Abbildungen der Seite


Nullum bonum philosophiâ optabilius, nullum præstantius.


There is nothing more to be wished for, there is nothing more excellent, than philosophy.

I HAVE always found that natural philosophy, when treated in an intelligible and unaffected manner, without the terms too generally brought into discourses on it, merely to shew that the author is acquainted with them, becomes as familiar and as agreeable a subject of common disquisition, as any in the whole round of sci


Under this persuasion I shall venture to make a topic of this kind the subject of to-day's entertainment; and though I shall, on this occasion, contradict, at my first setting out, the common opinion of the world in regard to a thing supposed universally known, I am in no doubt but I shall be able, in the short limits of this paper, to prove what I assert: and I am well convinced that, after that, none of my readers will continue in known error.

The falling of the dew is a phrase received

in all languages, among all people, learned and ignorant; and all express by it their opinion, that those drops of water which we find in mornings and evenings on the grass and herbage of the fields, have descended from the upper regions of the air. On the contrary, I assert, not as an opinion, but as a certainty, that these drops of dew, never, in this state, were higher above the earth than we see them, and that they do not descend from on high at all, but rise out of the earth, and never, as dew, fall to it again.

There is, indeed, no lawin nature by which dew could be formed, as it has been generally understood to be; but all the established doctrines of philosophy and mechanics concur in the production and formation of it on this plan. The earth is, to some considerable depth, always more or less moist; the action of the sun heats the earth's surface, and heat must raise that moisture up in vapour: the heat occasioned by the sun will conti. nue, though in a more remiss degree, during the whole night; and while it continues, vapours will also continue to be raised; it is evident, therefore, that vapours are rising all day and all night from the earth. What rise in the day-time are dispersed and evaporated by the heat of the air, as soon as raised, and we see nothing

of them; but what rise in the absence of the sun, and in a cooler state of the air, form themselves into drops according to the known laws of at: traction.

Such, then, is the nature and origin of dew; it is water raised in form of vapour from the earth, in consequence of its being heated by the sun; it collects itself into drops on any thing proper to receive and retain it; 'or it hangs in the lower regions of the air, in form of a fog or mist, till the sun's rays evaporate and dissipate it. Such are the assertions of the Inspector in regard to dew; the facts which led to, and will be found to support them are these. The late Lord Petre, with whom I had the honour to enjoy a particular intimacy, had engaged me to spend a part of the last summer of his life at his house in Essex. He was as fond as myself of experiments that tended to some obvious purpose, and accompanied my observations during that whole period. One of these was an experiment in regard to the quantity of dew suspended in the air at the different periods of the night. The manner of experimenting this, was by hanging up several bundles of tow at different heights in the air, and weighing them, from time to time, as they became more and more wetted by it. We evidently found from

this, that the dew impregnated the air in greater quantities in the beginning of the night than at any other time; the increase of moisture growing less and less to the morning.

Additionally to this, however, I discovered that those bundles of tow which had hung lowest, or nearest the earth, were wet sooner than those which were placed higher. From this circumstance I alleged, that the dew did not descend from the air, but ascend from the earth. The thought at first startled his lordship, but we determined to give it a fair trial. We suspended a large square of glass flatwise, by a string, from a horizontal pole laid over the tops of two distant trees in the garden ; and we found its lower surface became wet sooner than

its upper.

A large tree had just at this time been transplanted by this nobleman's order, and was supported erect by three poles of thirty feet high, which were fixed with their tops at its trunk, and their bottoms at a considerable distance from its root in the earth. A carpenter was employed to make grooves at three foot distance, all the way up each of these poles, for the reception of plates of glass : a number of these plates, of four or five inches in diameter, were fixed by their edges in those grooves; and as they were so placed as not to obstruct the passage of vapours either from above or below to one another, we knew it must be easy, by observing which of these, and which surface of those, was wetted first, to determine whether the dew rose or fell. The whole apparatus was fixed in the day-time, and the gardener's steps were placed near for making the observation.

The evening proved windy, and I excepted against it, as improper for the observation, since it was evident the course of the vapours either way must be disturbed: the same objection held against many successive nights; at length there was a perfect calm one; we were up great part of the night at the observation, repeating it occasionally on one of the posts, by wiping the glasses. We found the under surface of the lowest plates first wet, after that the upper surface of the same plates, but much less so; after these the lower surface of the second plates, then their upper; then the lower surface of the third set from the ground, and so in perfect regularity.

When both surfaces of all these were thoroughly wetted, we mounted the steps and examined the upper plates; all these we found perfectly dry; they continued so for some time,

« ZurückWeiter »