Графични страници
PDF файл
ePub

it was the same to any considerable depth. For this determination, I procured at the dock-yard, by the permission of the Navy Board, ten pieces of oak a foot long, and about six inches in diameter, which were specifically heavier than water, and ten similar pieces of larch, which had a much less specific gravity than water. These were connected, two and two together, by small chains of different lengths, from fourteen feet long to two feet, so that each pair acted like a hook and quill, and they were so adjusted, by small weights of lead, that only the upper surface of the larch cylinders was above water ; there were, moreover, two larch cylinders, which floated merely on the surface without any appending oak cylinders. The upper part of the larch cylinders were painted of different colours and forms, so that each might be distinguished from the rest.

These were all thrown into the middle of the stream opposite Woolwich Dock-yard, at about one hour after low water ; they, of course, immediately proceeded with the current, and I accompanied them in a boat to register their progress. It was presumed, that, if the upper and lower parts of the stream had different velocities, that the deeper floats would be left behind those which were less deep; and these latter behind those which were merely on the surface : but that if the whole body of water had a common velocity, they would all proceed together.

It was soon found that the effect of currents at different depths, caused the floats to separate considerably from each other, but still their progressive velocity up the river was nearly the same; so nearly, indeed, that after following them for more than two hours, they all passed the same line, opposite Greenwich Hospital, within one minute of each other ; although in their progress, some of them had passed under the keels of vessels, which intercepted their path.

We now waited till the time of high-water, and then immersed them again opposite the Hospital. The case was now very different ; for we found the deeper Aoats still ascending the river, others descending, and others nearly motionless ; so that we had some difficulty to collect them. This, however, we did, in about half an hour, when they were again set afloat, and they now, as before, proceeded pretty uniformly, and passed a line of the river at Woolwich (except two which had gone ashore) within about the same interval as before.

It follows, therefore, that, except for a short interval about the time of ebb and flood, the whole body of water is moving with the same, or yery nearly the same, velocity; and hence the quantity which passes any section of the river during ebb and flood may be pretty accurately estimated, the section being given.

Without entering into a minute examination of all the preceding particulars, some of the consequences of the removal of the bridge may be easily foreseen.

For example, since the high water-mark is a dead level from Blackwall to the London Docks, and thence to the bridge, abstracting only 1$ inch at the bridge (which is unquestionably attributable to the fall at that place); and since it is also a dead level from the bridge to Richmond, there can be no doubt that the same law will obtain when the bridge is removed ; so that at a medium spring-tide, we may expect the high water-mark from the bridge upwards to Richmond to be higher by about 13 inches than it is at present, and from Richmond towards Teddington this rise will gradually diminish, till it is lost in the general ascent of the bed of the river to that place.

With respect to the low water-line, it will fall very considerably below its present mark immediately above bridge, but not, perhaps, to the amount of the present head, viz. 5 feet 4 at a medium spring-tide, because the quantity of water which will

pass

the bridge, during the ebb, will be considerably greater than at present, and will, of course, acquire a new velocity consistent with the new circumstances, and consequently a new slope; so that it is possible the new low water-line, at the site of the bridge, will fall between the present low water-mark above, and that below the bridge, but much nearer to the latter than the former.

The additional quantity of water which will pass and repass the new bridge each tide, will consist of all that which will be admitted upwards, more than at present, at high-water, and of the greater part of that which is now dammed up above bridge at low water, which will together amount, perhaps, to about oneseventh of that which now passes. This, of course, will increase the velocity both of the flood and ebb tides, and have a tendency to deepen the river from the bridge upwards; but the navi,

gation for large barges, it is presumed, will, notwithstanding, be impeded for an hour or two each tide, at and during the time of low-water, particularly at spring-tides.

It was not, however, my intention, in this paper, to do more than record the preceding particulars, for the purpose of comparison hereafter. I shall not, therefore, enter farther

upon

the probable changes which the removal of the bridge may occasion; a short time will decide the question, by the best of all tests, actual experience.

On the Afinities of the Empetreæ, a natural Group of Plants.

By Mr David Don, Libr. L. S. &c. Communicated by the Author.

I am aware that it has already been proposed to separate Empetrum from the Erice@*; but the mere removal of it from that family, was nothing more than what Jussieu himself had previously done. That its relative affinities have hitherto been entirely overlooked, no one who has given the least attention to the investigation will for a moment question ; and it would only be a waste of time to attempt to point out the discrepances between the Empetreæ and Ericea, or between them and Conifera, of which Nuttall is disposed to consider them as a section ti for, with the exception of a slight resemblance in habit, there is really neither analogy nor affinity between them; nor do the Empetree even belong to the same natural class with either of these families. I have, however, lately discovered a remarkable affinity between this group and Euphorbiacea, as well as Celastrinæ, which it is my principal object in this paper to point out; but, as they appear to me to be more intimately allied to the former, the comparison will be chiefly confined to these two tribes. The Euphorbiaceæ and Empetreæ agree, therefore, in the imbricate æstivation of their calyx ; in the stamens being opposite to the divisions of the calyx, and both these being of an

