United States forms no exception to the observation which applies to other parts of the globe, viz: that in approaching the tropics the vegetable forms become not only more rich in their colouring, and more luxuriant in their growth, but also that the number of the species is greatly increased. To confirm what has just been said, I need only repeat, that Mr. Elliott, in his Flora of South Carolina and Georgia, enumerates as found in these two states alone, about 300 more Phenogamous plants, than belong to all the Northern and Middle States. It may also be added, that of 619 genera, described by Elliott as belonging to South Carolina and Georgia, 132 are not found in the Flora of the Northern States; and that of 592 genera enumerated in Torrey's Compendium, 73 are not found in the work of Elliott, though some of these occur in Virginia.

Proceeding to the North, the species just cited as characteristics of the Southern vegetation, gradually disappear; and as we reach Virginia, we meet with other forms indicative of a more northern region. This may be said to be the neutral ground between the north and the south, and does not assume the decided character of either.

As we reach Pennsylvania, New-Jersey, and New-York, a change in the aspect of the forests is observable. The Pinus rigida, L. (pitch pine,) Pinus strobus, L. (white pine,) Pinus canadensis, L. (hemlock spruce,) which in the south are confined to the mountainous ranges, and then stinted in their growth, rise to their native height on the lowlands. The same may be said of the Pinus alba, Aiton, (white spruce,) Pinus nigra, Aiton, (black spruce,) and Pinus microcarpa, Lamb. P. pendula, (red and black larch.). Here, on the banks of streams, in marshes and elsewhere, are the Thuya occidentalis, L. (white cedar,) Betula papyracea, Mich. B. populifolia, Aiton, (white birch,) Betula lenta, L. (black birch,) and Betula excelsa, Aiton, (yellow birch,) which, if found at all in the Southern States, are confined to the Alleghanies. Of the smaller trees, the

Willows are also peculiar in this respect.

Of from 40 to 50 species,

at present known as belonging to the United States, not more than six are found much south of Pennsylvania; and Mr. Elliott remarks, that the Salix nigra, L. (black willow,) is the only species which is found in the low country of Carolina, except the exotic, S. babylonica, and the S. vitellina, which are occasionally cultivated in gardens. Among other plants, peculiar to the Northern Atlantic region, may be named Xylosteum ciliatum, Pursh, Diervilla canadensis, Rhodora canadensis; several species of Ribes, Celastrus,

scandens, Scheuchzeria palustris; several species of Epilobium, Pyrola and Hudsonia, which are not represented in the Southern region, if we except a single species of Epilobium, found on the

mountains.

In passing to the west from the Alleghanies, new vegetable forms appear, though a similarity of latitude prevails. This difference be comes striking as we cross the Mississippi. In the forests we now find an abundance of the Celtis crassifolia, (hack berry,) Acer negundo, (ash leaved maple,) Quercus macrocarpa, (over-cup-oak,) Asimina triloba, (papaw,) several species of Aesculus, (buckeye,) viz: Ae. flava, pavia, macrostachya, &c. the coffee tree, Gymno cladus canadensis, Gleditschia triacanthos, (honey locust,) Juglans olivaformis, (peccan,) and J. nigra, Cercis canadensis, (red bud ;) and on the banks of streams, Salix longifolia, Populus angulata, (cotton wood.) These, although not all peculiar to this section, form prominent features in its vegetation. Among the more rare productions of the Western forests, we may also mention the Yellow Wood, or Bois d'arc, Maclura aurantiaca, Nutt. extending westward to the banks of the Little Missouri, and which is so extensively employed by the natives of that section. Among the more humble plants, the splendid Rosa rubifolia, R. Brown, which is abundant on the margins of prairies, may be adverted to; as also several species of Monarda, Penstemon, Oenothera and Nuttallia, Troximon glaucum, Nutt. and T. marginatum, Collinsia verna, Mahonia aquifolum, Nutt. Glycyrrhiza lepidota, Nutt. (wild liquorice,) found at St. Louis, on the Platte river and Rocky Mountains, and several species of Verbena, &c. &c.

This varied and interesting vegetation, continues until we reach the chain of mountains which traverse the Western part of our continent, on which, among many alpine plants which are found on the mountains in the Eastern part of this continent, in Europe, and in Asia, are some which are peculiar. But, as has been remarked, our knowledge concerning this tract, is still too limited to warrant any general remarks or comparisons.

It would be interesting to notice the features of the vegetation in the states on the Gulf of Mexico, viz: Alabama, Mississippi and Louisiana, and to compare them with those of Georgia and South Carolina; but we are still in want of materials for this purpose. Yet, as far as I can judge from a catalogue published under the di

rections of Mr. Cornelius, a great similarity prevails; and it is probable, that this continues westward to the mountains of Mexico.

