mined by the indications of the barometer, not neglecting other appearances.

Thus when vessels are bound westward, in the temperate latitudes, and have southeasterly winds with a falling barometer, they should steer to the northward and westward, instead of keeping their direct course; and when the wind has veered to the northeast quarter they may resume their true course, with a fair wind which will veer northward; but if finally compelled to heave to with the wind northeasterly or northerly, they should then take the port tack, so as to come up to the wind, in its further changes. This curved course will be found to favor a speedy passage, in most cases, as it gives a fair wind of longer continuance, by placing the ship in the left side of the storm path, and in a position which renders the subsequent northwesterly wind more available. But in case of a gale's hauling southward and westward, the ship, when headed off from her course, should be hove to on the starboard tack, being in the right hand side of the storm path. The ship will then come up to the sea, as the wind veers by the west towards the northwest.

It will at once be seen that in revolving winds a direct course is not always most conducive to a quick passage, but such variable course should be preferred as will render available the succeeding changes of the wind; which changes, whether by south or north, sometimes depend on the course of the vessel.*

The foregoing statements and suggestions are equally applicable in the southern hemisphere, with only this difference; viz., that in the actual courses of the winds and storms, south is there always substituted for north; east and west remaining the same. Hence, the practice must be varied accordingly.

These practical deductions accord with the statements and diagrams which I have published in 1831 and subsequent years. Storm figures of this kind, better elaborated, have also been given by Col. REID, in his work, accompanied with remarks on lying to, and by him and Mr. PIDDINGTON have been placed on cards, and on plates of horn or glass, in order that a mariner may determine the place of a vessel in a storm, by placing the figure on the face of his chart, in such manner as to coincide, on the outer

*

See, also, Col. REID's valuable Note on Progressive Revolving Winds and the Advantages of Sailing on Curved Courses; Jameson's Ed. New Phil. Journal for July, 1846. Also, Remarks on Lying to, in the Messrs. Blunt's forthcoming edition of the Young Seaman's Sheet Anchor.

circle, with the observed direction of the storm-wind, at the first freshening or commencement of the gale. In this manner the geographical position and coming changes of the storm may be apprehended by those who may not fully comprehend the law of the wind's rotation.*

These storm figures and their uses, may be exemplified in the annexed diagrams.†

Fig. 6.-Storm Figure for Northern Hemisphere.

a, General Course of the Storm in the low latitudes; changing successively to b, which is the general course in the Temperate latitudes.

DIRECTIONS.-First, mark the position of the ship on the Chart, at the beginning of a gale, and then place this figure to the southward of such position, with the needle pointing to the North, and in such location on the Chart that one of the wind arrows in the outer circle will conform to the actual direction of the wind. This will show nearly the true position of the storm at that time. Then move forward the figure in the direction in which gales commonly advance in that latitude and locality, but without turning the figure. The arrows which are thus brought in succession over the ship's place, will show the changes of wind which may be expected, in the further progress of the gale; and also, into what portion of the storm the vessel will be likely to fall, in her then position, and what changes of the ship's course will be likely to favor her safety and the further prosecution of the voyage.

* See Col. REID's work, first edition, pp. 5-7 and 424-427. Weale, London, 1838. Also, Horn Book of Storms, for the Indian and China Seas, by HENRY PIDDINGTON: Ostell & Lepage, Calcutta ; W. H. Allen, London, 1845. I have lately received from Mr. PIDDINGTON his Thirteenth Memoir, which relates to the hurricane of the Charles Heddle, before mentioned, and is well worthy of the attention of both navigators and meteorologists.

† See, also, Bowditch's Navigator, edition of 1839, pp. 441, 442; edition of 1845, PP. 440, 441.

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a, General Course of the storm in low latitudes, south of the Equator; changing, on its approach to the tropic, to b, which is the general course in the southern Temperate latitudes.

DIRECTIONS.-Place the figure or storm card as before, with the needle pointing to the north, and follow out the directions as before given. The size of the storm figure, for use, may be drawn to the common proportions of a storm on the Chart.

In the Atlantic ports of the United States, the approach of a gale, when the storm is yet on the Gulf of Mexico, or in the southern or western states, may be made known by means of the electric telegraph; which, probably, will soon extend from Maine to the Mississippi. This will enable the merchant to avoid exposing his vessel to a furious gale soon after leaving her port. By awaiting the arrival of a storm and promptly putting to sea with its closing winds, a good offing and rapid progress will be secured by the voyager.

However useful the knowledge of storms may prove, no one will expect the tempest to be disarmed of its power. Nor can disasters in navigation be in all cases avoided. But, contemplating this subject in its relations to the thousands of lives and the millions of property which are lost by shipwreck, almost annually, we cannot doubt that much of this loss might be prevented, by the exercise of timely and intelligent precaution. Indeed, the practical value of accurate knowledge and investigation, in all branches of science, is generally admitted; and in so important a matter as that of the rotation and progression of storms, it will not be estimated too highly.

