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living animal as the viper is. It is said, indeed, that frogs and other creatures have been found in his belly; but the truth of this is what I am desirous of knowing, and as this is the season for their making their appearance after the sleep of the winter, I shall be obliged to any curious naturalist, that will open a few of them this spring, and look into their stomachs, to inform us what he finds there, for at present ! can hardly think, if he feeds upon animal food at all, that it can be any thing more than worms and insects; for since he is not armed with poison, it is very difficult to conceive how he can master and manage any larger animal, though his gullet, I suppose, is as capable of distention for the swala lowing either of a mouse or small bird, as is the viper's.
Yours, &c. 1757, March.
XXVII. On the Phenomenon of Dew.
THÉ dispute concerning the origin of dew seems as yet tą be left undetermined. Some philosophers have insisted that it falls from the middle region of the air, others as strenuously assert that it rises from the bowels of the earth in vapour, which never reaches the middle region of the air, but falls back condensed into water, after having risen a compa, ratively small distance above the earth's surface.
The former of these alledge, in fayour of their opinion, ç that it is most natural; that we see the rain, which is of the same nature with dew, descending from the superior regions; and consequently ought not to suppose that the dew has any other origin, since it differs no otherwise from small rain, or misling, than in degree. That the atmosphere is continually replete with a vast quantity of vapours; and that, when the solar heat is withdrawn, the cold which occupies the superior regions immediately condenses and precipitates them, if not dissipated by the wind, in form of dew: and that those þubbles or vesicules, though imperceptible to us while separate, easily gather into larger drops (when they fall) by their own attraction; and are, in that state, found on grass, and on the herbs of the field and gar, den, in the morning, where they remain till they are again exhaled by the sun.'
Those of the contrary party say, “That exhalations are continually flying off from the earth; being raised either
the solar or subterraneous heat, or both. That these evaporations do not cease even in the night. That, during the heat of the day, these vapours, being specifically lighter than the circumambient air, are dissipated in their ascent; but, in the night, they rise not far above the ground, being immediately condensed and precipitated again by the cold. That though they cannot boast of the universality of their opinion, yet they hope it is established upon a surer foundation than the other; as they have had recourse to experiments, the most rigid tests of truth. That M. Dufay, in particular, being resolved to try the grand question whether dew did or did not first ascend in vapour, reduced it to this simple process. He considered, that if the dew did ascend it must wet a body placed lower, sooner than one placed higher, and its under part sooner than its upper; and upon these principles, he tried the following experiments. He placed two ladders, with their tops resting against each other, their feet at a considerable distance, and their height 32 feet. To the steps of these ladders he fastened squares
of glass, in such a manner as not to hang over each other. On trial, he found it exactly as he expected; the lower surface of the lowest square being first wetted, then its upper surface; then the lower surface of the second square; and so on gradually through the whole series."
These are some of the strongest arguments produced on each side in confirmation of each hypothesis. But perhaps neither side has been so fortunate as, upon the whole, to hit upon the true account, nor examined it so narrowly as to preclude any
future discoveries. I am, however, apt to believe, after repeated trials, that part of the dew does really fall. Í say part, for I hope to make it appear that a great deal of it, perhaps one half, except in thick foggy nights, rises, But when I say, rises, let it be noted that I do not mean in form of vapour; but in manner of perspiration from grass plants, and other herbage; the truth of which position the following experiments will, I hope, in a great measure, put beyond dispute.
EXPER. I. About an hour before sun-set, I inverted a large tub or vat upon some fine fresh grass, and stopped it so close at the bottom that it could have no communication with the external air. Upon examination in the morning I found the grass under the tub, to my surprise, charged as plentifully with dew, as that which was uncovered all around it: but the sphețules or drops, though equal in size, were only on the summits of the blades.--N.B. In a windy night there is seldom any dew, or very little; but the wind never affects the
grass at all; the drops being as large then, as at any other time.
II. The former experiment I repeated, but with this addition; under the tub, I suspended a large pane of glass horizontally about a foot, and a little tuft of wool at the same distance, from the ground; I also suspended another pane of glass and another little tuft of wool over the tub, exposed to the air. In the morning I found the grass as before, Glass and wool under the vessel perfectly dry; but that over it very
III. Made a great many trials on some gross garden plants, such as cabbages, coleworts, brocoli, and several others of the same species, by covering them with the same vessel, In the morning the edges of their leaves were always charged with large round drops; each drop dependent from the extremity of one of its ribs or fibres. When I traced my finger over the surface of the leaf, I could not be certain whether it was wet or not; but the surfaces of those that were uncovered were bedewed very plentifully.
IV. About ten o'clock in the forenoon, when the dew was all exhaled and the grass quite dry, I inverted the tub again; taking care always if it was not in a shady place, to cover it with something that might hinder the sun beams from penetrating; and, in a few hours time, I found the summits of every blade of grass,except those that werewithered, loaden with as large drops as would have been in the same space of time in the night, or perhaps larger. This experiment always succeeded in perfect regularity.
