hundred to fifteen hundred diameters, with an ocular magnifying ten times; Ross's was the feeblest, that of Spencer

the strongest.

The angular opening was first measured with great accuracy, and found as follows:

These measurements were all verified by the respective owners of these lenses.

The objects examined were the most difficult test-objects among the silicious infusoria, as the Navicula angulata, one of the species of Gramatophora, and a Navicula called the Amici test. The first two were in balsam.

The lenses were first attached to one of Nachez's mounting, and the best adjustment of oblique light used that this instrument affords. The difference in the effect of the three lenses was very slight, all failing to show the lines on the Gramatophora, or on the Amici test. As, notwithstanding the admirable arrangement of Nachez's instrument for working purpose, we do not get the extreme obliquity of light which is required for examining their fine lines, I had them all arranged in a mounting of Amici, which furnishes the necessary obliquity of light. Thus arranged, the lines on the Gramatophora were distinctly and beautifully seen by all, with slight advantages in favour of Spencer and Ross, the former of which magnified them most.

The Amici test was next tried, which resulted in Ross showing the lines with perfect satisfaction; Spencer shewing them, but not quite so well; Nachez still less distinctly.

I would remark that this difference between the lenses appears to be owing entirely to difference in the angle of opening; for where a very oblique light is necessary to shew lines, the lenses must be so constructed as to admit this light. I would also state that Nachez's system lacks an adjustment which the others have, by which the relative position of the lenses can be changed, so as to compensate for the thickness of the glass which covers the object, and which appears VOL. LI. NO. CI.-JULY 1851.

D

favourable to the examination of those delicate tests.

the examination of globules we could not perceive any appreciable difference between the lenses.

I would here remark, in justice to M. Nachez, that he deserves much praise for the manner in which he has improved the microscope in France without augmenting the cost of the instrument; and out of England he is undoubtedly the best maker in Europe. To furnish an idea of what he has done to diminish the cost of a good instrument, I will compare the price of the objectives which have been the subject of the experiments.

And what is still more, he is constantly improving his lenses, without adding to their expense.

The lower powers of these makers were examined without finding any sensible difference in the defining effects of them, and what little there was, was in favour of Spencer. The field of the three differed; Nachez's being the least, and Spencer's the greatest. We cannot bestow too much praise on our American maker for the immense progress which he has made in the construction of objective lenses; and it is to be regretted that he has not chosen a better mounting for them than that of Chevalier, which is very defective, and prevents good glasses from showing their best effects.

I had intended making some remarks on oblique light, which has come very much in use lately in observing lines and points on certain objects; but it will be better for me to defer it. I would simply remark, that much caution is necessary in using it, as it will not always give correct distances between lines.-American Journal of Science and Arts, vol. xi., No. 32, p. 277.

On the Currents of the Atlantic, and the Existence of the North-West Passage.

Lieutenant Maury says, that in studying the system of oceanic circulation he had found it necessary to set out with a very obvious and simple principle-viz., that from whatever part of the ocean a current was found to run, to the same part a current of equal volume was obliged to return. Upon this principle was established the whole system of currents and counter-currents. It is not necessary to associate with oceanic currents the idea that they must of necessity, as on land, run from a higher to a lower level. So far from this being the case, some currents of the sea actually run up hill, while others run on a level. The Gulf Stream was of the first class. The bottom of this stream he had shewn some years since to be an inclined plane, running upwards. If the Gulf Stream was two hundred fathoms deep in the Florida Pass, and but one hundred fathoms off Hatteras, it is evident that the bottom would be uplifted 100 fathoms within that distance, and that, while the bottom of the Gulf Stream was up hill, the top preserved the water-level, or nearly so. The currents which run from the Atlantic into the Mediterranean, and from the Indian Oce an into the Red Sea, were the reverse of this, that the bottom of the current was a water-level, and the top an inclined plane running down hill. The Red Sea, for example, lies for the most part in a rainless and riverless district. It may be compared to a long narrow trough. It is about one thousand miles long, extending nearly north and south, from latitude 12° or 13°, to the parallel of 30° N. The evaporation from its surface is immense, and may be safely assumed to equal a rate of twotenths of an inch per day. Now, if we suppose the current which runs into this sea to average from mouth to head twenty miles a day, it would take the water fifty days to reach the head of it. If it lose two-tenths of an inch from its surface daily by evaporation, by the time it reached the isthmus of Suez it would have lost ten inches from its surface. Thus the waters of the Red Sea ought to be lower

