Memoirs of Benjamin Franklin Volume II Part 27

What signifies philosophy that does not apply to some use? May we not learn from hence that black clothes are not so fit to wear in a hot sunny climate or season as white ones; because in such clothes the body is more heated by the sun when we walk abroad, and are, at the same time, heated by the exercise, which double heat is apt to bring on putrid dangerous fevers? That soldiers and seamen, who must march and labour in the sun, should in the East or West Indies have a uniform of white? That summer hats for men or women should be white, as repelling that heat which gives headaches to many, and to some the fatal stroke that the French call the _coup de soleil_? That the ladies' summer hats, however, should be lined with black, as not reverberating on their faces those rays which are reflected upward from the earth or water? That the putting a white cap of paper or linen _within_ the crown of a black hat, as some do, will not keep out the heat, though it would if placed _without_? That fruit-walls, being blacked, may receive so much heat from the sun in the daytime as to continue warm in some degree through the night, and thereby preserve the fruit from frosts or forward its growth? with sundry other particulars of less or greater importance, that will occur from time to time to attentive minds.

B. FRANKLIN.

_To the same._

ON THE EFFECT OF AIR ON THE BAROMETER. AND THE BENEFITS DERIVED FROM THE STUDY OF INSECTS.

Craven-street, June 11, 1760.

'Tis a very sensible question you ask, how the air can affect the barometer, when its opening appears covered with wood? If, indeed, it was so closely covered as to admit of no communication of the outward air to the surface of the mercury, the change of weight in the air could not possibly affect it. But the least crevice is sufficient for the purpose; a pinhole will do the business. And if you could look behind the frame to which your barometer is fixed, you would certainly find some small opening.

There are, indeed, some barometers in which the body of the mercury in the lower end is contained in a close leather bag, and so the air cannot come into immediate contact with the mercury; yet the same effect is produced. For the leather, being flexible, when, the bag is pressed by any additional weight of air, it contracts, and the mercury is forced up into the tube; when the air becomes lighter and its pressure less, the weight of the mercury prevails, and it descends again into the bag.

Your observations on what you have lately read concerning insects is very just and solid. Superficial minds are apt to despise those who make that part of the creation their study as mere triflers; but certainly the world has been much obliged to them. Under the care and management of man, the labours of the little silkworm afford employment and subsistence to thousands of families, and become an immense article of commerce. The bee, too, yields us its delicious honey, and its wax useful to a mult.i.tude of purposes. Another insect, it is said, produces the cochineal, from whence we have our rich scarlet dye. The usefulness of the cantharides, or Spanish flies, in medicine, is known to all, and thousands owe their lives to that knowledge. By human industry and observation, other properties of other insects may possibly be hereafter discovered, and of equal utility. A thorough acquaintance with the nature of these little creatures may also enable mankind to prevent the increase of such as are noxious, or secure us against the mischiefs they occasion. These things doubtless your books make mention of: I can only add a particular late instance, which I had from a Swedish gentleman of good credit. In the green timber intended for shipbuilding at the king's yard in that country, a kind of worms was found, which every year became more numerous and more pernicious, so that the ships were greatly damaged before they came into use. The king sent Linnaeus, the great naturalist, from Stockholm, to inquire into the affair, and see if the mischief was capable of any remedy. He found, on examination, that the worm was produced from a small egg, deposited in the little roughnesses on the surface of the wood, by a particular kind of fly or beetle; from whence the worm, as soon as it was hatched, began to eat into the substance of the wood, and, after some time, came out again a fly of the parent kind, and so the species increased. The season in which the fly laid its eggs Linnaeus knew to be about a fortnight (I think) in the month of May, and at no other time in the year. He therefore advised, that some days before that season, all the green timber should be thrown into the water, and kept under water till the season was over. Which being done by the king's order, the flies, missing the usual nests, could not increase, and the species was either destroyed or went elsewhere: and the wood was effectually preserved, for after the first year it became too dry and hard for their purpose.

There is, however, a prudent moderation to be used in studies of this kind. The knowledge of nature may be ornamental, and it may be useful; but if, to attain an eminence in that, we neglect the knowledge and practice of essential duties, we deserve reprehension. For there is no rank in natural knowledge of equal dignity and importance with that of being a good parent, a good child, a good husband or wife, a good neighbour or friend, a good subject or citizen, that is, in short, a good Christian. Nicholas Gimcrack, therefore, who neglected the care of his family to pursue b.u.t.terflies, was a just object of ridicule, and we must give him up as fair game to the satirist.

B. FRANKLIN.

_To Dr. Joseph Priestley._

EFFECT OF VEGETATION ON NOXIOUS AIR.

* * That the vegetable creation should restore the air which is spoiled by the animal part of it, looks like a rational system, and seems to be of a piece with the rest. Thus fire purifies water all the world over.

It purifies it by distillation, when it raises it in vapours, and lets it fall in rain; and farther still by filtration, when, keeping it fluid, it suffers that rain to percolate the earth. We knew before that putrid animal substances were converted into sweet vegetables when mixed with the earth and applied as manure; and now, it seems, that the same putrid substances, mixed with the air, have a similar effect. The strong, thriving state of your mint, in putrid air, seems to show that the air is mended by taking something from it, and not by adding to it.