* Nutt. Gen. 2. p. 233.

of Nutt. I. c.

equal and definite number ; in having bilocular anthers; in their superior ovarium; in the plurality of styles; in their divided stigmas; and, lastly, in the arrangement of the ovula, and presence of a copious albumen. The embryo is also nearly the length of the albumen, and its cylindrical form brings it close to that of Phyllanthus. The male inflorescence of Empetrum album has a striking analogy to that of Buxus *, whose calyx consists of 3 or 4 leafets, with the stamens equal in number, and placed opposite to, not alternating with them ;-a circumstance which proves that this organ in Buxus is a true calyx, and not, as Linnæus regarded it, a corolla. In separating the Empetrea from Euphorbiacea, the principal character relied on is their erect embryo; for in habit they are not far removed from Micranthea of Desfontaines, some species of Phyllanthus, or even of Euphorbiu itself. In Phyllanthus, the calyx consists of 6 segments: the filaments are 3, closely united together; and the anthers are 4 in number, 3 of which are lateral and alternate with the inner divisions of the calyx, which are probably to be regarded as petals.

The fourth anther, which may be considered as spurious, is placed directly in the centre of the 3 lateral ones. In a decandrous species of this genus from Mexico, each of the filaments is trichotomous, and each branch bears an anther of the usual structure, which, together with the central one, augments the number to 10. In the female flower, the styles are 3, united at the base, and the stigmas are divided into two lobes. The fruit is 3-sided and 3-celled, each cell containing two seeds placed parallel, and opening at the angle by a longitudinal suture; these sutures are immediately perpendicular to the styles, and placed opposite the exterior segments of the calyx. I ought to have before remarked, that the form and structure of the anthers of Euphorbia and Empetrea are exactly similar. The monophyllous calyx ; the non-separation of the sexes; the presence of a perigynous disk ; and the flat, somewhat foliaceous cotyledons,-appear to separate sufficiently the Celastrina from the Empetrea. The distinctions between them and the Rhamnee are still more apparent, however; for in them, the stamens are placed opposite the petals, and the æstivation of the calyx is valvular,characters of primary importance in a natural classification. The embryo in Rhamneæ and Celastrinæ agrees exactly in form and structure. Mr Brown has very properly placed Phylica among the Rhamneæ, although I have known some who, merely from the fruit being inferior, were disposed to remove it from that family. It is evidently intimately allied to Pomaderris, both in habit and characters, and it is equally evident, that the fruit being inferior, is a distinction more apparent than real; for in Pomaderris, and even in some species of Rhamnus, the tube of the calyx coheres with the ovarium; and could we, for example, suppose an equal elongation of the tube of the calyx in these, as in Phylica, we should then have the situation of the fruit precisely the same. The apparently simple stigma in Phylica is not very different from the triple one of Pomaderris ; for there is evidently an indication of three distinct lobes.

* In the Prodromus Flore Nepalensis, I have very briefly noticed a plant under the name of Buxus Saligna, and which I then suspected would eventually prove a distinct genus; but materials are still wanting to determine this point satisfactorily. From the very imperfect description given by me, Mr Lindley, in a late number of the Botanical Register, has been induced to suspect its being only a variety of his Sarcoccoca pruniformis ; but the following description will shew that it has very little affinity to that plant.

Buxus SALIGNA, Don, Prodr. Fl. Nep. p. 63. Flores dioici ? Fem. Calyx squamis pluribus (6-8). Ovarium ellipticum,

3-loculare: ovulis solitariis. Stigmata 3 (raro 4) lanceolato-subulata, acuta, revoluta, suprà convexa, tomentosa, sulco exarata, subtùs nuda. Fructus (immaturum tantùm vidi) 3-locularis (rarò 4-locularis), stigmatibus persistentibus rostratis, et inter rostra foramine dehiscens : loculis monos permis. Dissepimenta membranacea. Frutex erectus, ramosissimus, frondosus, sem. pervirens. Folia alterna, nunc rarò subopposita, petiolata, angustè lanceolata, acuminata, integerrima, basi acuta, margine reflectente, pagina utraque diversâ (ut in Buxo), coriacea, enervia, glaberrima, nitida, subtùs venis parum conspicuis, 3-pollicaria, semiunciam lata. Pedunculi axillares, breves, divisi, pluriflori, subracemosi, cernui.

EMPETREÆ, Nutt.

ERICEIS AFFINIA, Juss. Flores dioici. Masc.-Calyx 3- (rarò 2-) phyllus, æstivatione imbricata, basi nu

dus v. squamis (4-6) duplici ordine imbricatis munitus. Petala 3 (rarò 2) hypogyna, foliolis calycinis alterna, ungui brevi,

« ПредишнаНапред »