at the isthmus of Suez than at the straits of Babelmandel. They ought to be lower from two causes-viz., evaporation and temperature; for the temperature of that sea is necessarily lower at Suez, in latitude 30°, than at Babelmandel, in latitude 13°. To make this quite clear, suppose the channel of the Red Sea to have no water in it, and a wave ten feet high to enter the straits, and flow up this channel at the rate of twenty miles a day for fifty days, losing daily by evaporation two-tenths of an inch, it is easy to perceive that at the end of the fiftieth day it would not be so high, by ten inches, as it was the first day it commenced to flow. The top of this sea, therefore, is probably an inclined plane. But the salt water which has lost so much of its freshness by evaporation, becomes salter, and therefore heavier. The lighter water at the straits cannot balance the heavier, colder, and salter water at the isthmus, and therefore the heavier water must either run out as an under-current, or it must deposit its surplus salt, and thus gradually make the bottom of the Red Sea a salt-bed. As we know that this latter process is not going on, we infer that there is from the Red Sea an under or outer current, as from the Mediterranean through the Straits of Gibraltar. The rivers which discharge into the Mediterranean are not sufficient to supply the waste of evaporation, and it is by this under-current that the salt carried in from the ocean is returned to it again; were it not so, the bed of that sea would be a mass of solid salt. Thus it is that by a system of compensation the equilibrium of the seas is maintained.

Lieutenant Maury said that he had noticed this fact, that, inasmuch as the Gulf Stream was a bed of warm water, lying between banks of cold water, the warm water was lighter, and therefore the surface of the Gulf Stream was in the shape of a double inclined plane, like the roof of a house, down which there was a shallow surface or roof current, from the middle towards either edge of the stream. This fact had been confirmed in a singular way: a person who had been engaged on the coast survey, with observations on the Gulf Stream, had noticed that when he tried the current in a boat, he found it sometimes one way, sometimes

another, but scarcely ever in the true direction; whereas the vessel, which drew more water, shewed it constantly in one direction.

Lieutenant Maury also called attention to this remarkable fact, that though there be well known currents which bring immense volumes of water into the Atlantic, we know of none which carry it out again, and which, according to the principle before stated, ought to be found running back from that ocean. The La Plata, the Amazon, the Mississippi, and St Lawrence, with many other rivers, run into this very small ocean, and it is not probable that all of these waters are taken up from it again by evaporation; yet the sea is not full. Where does the surplus go? The ice-bearing current, from Davis' Straits, which is counter to the Gulf Stream, moves an immense volume of water down towards the equator. The ice-bearing current which runs from the Antarctic regions, and passes near Cape Horn into the Atlantic, and the Lagullas current which sweeps into it around the Cape of Good Hope, both move immense volumes of water, and bear it along also towards the equator. This water must get out again, or the Atlantic would be constantly rising. A part of the Gulf Stream runs around North Cape into the Arctic Ocean. The thermal charts now in process of construction, under the direction of Lieutenant Maury, at the National Observatory, prove this, as also the charts of Professor Dove of Berlin. But this current probably performs its circuit of the Arctic Ocean, and returns to the Atlantic with increased volume. The great rivers of Northern America, Asia, and Europe, that empty into the Frozen Ocean, as well as the current from the Pacific into Behring's Straits, all sources of supply, serve, in the opinion of Lieutenant Maury, to swell the current down from Baffin's Bay through Davis' Straits into the Atlantic.

That there was an open water communication, sometimes at least, from Behring's Straits to Baffin's Bay, had been all but proved by the results of investigations undertaken about ten years ago, at the National Observatory, with regard to the habits, migrations, &c. of the whale. These investigations were conducted in such a manner as to shew, by a