the juice found in vegetables.

We observed, when treating of Plants, that it has been long disputed whether the sap of plants be analogous to the blood of animals, and circulates in the same manner. We also mentioned the conclusions that Dr Hales drew from his numerous experiments, which were all in opposition to the doctrine that the sap circulates. As the subject is curious and interesting, and as additional light has been thrown upon it of late years, we wish to communicate it to our readers as fully as our limits will permit.

As the vegetable economy is still but imperfectly understood, and experiments made for tracing the motion of the sap may lead to important discoveries, we are happy to find, that of late years this subject has been again revived. Dr Walker, professor of Natural History in the university of Edinburgh, has published in the first volume of the Philosophical Transactions of Edinburgh an account of a course of very accurate and ingenious experiments, accompanied with observations and conclusions made with a caution which inspires confidence, and is indeed worthy of a disciple of Bacon. He is the first person, as far as we know, who thought of comparing the thermometer with the motion of the sap.

It is well known that in the spring vegetables contain a great quantity of sap; and there are some trees, as the birch and plane, which, if wounded, will discharge a great portion of it. Whence is this moisture derived? Whether is it imbibed from the atmosphere, or does it flow from the soil through the roots? These are the questions which require first to be answered; and Dr Walker's experiments enable us to answer them with confidence.

He selected a vigorous young birch, 30 feet high and 26 inches in circumference at the ground. He bored a hole just above the ground on the 1st of February, and cut one of its branches at the extremity. He repeated this every second day; but no moisture appeared at either of the places till the 5th of May, when a small quantity flowed on making an incision near the ground. He then cut 21 incisions in the trunk of the tree, on the north side, at the distance of a foot from one another, and reaching from the ground to the height of 30 feet. The incisions were solid triangles, each side being an inch long and an inch deep, and penetrating through the bark and wood. Dr Walker visited the tree almost every day for two months, and marked exactly from which of the incisions the sap flowed. He observed that it flowed from the lowest incision first, and gradually ascended to the highest. The following table will show the progress of the sap upwards, and its correspondence with the thermometer.

The first column is the day of the month on which the observation was made; the second expresses the number of incisions from which the sap flowed on the day of the month opposite; and the third column the degree of the thermometer at noon. Some days are omitted in March, as the incisions, though made on the 5th, did not bleed till the 11th. Some days are also passed over in April, because no observation was made on account of rain.

| March | N.ofI. | Ther. Noon. | March | N.ofI. | Ther. Noon. | |-------|--------|-------------|-------|--------|-------------| | 5 | | | 30 | | | | 11 | 2 | 49 | 31 | 7 | 62 | | 12 | 2 | 49 | | | | | 13 | 1 | 44 | April | 2 | 7 | | 14 | 4 | 48 | | 4 | 10 | | 15 | 5 | 52 | | 7 | 11 | | 16 | 5 | 47 | | 8 | 11 | | 17 | 4 | 44 | | 9 | 12 | | 18 | 5 | 47 | | 10 | 13 | | 19 | 6 | 48 | | 11 | 13 | | 20 | 5 | 44 | | 12 | 13 | | 21 | 7 | 48 | | 13 | 13 | | 22 | 7 | 45 | | 14 | 14 | | 23 | 8 | 46 | | 15 | 14 | | 24 | 9 | 47 | | 16 | 16 | | 25 | 9 | 42 | | 18 | 16 | | 26 | 7 | 39 | | 19 | 17 | | 27 | 8 | 45 | | 20 | 19 | | 28 | 8 | 49 | | 21 | 20 | | 29 | 8 | 46 | | 22 | 21 |

Dr Walker found that the sap ascends through the wood, and still more copiously between the wood and the bark; but none could be perceived ascending through the pith or the bark. He found also, that when the thermometer at noon is about 49°, or between 46° and 50°, the sap rises about one foot in 24 hours; that when the thermometer is about 45° at noon, it ascends about... one foot in two days; and that it does not ascend at all unless the mid-day heat be above 40°. He observed that it moves with more velocity through young than through old branches. In one young branch it moved through seven feet in one day, the thermometer being at 49°, while it moved in the trunk of the tree only seven feet in seven days. Dr Walker has thus explained the reason why the buds on the extremities of branches unfold first; because they are placed on the youngest wood, to which the sap flows most abundantly.

The effects produced by the motion of the sap deserve to be attended to. In those parts to which it has mounted, the bark easily separates from the wood, and the ligneous circles may, without difficulty, be detached from one another. The buds begin to swell and their scales to separate, while those branches to which the sap has not ascended remain closely folded. When the sap has reached the extremities of the branches, and has thus pervaded the whole plant, it is soon covered with opening buds and leaves to bleed. The bleeding leaves start in the upper parts of the tree, and in the lower parts successively downwards, and the wood becomes dry. An inverted branch flows more copiously when cut than those which are erect. This is a proof that the ascent of the sap is not occasioned by capillary attraction, for water which has risen in a small glass tube by this attraction will not descend when the tube is inverted.

It is evident that there is an intimate connection between heat and the ascent of the sap. It did not begin to flow till the thermometer stood at a certain point; when it fell below 40°, it was arrested in its progress. The south side of the tree, when the sun was bright, bled more profusely than the north side; and at sun-set the incisions at the top ceased to bleed, where it was exposed most to the cold air, while it still continued to flow from the incisions next to the ground; the ground retaining its heat longer than the air.

siege, is a trench, or an approach made under cover of 10 or 12 feet broad, when the besiegers come near the place, and the fire from the garrison grows so dangerous that they are not able to approach uncovered.—There are several sorts of saps; the single, which has only a single parapet; the double, having one on each side; and the flying, made with gabions, &c. In all saps traverses are left to cover the men.