I hope this will give some check to the rage of destroying trees that grow near houses, which has accompanied our late improvements in gardening, from an opinion of their being unwholesome. I am certain, from long observation, that there is nothing unhealthy in the air of woods; for we Americans have everywhere our country habitations in the midst of woods, and no people on earth enjoy better health or are more prolific.

B. FRANKLIN.

_To Dr. John Pringle._

ON THE DIFFERENCE OF NAVIGATION IN SHOAL AND DEEP WATER.

Craven-street, May 10, 1768.

You may remember, that when we were travelling together in Holland, you remarked that the trackschuyt in one of the stages went slower than usual, and inquired of the boatman what might be the reason; who answered, that it had been a dry season, and the water in the ca.n.a.l was low. On being asked if it was so low as that the boat touched the muddy bottom, he said no, not so low as that, but so low as to make it harder for the horse to draw the boat. We neither of us, at first, could conceive, that if there was water enough for the boat to swim clear of the bottom, its being deeper would make any difference; but as the man affirmed it seriously as a thing well known among them, and as the punctuality required in their stages was likely to make such difference, if any there were, more readily observed by them than by other watermen who did not pa.s.s so regularly and constantly backward and forward in the same track, I began to apprehend there might be something in it, and attempted to account for it from this consideration, that the boat, in proceeding along the ca.n.a.l, must in every boat's length of her course move out of her way a body of water equal in bulk to the room her bottom took up in the water; that the water so moved must pa.s.s on each side of her and under her bottom to get behind her; that if the pa.s.sage under her bottom was straitened by the shallows, more of that water must pa.s.s by her sides, and with a swifter motion, which would r.e.t.a.r.d her, as moving the contrary way; or, that the water becoming lower behind the boat than before, she was pressed back by the weight of its difference in height, and her motion r.e.t.a.r.ded by having that weight constantly to overcome. But as it is often lost time to attempt accounting for uncertain facts, I determined to make an experiment of this when I should have convenient time and opportunity.

After our return to England, as often as I happened to be on the Thames, I inquired of our watermen whether they were sensible of any difference in rowing over shallow or deep water. I found them all agreeing in the fact, that there was a very great difference, but they differed widely in expressing the quant.i.ty of the difference; some supposing it was equal to a mile in six, others to a mile in three, &c. As I did not recollect to have met with any mention of this matter in our philosophical books, and conceiving that if the difference should really be great, it might be an object of consideration in the many projects now on foot for digging new navigable ca.n.a.ls in this island, I lately put my design of making the experiment in execution in the following manner.

I provided a trough of planed boards fourteen feet long, six inches wide, and six inches deep in the clear, filled with water within half an inch of the edge, to represent a ca.n.a.l. I had a loose board, of nearly the same length and breadth, that, being put into the water, might be sunk to any depth, and fixed by little wedges where I would choose to have it stay, in order to make different depths of water, leaving the surface at the same height with regard to the sides of the trough. I had a little boat in form of a lighter or boat of burden, six inches long, two inches and a quarter wide, and one inch and a quarter deep. When swimming, it drew one inch water. To give motion to the boat, I fixed one end of a long silk thread to its bow, just even with the water's edge; the other end pa.s.sed over a well-made bra.s.s pully, of about an inch diameter, turning freely on a small axis; and a shilling was the weight. Then placing the boat at one end of the trough, the weight would draw it through the water to the other.

Not having a watch that shows seconds, in order to measure the time taken up by the boat in pa.s.sing from end to end, I counted as fast as I could count to ten repeatedly, keeping an account of the number of tens on my fingers. And as much as possible to correct any little inequalities in my counting, I repeated the experiment a number of times at each depth of water, that I might take the medium. And the following are the results:

Water 1-1/2 inches deep. 2 inches. 4-1/2 inches.

1st exp. 100 94 79 2d " 104 93 78 3d " 104 91 77 4th " 106 87 79 5th " 100 88 79 6th " 99 86 80 7th " 100 90 79 8th " 100 88 81 --- --- --- 813 717 632 --- --- --- Medium 101 Medium 89 Medium 79

I made many other experiments, but the above are those in which I was most exact; and they serve sufficiently to show that the difference is considerable. Between the deepest and shallowest it appears to be somewhat more than one fifth. So that, supposing large ca.n.a.ls, and boats, and depths of water to bear the same proportions, and that four men or horses would draw a boat in deep water four leagues in four hours, it would require five to draw the same boat in the same time as far in shallow water, or four would require five hours.

Whether this difference is of consequence enough to justify a greater expense in deepening ca.n.a.ls, is a matter of calculation, which our ingenious engineers in that way will readily determine.

B. FRANKLIN.

_To Oliver Neale._

ON THE ART OF SWIMMING.

I cannot be of opinion with you, that it is too late in life for you to learn to swim. The river near the bottom of your garden affords a most convenient place for the purpose. And as your new employment requires your being often on the water, of which you have such a dread, I think you would do well to make the trial; nothing being so likely to remove those apprehensions as the consciousness of an ability to swim to the sh.o.r.e in case of an accident, or of supporting yourself in the water till a boat could come to take you up.

