trickles down the twigs and boughs, so as to make the ground below quite in a float. In Newton-lane, in October, 1775, on a misty day, a particular oak in leaf dropped so fast that the cart-way stood in puddles, and the ruts ran with water, though the ground in general was dusty.*

In some of our smaller islands in the West Indies, if I mistake not, there are no springs or rivers; but the people are supplied with that necessary element, water, merely by the dripping of some large tall trees, which, standing in the bosom of a mountain, keep their heads constantly enveloped with fogs and clouds, from which they dispense their kindly, neverceasing moisture; and so render those districts habitable by condensation alone. †

* The house in which we resided in Fife was built on a greenstone rock, on the south brow of the high ground overlooking the beautiful river Leven, about two hundred feet above its level, and five hundred feet distant from it. We there remarked, that, even in closets in the garrets, shoes, and all kinds of leather, soon became mouldy, which could be produced only by the moisture generated by the trees, which in thick groves closely surrounded the house. .ED.

†There are no rivulets, or springs, in the island of Ferro, the westmost of the Canaries, except on a part of the beach, which is nearly inaccessible. To supply the place of a fountain, however, Nature, ever bountiful, has bestowed upon this island a species of tree, unknown to all other parts of the world. It is of moderate size, and its leaves are straight, long, and evergreen. Around its summit a small cloud perpetually rests, which so drenches the leaves with moisture, that they continually distil upon the ground a stream of fine clear water. To these trees, as to perennial springs, the inhabitants of Ferro resort; and are thus supplied with an abundance of water for themselves and for their cattle.

The trunk of this tree is about nine feet in circumference; the top branches are not higher than thirty feet from the ground; the circum ference of all the branches together is one hundred and twenty feet; the branches are thick, and extended, the leaves being about three feet nine inches from the ground. Its fruit is shaped like that of the oak, but tastes like the kernel of a pine apple, and the leaves resemble those of the laurel, but are longer, wider, and curved.

This

Trees require a great quantity of water to supply their organs. is given off in perspiration by their leaves. In the experiments of Hales on the quantity of water taken up by plants, it was found that a peartree, which weighed seventy-one pounds, absorbed fifteen pounds of water in six hours; and that branches of an inch diameter, and from five to six feet high, sucked up from fifteen to thirty ounces in twelve hours. When these were stript of their leaves, they only sucked up one ounce in twelve hours.

The white birch tree, betula alba, is noted on account of the wine that is extracted from it, and is said to possess the medical qualities of an antiscorbutic, deobstruent, and diuretic. The method of bleeding the tree is performed thus: -About the beginning of March, an oblique cut is

Trees in leaf have such a vast proportion more of surface than those that are naked, that, in theory, their condensations should greatly exceed those that are stripped of their leaves : but, as the former imbibe also a great quantity of moisture, it is difficult to say which drip most: but this I know, that deciduous trees, that are entwined with much ivy, seem to distil the greatest quantity. Ivy leaves are smooth, and thick, and cold, and therefore condense very fast; and besides, evergreens imbibe very little.* These facts may furnish the intelligent with hints concerning what sorts of trees they should plant round small ponds that they would wish to be perennial; and shew them how advantageous some trees are in preference to others.

Trees perspire profusely, condense largely, and check evaporation so much, that woods are always moist; no

made, almost as deep as the pith, under some well spreading branch, into which a small stone or chip is inserted to keep the lips of the wound open. To this orifice a bottle is attached to collect the flowing juice, which is limpid, watery, and sweetish, but retains something of both the taste and odour of the tree. One tree affords two or three gallons a day; at the same time, it receives no perceptible injury from being thus bled, from which it would appear, that much of its moisture has at other times been given off through its leaves; and, in all probability, it acquires an increased action to supply the extra quantity which is thus drained from it. - Ec.

*There can be little doubt, that the moisture of climate is greatly influenced by trees. It has been remarked, after cutting down forests, particularly on high grounds, that the quantity of rain has been lessened, by diminishing, it is supposed, the attraction between the earth and the clouds. This fact has been experienced on a large scale in America. In Kentucky there are many brooks, now completely dry in summer, which afforded an abundant supply of water all the year round about twenty-five or thirty years ago; and, in some parts of the state of New Jersey, where the woods have been extensively cleared away, many streams have altogether disappeared.

The climate of Britain, it is very generally believed, has deteriorated by becoming much more changeable than it was sixty years ago. This has, with much probability, been attributed to the extent of planting, to the introduction of green crops, and abolition of fallows in an improved system of agriculture. Mr Murray is of opinion, that trees, by condensing the moisture of the air in foggy weather, materially affect the climate, and that thickly wooded countries must necessarily be colder, and more humid than naked savannahs. Trees are, therefore, it would seem, ready conductors of aërial electricity, the climate being improved when woods are cleared away, and becoming more moist by planting. This fact receives corroboration from the history of our own country, as well as from that of North America. — ED.

wonder, therefore, that they contribute much to pools and

streams.

That trees are great promoters of lakes and rivers, appears from a well-known fact in North America; for, since the woods and forests have been grubbed and cleared, all bodies of water are much diminished: so that some streams, that were very considerable a century ago, will not now drive a common mill.* Besides, most woodlands, forests, and chases, with us, abound with pools and morasses, no doubt for the reason given above.+

To a thinking mind, few phenomena are more strange than the state of little ponds on the summits of chalk hills, many of which are never dry in the most trying droughts of summer; on chalk hills, I say, because in many rocky and gravelly soils, springs usually break out pretty high on the sides of elevated grounds and mountains; but no person acquainted with chalky districts will allow that they ever saw springs in such a soil but in valleys and bottoms, since the waters of so pervious a stratum as chalk all lie on one dead level, as well-diggers have assured me again and again.‡

Now, we have many such little round ponds in this district; and one in particular on our sheep-down, three hundred feet above my house; which, though never above three feet deep in the middle, and not more than thirty feet in diameter, and

* Vide Kalm's Travels to North America.