V. At mid-day I made the same experiment on some of the before-mentioned plants. The result was the same with Exp. III. but the drops were larger, and none were discernable either on the upper or under surfaces.
VI. Exposed a sqnare of glass, some pieces of cloth, wool, dry wood, &c, on the top of a building, about 60 feet from the ground; all which, in the morning, were very copiously wetted on their upper surfaces, but not underneath,
From these experiments, particularly the 2d and 6th, and part of the 3d, it appears, that some part of the dew actually falls; and, from the 1st, 4th, and 5th, and part of the 2d and 3il, that no small quantity of it rises; that is, perspires. It appears also from the 4th, that it rises by perspiration from the plants themselves, for if it had risen in vapour from the earth, it would have been found on the withered blades as well as the rest.
It seems to be a point pretty well agreed, by the naturalists, that there is a Circulation, or distribution, of the sap, or nu
tritious juices, in vegetables, something similar or analogous to that of the blood in animal bodies : and if so, why may not the vegetables, as well as the animals, have some way or other of sweating out the redundant juices ? That there is indeed something in all of them analogous to perspiration in animals is highly probable; but that it is sensible in some, the 4th and 5th experiments plainly evince. And of these secretions we should be witnesses, day as well as night, did. not the sun at that time, exhale the moisture as fast as it exsudates, nay several times faster, for when the heat is extreme, it exhausts the vessels of their nutrimental juice to such a degree, that the plant languishes and droops till the sun retires, and the waste is again made up by a fresh supply from the root. It seems to be these secretions which keep the common cabbage fresh and cool in the very hottest day; for did it not evacuate this cooling fluid in such large quantities, being such a gross and succulent plant, it would quickly languish and become quite faccid. Of the truth of this any one may be convinced, by cutting one directly through the middle; for, upon examining the several plicatures or folds, they will be found plentifully stored with drops of dew.
But the most remarkable instance of evacuations of this kind, in plants, is the Nepenthes. At the extremities of the leaves of this plant are certain vessels of a considerable bigness, on purpose to receive and preserve the superfluous juices, which it discharges in great abundance. A particular account of this wonderful plant may be seen in the 25th No. of Eden; from which I shall make the following extract, as it is very much to my purpose. " Glands of the secretory kind are very common in plants, though rarely conspicuous. They cover the whole stalk in the diamond inasembryanthemum; in the urena, they are situated on the back of the leaf; and, in the sunder, on its upper surface. All these secrete a watery fluid, but it is in few instances that it is detained in a kind of vessel. We see it so, however, in the leaves of the saracena ; in the maregravia it is lodged in a kind of vessels raised from the centre of the umbel; and in the nepenthes, not in the leaf itself, but in a peculiar appendage. We see the sunder, a minute plant, throw out its redundant moisture in big round drops. In the Æthiopian çalla, when over supplied with water, the fine and slender extremities of the leaves sweat out the load in a continual succession: this Comeline saw in Holland, as well as severa! persons in England. In the American hart's-tongue, the same incident propagates the plant. The fine and small end of the leaf is bent to the earth by the weight of the drop it gradually secretes; another and another follows, as it remains in that situation, and the plant, being full of life, takes root there, and prouinces a new stock, itself fixed to the earth by roots at each extremity. These are known instances of a secretion of this kini, though not generally understood; and this in the nepenthes is little more. It grows in thick forests, where its long fibres supply it well with water, and where no sun comes to exhale it." 1757, Oct.
XXVIII. Observations on the Gossamer.
March 13, 1759. I Do not remember to have met with a full and clear account, in any
ancient or modern writer, of a remarkable phenomenon in nature, commonly called the Gossamer. I hope therefore the following remarks will not be unacceptable to the public, especially to the lovers of natural philosophy.
The Gossamer is a fine filmy substance, like cobwebs, which is seen to float in the air, in clear sunny days in autumn; but much more observable in stubble-fields, and upon furze, and other low bushes. I often used to wonder from whence such a quantity of those tine threads could come, which I had frequently taken notice of in the stubblefields about Wandsworth, and on the furze bushes on Wimbledon and Putney commons. Yet I thought, that, as they had the appearance of the work of Spiders, I might find some such creatures in, or about them. I examined therefore the ground in the stubbles, and the bushes, on which they hung the thickest, with great diligence, but could not discover any thing like spiders, in those places, though I concluded there must be thousands of them somewhere, to be capable of making such multitudes of fine webs, and sometimes for inany days together. Now it happened that a while after (not having been able to satisfy myself in my inquiries on this subject) as I was reading over Mr. Ray's letters, I found what I had been puzzling myself about so long to no pur. pose.
That sagacious naturalist, about the year 1968, in a letter