I do not know how far corks or bladders may be useful in learning to swim, having never seen much trial of them. Possibly they may be of service in supporting the body while you are learning what is called the stroke, or that manner of drawing in and striking out the hands and feet that is necessary to produce progressive motion. But you will be no swimmer till you can place some confidence in the power of the water to support you; I would therefore advise the acquiring that confidence in the first place, especially as I have known several who, by a little of the practice necessary for that purpose, have insensibly acquired the stroke, taught, as it were, by nature.

The practice I mean is this. Choosing a place where the water deepens gradually, walk coolly into it till it is up to your breast; then turn round, your face to the sh.o.r.e, and throw an egg into the water between you and the sh.o.r.e. It will sink to the bottom, and be easily seen there, as your water is clear. It must lie in water so deep as that you cannot reach it to take it up but by diving for it. To encourage yourself in order to do this, reflect that your progress will be from deeper to shallower water, and that at any time you may, by bringing your legs under you and standing on the bottom, raise your head far above the water. Then plunge under it with your eyes open, throwing yourself towards the egg, and endeavouring, by the action of your hands and feet against the water, to get forward till within reach of it. In this attempt you will find that the water buoys you up against your inclination; that it is not so easy a thing to sink as you imagined; that you cannot, but by active force, get down to the egg. Thus you feel the power of the water to support you, and learn to confide in that power; while your endeavours to overcome it and to reach the egg teach you the manner of acting on the water with your feet and hands, which action is afterward used in swimming to support your head higher above water, or to go forward through it.

I would the more earnestly press you to the trial of this method, because, though I think I satisfied you that your body is lighter than water, and that you might float in it a long time, with your mouth free for breathing, if you would put yourself in a proper posture, and would be still and forbear struggling, yet, till you have obtained this experimental confidence in the water, I cannot depend on your having the necessary presence of mind to recollect that posture and directions I gave you relating to it. The surprise may put all out of your mind. For though we value ourselves on being reasonable, knowing creatures, reason and knowledge seem, on such occasions, to be of little use to us; and the brutes, to whom we allow scarce a glimmering of either, appear to have the advantage of us.

I will, however, take this opportunity of repeating those particulars to you which I mentioned in our last conversation, as, by perusing them at your leisure, you may possibly imprint them so in your memory as, on occasion, to be of some use to you.

1. That though the legs, arms, and head of a human body, being solid parts, are specifically something heavier than fresh water, yet the trunk, particularly the upper part, from its hollowness, is so much lighter than water, as that the whole of the body, taken together, is too light to sink wholly under water, but some part will remain above until the lungs become filled with water, which happens from drawing water into them instead of air, when a person, in the fright, attempts breathing while the mouth and nostrils are under water.

2. That the legs and arms are specifically lighter than salt water, and will be supported by it, so that a human body would not sink in salt water, though the lungs were filled as above, but from the greater specific gravity of the head.

3. That, therefore, a person throwing himself on his back in salt water, and extending his arms, may easily lie so as to keep his mouth and nostrils free for breathing; and, by a small motion of his hands, may prevent turning if he should perceive any tendency to it.

4. That in fresh water, if a man throws himself on his back near the surface, he cannot long continue in that situation but by proper action of his hands on the water. If he uses no such action, the legs and lower part of the body will gradually sink till he comes into an upright position, in which he will continue suspended, the hollow of the breast keeping the head uppermost.

5. But if, in this erect position, the head is kept upright above the shoulders, as when we stand on the ground, the immersion will, by the weight of that part of the head that is out of water, reach above the mouth and nostrils, perhaps a little above the eyes, so that a man cannot long remain suspended in water with his head in that position.

6. The body continuing suspended as before, and upright, if the head be leaned quite back, so that the face look upward, all the back part of the head being then under water, and its weight, consequently, in a great measure supported by it, the face will remain above water quite free for breathing, will rise an inch higher every inspiration, and sink as much every expiration, but never so low that the water may come over the mouth.

7. If, therefore, a person unacquainted with swimming, and falling accidentally into the water, could have presence of mind sufficient to avoid struggling and plunging, and to let the body take this natural position, he might continue long safe from drowning till perhaps help would come. For as to the clothes, their additional weight, while immersed, is very inconsiderable, the water supporting it, though, when he comes out of the water, he would find them very heavy indeed.

But, as I said before, I would not advise you or any one to depend on having this presence of mind on such an occasion, but learn fairly to swim, as I wish all men were taught to do in their youth; they would, on many occurrences, be the safer for having that skill, and on many more the happier, as freer from painful apprehensions of danger, to say nothing of the enjoyment in so delightful and wholesome an exercise.

Soldiers particularly should, methinks, all be taught to swim; it might be of frequent use either in surprising an enemy or saving themselves.

And if I had now boys to educate, I should prefer those schools (other things being equal) where an opportunity was afforded for acquiring so advantageous an art, which, once learned, is never forgotten.

B. FRANKLIN.

_To Miss Stephenson._