For the diminution of some of the lakes and rivers of America, we must seek other causes. About a thousand rivers and streams empty themselves into Lake Superior, sweeping into it earth, primitive boulder stones, and drift timber, which sometimes accumulate so much as to form islands in the estuaries. A lignite formation, indeed, is said to be now in progress, similar to that of Bovey in Devonshire. Within a mile of the shore, the water is about seventy fathoms; within eight miles, one hundred and thirty-six fathoms; and the greatest depth of the lake, farther from the shore, is unknown. Lake Erie, from similar causes, is gradually becoming shallower. Long Point, for example, has, in three years, gained no less than three miles on the water. - ED.

In making wells at Modena, in Italy, the workmen dig through several strata of soils, till they come to a very hard kind of earth, much resembling chalk; here they begin the mason-work, and build a wall, which they carry on at their leisure till they finish it, without being interrupted with one drop of water, and without any apprehension of not finding it when they come to make the experiment. The wall being completed, they bore through the bed of chalk, at the bottom, with a long auger, but take care to ascend from the pit before they draw out the instrument again: which when they have done, the water springs up into the well, and in a little time rises to the brim-nay, sometimes overflows the neighbouring grounds. En.

198 PONDS ON THE SUMMITS OF CHALK HILLS.

containing pernaps not more than two or three hundred hogsheads of water, yet never is known to fail, though it affords drink for three hundred or four hundred sheep, and for at least twenty head of large cattle besides. This pond, it is true, is overhung with two moderate beeches, that, doubtless, at times, afford it much supply; but then we have others as small, that, without the aid of trees, and in spite of evaporation from sun and wind, and perpetual consumption by cattle, yet constantly maintain a moderate share of water, without overflowing in the wettest seasons, as they would do if supplied by springs. By my journal of May, 1775, it appears that "the small and even considerable ponds on the vales are now dried up, while the small ponds on the very tops of hills are but little affected." Can this difference be accounted for from evaporation alone, which certainly is more prevalent in bottoms? or rather have not those elevated pools some unnoticed recruits, which in the night-time counterbalance the waste of the day; without which, the cattle alone must soon exhaust them? And here it will be necessary to enter more minutely into the cause. Dr Hales, in his Vegetable Statics, advances, from experiment, that the moister the earth is, the more dew falls on it in a night; and more than a double quantity of dew falls on an equal surface of moist earth." Hence we see that water, by its coolness, is enabled to assimilate to itself a large quantity of moisture nightly by condensation ; and that the air, when loaded with fogs and vapours, and even with copious dews, can alone advance a considerable and neverfailing resource. * Persons that are much abroad, and travel early and late, such as shepherds, fishermen, &c. can tell what prodigious fogs prevail in the night on elevated downs, even in the hottest parts of summer; and how much the surfaces of things are drenched by those swimming vapours, though, to the senses, all the while, little moisture seems to fall.

*Fogs are much more frequent in cold seasons, and cold countries, than in such as are warm; because, in the former, the aqueous particles, being condensed almost as soon as they proceed from the surface of the earth, are incapable of rising into the higher portions of the atmosphere. If the cold be augmented, the fog freezes, attaching itself in small icicles to the branches of trees, and to the hair and clothes of persons exposed to it, to the blades of grass, and other substances.- ED.

LETTER LXXII.

TO THE HON. DAINES BARRINGTON.

SELBORNE, April 3, 1776. DEAR SIR, Monsieur Herissant, a French anatomist, seems persuaded that he has discovered the reason why cuckoos do not hatch their own eggs; the impediment, he supposes, arises from the internal structure of their parts, which incapacitates them for incubation. According to this gentleman, the crop, or craw, of a cuckoo, does not lie before the sternum at the bottom of the neck, as in the gallinæ, columbæ, &c. but immediately behind it, on and over the bowels, so as to make a large protuberance in the belly.*

Induced by this assertion, we procured a cuckoo; and, cutting open the breast-bone, and exposing the intestines to sight, found the crop lying as mentioned above. This stomach was large and round, and stuffed hard, like a pin-cushion, with food, which, upon nice examination, we found to consist of various insects; such as small scarabs, spiders, and dragonflies; the last of which we have seen cuckoos catching on the wing, as they were just emerging out of the aurelia state. Among this farrago also were to be seen maggots, and many seeds, which belonged either to gooseberries, currants, cranberries, or some such fruit; so that these birds apparently subsist on insects and fruits; nor was there the least appearance of bones, feathers, or fur, to support the idle notion of their being birds of prey.†

The sternum in this bird seemed to us to be remarkably short, between which and the anus lay the crop, or craw, and, immediately behind that, the bowels against the back-bone.

It must be allowed, as this anatomist observes, that the crop, placed just below the bowels, must, especially when full, be in a very uneasy situation during the business of incubation ; yet the test will be, to examine whether birds that are actually known to sit for certain are not formed in a similar manner.

*Histoire de l'Academie Royale, 1752.

Sir William Jardine says, that when cuckoos have fed much on some of the large hairy caterpillars so common on the northern moors, the stomach becomes coated with the short hairs, which may have given rise to the opinion that they are predatory. But has not Sir William mistaken the fibrous structure of the stomach for these hairs? Its American congeners, the yellow-billed cuckoo, and the black-billed cuckoo, rob birds of their eggs; and the latter feeds on fresh water shell-fish. — ED.