumb and finger to pinch the book strongly, so that you may draw it out. As it leaves the other books, turn the instrument a quarter round, so that the book may lie flat and rest on its side upon the under lath or finger. The knots on the sinew will help you to keep it tight and close to the side of the arm as you take it down. hand over hand, till the book comes to you; which would drop from between the thumb and finger if the sinew was let loose.

All new tools require some practice before we can become expert in the use of them. This requires very little.

Made in the proportions above given, it serves well for books in duodecimo or octavo. Quartos and folios are too heavy for it; but those are usually placed on the lower shelves within reach of hand.

The book taken down, may, when done with, be put up again into its place by the same machine.

TO BENJAMIN VAUGHAN.

On the Effects of Lead upon the Human Constitution.

DEAR FRIEND,

Philadelphia, 31 July, 1786.

I recollect, that, when I had last the pleasure of seeing you at Southampton, now a twelvemonth since, we had some conversation on the bad effects of lead taken inwardly; and that at your request I promised to send you in writing a particular account of several facts I then mentioned to you, of which you thought some good use might be made. I now sit down to fulfil that promise.

The first thing I remember of this kind was a genera! discourse in Boston, when I was a boy, of a complaint from North Carolina against New England rum, that it poisoned their people, giving them the dry bellyache, with a loss of the use of their limbs. The distilleries being examined on the occasion, it was found that several of them used leaden still-heads and worms, and the physicians were of opinion, that the mischief was occasioned by that use of lead. The legislature of Massachusetts thereupon passed an act, prohibiting, under severe penalties, the use of such still-heads and worms thereafter.

In 1724, being in London, I went to work in the printing-house of Mr. Palmer, Bartholomew Close, as a compositor. I there found a practice, I had never seen before, of drying a case of types (which are wet in distribution) by placing it sloping before the fire. I found this had the additional advantage, when the types were not only dried but heated, of being comfortable to the hands working over them in cold weather. I therefore sometimes heated my case when the types did not want drying. But an old workman, observing it, advised me not to do so, telling me I might lose the use of my hands by it, as two of our companions had nearly done, one of whom, that used to earn his guinea a week, could not then make more than ten shillings, and the other, who had the dangles, but seven and sixpence. This, with a kind of obscure pain, that I had sometimes felt, as it were in the bones of my hand when working over the types made very hot, induced me to omit the practice. But talking afterwards with Mr. James, a letter-founder in the same Close, and asking him if his people, who worked over the little furnaces of melted metal, were not subject to that disorder; he made light of any danger from the effluvia, but ascribed it to par

ticles of the metal swallowed with their food by slovenly workmen, who went to their meals after handling the metal, without well washing their fingers, so that some of the metalline particles were taken off by their bread and eaten with it. This appeared to have some reason in it. But the pain I had experienced made me still afraid of those effluvia.

Being in Derbyshire at some of the furnaces for smelting of lead ore, I was told, that the smoke of those furnaces was pernicious to the neighbouring grass and other vegetables; but I do not recollect to have heard any thing of the effect of such vegetables eaten by animals. It may be well to make the inquiry.

In America I have often observed, that on the roofs of our shingled houses, where moss is apt to grow in northern exposures, if there be any thing on the roof painted with white lead, such as balusters, or frames of dormant windows, &c., there is constantly a streak on the shingles from such paint down to the eaves, on which no moss will grow, but the wood remains constantly clean and free from it. We seldom drink rainwater that falls on our houses; and if we did, perhaps the small quantity of lead, descending from such paint, might not be sufficient to produce any sensible ill effect on our bodies. But I have been told of a case in Europe, I forget the place, where a whole family was afflicted with what we call the dry bellyache, or colica pictorum, by drinking rain water. It was at a country-seat, which, being situated too high to have the advantage of a well, was supplied with water from a tank, which received the water from the leaded roofs. This had been drunk several years without mischief; but some young trees planted near the house growing up above the roof, and shedding their leaves upon it, it was supposed that an acid in those leaves had corroded the lead they covered,

and furnished the water of that year with its baneful particles and qualities.

When I was in Paris with Sir John Pringle in 1767, he visited La Charité, a hospital particularly famous for the cure of that malady, and brought from thence a pamphlet containing a list of the names of persons, specifying their professions or trades, who had been cured there. I had the curiosity to examine that list, and found that all the patients were of trades, that, some way or other, use or work in lead; such as plumbers, glaziers, painters, &c., excepting only two kinds, stonecutters and soldiers. In them I could not reconcile it to my notion, that lead was the cause of that disorder. But on my mentioning it to a physician of that hospital, he informed me that the stonecutters are continually using melted lead to fix the ends of iron balustrades in stone; and that the soldiers had been employed by painters, as laborers, in grinding of colors.

This, my dear friend, is all I can at present recollect on the subject. You will see by it, that the opinion of this mischievous effect from lead is at least above sixty years old; and you will observe with concern how long a useful truth may be known and exist, before it is generally received and practised on.

I am, ever, yours most affectionately,

B. FRANKLIN.

On Thermometers.

13 September, 1786.

THE two thermometers most generally in use at present, among the philosophers of Europe, are those of Réaumur and Fahrenheit. The French use Réaumur's, the English Fahrenheit's.

In their respective graduations, Réaumur marked his freezing point 0, Fahrenheit fixed his at 32 of his degrees above 0, and two of his degrees are just equal to one of Réaumur's. I know that in some instruments shis equality is not exact; but, in two which I have, the one Réaumur's, made by Cappy in Paris, the other Fahrenheit's, by Nairne, London, it is precisely so, they hanging together in the same room. And those work

men are famed for their exactness.

In reading, one frequently finds degrees of heat and cold mentioned, as measured by one or the other of those thermometers, and one is at a loss to reduce that least known to the other.

Rule.

Suppose the degree mentioned is 25 of Réaumur, which is 25 degrees above 0, or its freezing point, and you would know to what degree of Fahrenheit that

answers;

Double the 25, which will give you 50 of Fahrenheit's, and to them add 32, his number at the freezing point, and you will have 82, the degree of Fahrenheit's equal to 25 of Réaumur.

On the contrary, if you would reduce Fahrenheit to Réaumur, first subtract 32, and then take half of the remainder; thus taking 32 from 82, there remains 50, and the half of 50 is 25.

This answers in all cases where the degree is above the freezing point.

If below, double the degrees of Réaumur, and subtract them from the 32 of Fahrenheit, which will give you the equivalent degree of his scale. Thus, suppose it 5 below 0, or the freezing point of Réaumur; twice 5 is 10, which deducted from 32, Fahrenheit's freezing

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