be planted in hedgerows on walks or avenues, or for shade near a house, where nothing else is wanted to grow. The food for the worms, which produce the silk, is in the air, and the ground under the trees may still produce grass, or some other vegetable good for man or beast. Then the wear of silken garments continues so much longer, from the strength of the materials, as to give it greatly the preference. Hence it is that the most populous of all countries, China, clothes its inhabitants with silk, while it feeds them plentifully, and has besides a vast quantity, both raw and manufactured, to spare for exportation. Raw silk here, in skeins well wound, sells from twenty to twenty-five shillings per pound; but if badly wound is not worth five shillings. Well wound is, when the threads are made to cross each other every way in the skein, and only touch where they cross. Badly wound is, when they are laid parallel to each other; for so they are glued together, break in unwinding them, and take a vast deal of time more than the other, by losing the end every time the thread breaks. When once you can raise plenty of silk, you may have manufactures enough from hence. With great esteem, I am, my dear friend, yours affectionately, B. FRANKLIN."

1 Some of the author's friends in Philadelphia, engaged in raising silk, sent over a sample of their manufacture as a present to the Queen, which is alluded to in the following note:

"Dr. Franklin presents his respectful compliments to Sir John Pringle, and is much obliged to him for the trouble he has so kindly taken in the affair of the silk, and is very happy to learn that the Queen has graciously condescended to accept it with the purpose of wearing it. Her Majesty's countenance so afforded to the raisers of silk in Pennsylvania, where her character is highly revered, will give

CCCLXVIII

TO SAMUEL COOPER

LONDON, 30 September, 1769.

DEAR SIR:-Your favor of August 3d has given me great pleasure. I have only time now to acknowledge the receipt of it, but purpose to write fully by the next opportunity. I am just returned from France, where I found our dispute much attended to, several of our pamphlets being translated and printed there, among the rest my Examination' and the Farmer's Letters, with two of my pieces annexed, of which last I send you a copy. In short, all Europe (except Britain) appears to be on our side of the question. But Europe has its reasons. It fancies itself in some danger from the growth of British power, and would be glad to see it divided against itself. Our prudence will, I hope, long postpone the satisfaction our enemies expect from our dissensions. With sincere and great esteem, I am, dear Sir, &c., B. FRANKLIN.

CCCLXIX

ON VENTILATION

I do not know that we have in any author particular and separate directions concerning the ventilating them great encouragement to proceed in a measure which the British Parliament seems to have had much at heart, the procuring a supply of that valuable article from our colonies, for which at present large sums are paid to France, Spain, Italy, and the Indies."

1 Examination before the British House of Commons.

p. 171, under date of February 3, 1766.

See vol. iv.,

2 Written by John Dickinson, with a Preface by Dr. Franklin. See vol. iv., p. 437.

of hospitals, crowded rooms, or dwelling-houses, or the making of proper drains for carrying off stagnant or putrid water. The want of such general information on these subjects has induced me to endeavour to recollect all I can of the many instructive conversations I have had upon these matters with that judicious and most accurate observer of nature, Dr. Benjamin Franklin. I do this in hopes that either the Doctor himself or some other person well qualified for the task may follow the example set in so masterly a manner by Sir John Pringle, Baronet, when speaking on the preservation of the health of

seamen.

It has long been observed that if a number of persons are shut up in a small room, of which the internal air has little or no communication with the external, the respiration of those who are so confined renders by degrees the air of that room effete and unfit for the support of life.

Dr. Franklin was, if I mistake not, the first who observed that respiration communicated to the air a quality resembling the mephitic, such as the Grotto del Cane, near Naples. The air impressed with this quality rises only to a certain height, beyond which it gradually loses it. The amendment begins in the upper part, and descends gradually until the whole becomes capable of sustaining life. The Doctor confirmed this by the following experiment. He breathed gently through a tube into a deep glass mug, so as to impregnate all the air in the mug with this quality. He then put a lighted bougie into the mug, and upon touching the air therein the flame was

instantly extinguished. By frequently repeating this operation the bougie gradually preserved its light longer in the mug, so as in a short time to retain it to the bottom of it, the air having totally lost the bad quality it had contracted from the breath blown into it.

At the same time that the lower part of the air is thus affected, an acrid, noxious quality may be communicated to its upper part in the room, occasioned by the volatile putrescent effluvia of the persons enclosed therein. "It is surprising," says Sir John Pringle, in his Observations on the Diseases of the Army, fourth edition, p. 109, "in how few days the air will be corrupted in close and crowded wards; and what makes it hard to remedy the evil, is the difficulty of convincing either nurses or the sick themselves, of the necessity of opening the windows and doors at any time for a supply of fresh air."

It may be inferred from the above account of mephitic air, that such air can be but little altered by a ventilator in the ceiling of a room; and Dr. Franklin justly concluded, that in crowded rooms, and especially in bedrooms in dwelling-houses, a current of air should be kept up in the lower part of the rooms, to carry off what is thus affected. He approved of the use of chimneys for this purpose, especially when the current is quickened by a fire. Even when there is not any fire in the chimney, a current of air is constantly kept up in it, by its ascending or descending in the flue, as the weight of the internal or external air preponderates. This creates a kind of tide in the flue, conducing much to the healthiness of air in

rooms; and hence we may see the injudiciousness of having chimney-boards which fit closely, and thereby prevent a salutary circulation in the air. Hence also in warm weather we may account for liquors or other things kept in a chimney being cooled, and more so if means are used to create an evaporation around them.

Every person has an atmosphere of his own, heated by the warmth of his body, which can be dissipated only by motion in the circumambient air. Thus in warm weather, wind cools the body, by carrying off the personal atmosphere, and promoting at the same time a more free evaporation of the effluvia arising from the body. This creates a great degree of coolness on the skin. The personal atmosphere can be but little affected by a ventilator in the ceiling of a room, unless the admission of external air is so directed as to act principally on the air surrounding those in the room. Dr. Franklin, when consulted on ventilating the House of Commons, represented that the personal atmosphere surrounding the members might be carried off by making outlets in the perpendicular parts of the seats, through which the air might be drawn off by ventilators so placed as to accomplish this without admitting any by the same channels. It will appear from what has been said, that windows placed high in the walls of churches or rooms intended for large assemblies, can contribute but little towards correcting the mephitic quality of the lower part of the air, or towards carrying off the personal atmospheres.

The experiments made for ventilating crowded