In tracing the peculiarities of vegetation of the portion of the United States west of the Alleghanies, one circumstance should be remembered, as having a marked and powerful influence. It is, that this territory is watered by a magnificent stream, which holds a southerly course, through 20 degrees of latitude, to the Gulf of Mexico that this stream is fed by numerous tributaries from various parts of the Rocky Mountains on the west, and from the Alleghanies on the east, which pour into it the waters of the western part of New-York, as well as those of the mountains of Oregon. When we reflect what a vast number of plants flourish on the banks of these various streams, and that seeds are carried by the waters, and become fixed in the banks at other and distant places, we should not wonder if an uncommon uniformity of vegetation prevailed throughout this extensive region ;-uncommon, when compared with tracts of similar extent in other parts of the globe. An attention to this prominent feature in the geography of the western part of the United States, will also account for the fact mentioned by many travellers, that certain tropical forms reach a more northern latitude here than on the coast-and for the want of which, erroneous views have been entertained concerning its climate.

I have purposely omitted to notice, until the present time, the vegetation upon our mountainous tracts. It is now well established, that "vegetation, in ascending above the level of the sea, undergoes modifications, analagous to those which attend its progress from the line to either pole. With this distinction, that in the last case, the phenomena succeed by almost imperceptible gradations, while they crowd upon and follow each other in rapid succession, on the ascent of mountains. The height of 4 or 5000 yards in the hottest parts of the globe, produces changes as distinct as the 2000 leagues or more, which lie between the equator and the polar regions." (Mirbel, in the 2d volume of Brande's Journal.)

Observations of this kind, however, have been greatly extended by Humboldt and Bonpland, and by Decandolle. It has been estimated by the latter, that in the climate of France, 180 or 200 yards of elevation, act upon the mean temperature nearly in the ratio of a degree of latitude.

In our own country, the observations upon the vegetation of the mountains have not, heretofore, been conducted with all the accu

racy necessary to a full developement of this subject. But as far as can be judged from the materials which we possess, the same remarks will apply. It has already been stated, that in South Carolina and Georgia, various species of Pine, which in this latitude are found on the low lands, are there confined to the summits of the mountains. Drs. Bigelow and Boot, found on the summit of the White Hills, in lat. 44° 15' N. at 6000 feet above the level of the sea, natives of Lapland, Greenland and Labrador; as Epilobium alpinum, L. Empetrum nigrum, L. Ledum latifolium, Ait. Menziesia cærulea, Swartz, Rubus saxatilis, L. Diapensia, Lapponica, &c. So also those who crossed the Rocky Mountains, witnessed the same changes of vegetation.

The botanical examination of high mountainous tracts, whether in tropical or temperate regions, is therefore of extraordinary interest; as it affords, within a small compass, striking illustrations of many leading facts in geographical botany. At the base, vegetation is thrifty, the species are numerous and similar to those of surrounding regions-Upon ascending, these forms become stinted in their growth, and gradually disappear. Others succeed, still more dwarfish and hardy, and in their turn give place to the few representatives of the polar regions, which continue to the limit of eternal snow. In such a journey, therefore, the traveller passes as it were, in quick succession, from the equator to the poles.

A great peculiarity in the vegetation of the United States, and which strikes the traveller as well as the botanist, is the number and variety of its Forest Trees. This will be fully exhibited when we remark, that the single genus of Oak, comprehends within the United States, more species than Europe reckons within the whole amount of its trees. But this subject, together with a notice of the distribution of the Grasses, and other families of plants, will be noticed more in detail hereafter.

ART. IV. On some Modifications of the ELECTRO-MAGNETIC APPARATUS. By JOSEPH HENRY.

Read October 10, 1827.

The subject of Electro-Magnetism, although one of the most interesting branches of human knowledge, and presenting at this time the most fruitful field for discovery, is perhaps less generally understood, in this country, than almost any other department of natural science.

Our popular lecturers have not availed themselves of the many interesting and novel experiments with which it can so liberally supply them; and, with a few exceptions, it has not as yet been admitted as a part of the course of Physical Studies pursued in our higher institutions of learning. A principal cause of this inattention to a subject offering so much to instruct and amuse, is the difficulty and expense which formerly attended the experiments-a large galvanic battery, with instruments of very delicate workmanship, being thought indispensable. But this bar to the advancement of Electro-Magnetism no longer exists; several improvements having been made in the principles and arrangement of the apparatus, which tend considerably to simplify its construction and use. Mr. Sturgeon, of Woolwich, who has been perhaps the most successful in these improvements, has shown that a strong galvanic power is not essentially necessary, even to exhibit the experiments on the largest scale. On the contrary, he has proved that it may be almost indefinitely diminished, provided the magnetic force be proportionately increased. On this principle he has constructed a set of instruments, with large magnets and small galvanic elements, which from their size and the facility of their operations, are well calculated either for the private study or the public lecture room.*

Mr. Sturgeon's suite of apparatus, though superior to any other, as far as it goes, does not however form a complete set; as indeed it is plain that his principle of strong magnets cannot be introduced into every article required, and particularly into those intended to exhibit the action of the earth's magnetism on a galvanic current, or the operation of two conjunctive wires on each other. To form therefore a set of instruments, on a large scale, that will illustrate all the facts belonging to this science, with the least expense of galvanism, evidently requires some additional modification of the *Annals of Philosophy, New Series, vol. 12, page 375.