ART. XXX. On the Volcanoes of the Moon; by JAMES D. DANA. (Read before the Assoc. of Amer. Geologists and Naturalists, Sept., 1846.)

THE surface of the moon affords a most interesting subject for the study of the geologist. Though at a distance of many thousand miles, the telescope exhibits to us its structure with wonderful distinctness; and already, as a learned astronomer has observed, we are better acquainted with the actual heights of its mountains, than with those of our own planet.* Having an atmosphere of extreme rarity† (if any) and never obscured by clouds, its features are wholly open to view, and the eye aided with glasses, may wander over its rugged crags, survey its craters, its Alps and its Apennines, from their bases to their summits. Neither are there any sedimentary deposits, soil or vegetation,-for there can be none without water, and the igneous surface therefore is still its own naked self, exhibiting the results of ig

* M. Arago, Annuaire des Longitudes, pour l'an 1842, 2d ed., Paris, 1842.-P. 526, in an article on the Lunar Volcanoes, Arago says:-"Il est remarquable que grace au zèle et à l'exactitude d'Hevelius on ait connu la hauteur des montagnes de la Lune beaucoup plus tôt que la hauteur des montagnes de la Terre."

The evidence in favor of the existence of an atmosphere and of water in the moon, hitherto obtained, has not been deemed satisfactory. Herschel, at an eclipse, Sept. 5, 1793, observed the sharp horn of the limb of the moon, and says that it seemed perfectly regular; and that a deviation of a single second by the refraction of the solar light in the moon's atmosphere would not have escaped him. Phil. Trans., 1794, p. 39.-As stated in Beer and Mädler, (p. 133,) Schröter calculated the density of the supposed atmosphere to be one twenty-eighth the density of our own atmosphere; and Melanderhjelm demonstrated that the moon's atmosphere, judging from that of the earth, should have one thirty-sixth the density of our own atmosphere. But the above mentioned authors say that we have yet to prove that the moon has any atmosphere, adding that it must be very much more rare than the rarest gas on earth. They observe also that supposing our atmosphere to extend through space, its density half way to the moon, according to the Mariottian law of decrease, would be expressed by the fraction 10000, the denominator extending to ten thousand zeros. The singular observation occasionally made, that during the passage of the moon over a star, the star appears visible in front of the edge of the moon, before disappearing, may possibly indicate an extremely low atmosphere or surface vapors: but it has been attributed with much appearance of reason (Rep. Brit. Assoc., 1845, p. 5) to diffraction. The absence of any bodies of water on the moon is placed beyond doubt, both by actual telescopic examination and by inference from the absence of clouds. There are no streams, lakes or seas. An eminent astronomer has remarked that the heat of the surface exposed to the sun would occasion a transfer of any water the moon might contain to its dark side, and that there may be frosts in this part, and perhaps running water near the margin of the illumined portion. But in such a case, would not clouds appear about the margin at times in telescopic views?

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neous action in their simple grandeur, unaltered and uncomplicated by any attending operations. We may hope therefore to find some profit in contemplating for a few moments this land of the skies and although we may not look for very speedy “annexation," we may possibly gather some facts and ideas which the decree of Truth will annex to the domain of Science.

The moon, as we all know, has been minutely studied in a physical point of view, and already some important geological conclusions have been drawn from the facts it presents. The altitudes of its mountains were first estimated by Galileo,* and afterwards were mathematically calculated by Heveliust and Riccioli. Sir Wm. Herschel continued the investigations, and reported the probable activity of three of its volcanic mountains.‡ Mayer, Huth, Harding, and Schröter,§ and more lately Gruithuisen and W. G. Lohrmann, are other prominent names among those who have added largely to our knowledge of the moon's surface. More recently still, MM. Beer and Mädler have pursued this science of Selenography with wonderful perseverance and labor, and have given corrected results of all previous calculations, with magnificent maps of the moon's topography.¶ 1095 heights were carefully measured by them, and their features, to a great degree of accuracy, ascertained. These maps have afforded M. Elie de Beaumont some deductions alledged as supporting certain geological theories. James Nasmyth, Esq., in the Transactions of the Royal Astronomical Society for the present

* In the article referred to in the Annuaire des Longitudes, (p. 522,) Arago states that Clearchus, on the authority of Plutarch, described the moon as smooth and lustrous like a mirror. Democritus attributed the spots to inequalities of surface. Galileo first observed the lunar mountains with his telescope in 1610, and estimated their height at one twentieth of the diameter, giving 8800 metres for their altitude, which but little exceeds their actual height.

† J. Hevelius, Selenographia; fol., Gedani, 1647.

Phil. Trans. for 1780, p. 507, Astronomical Observations relating to the Moon: -for 1787, p. 229, An Account of Three Volcanoes in the Moon :—for 1794, p. 39, Account of some particulars observed during the late Eclipse (in 1793) of the Sun.

§ J. H. Schröter, Selenotopographische Fragmente zur genauern Kenntniss der Mondfläche ihrer erlittenen Veränderungen und Atmosphäre; 2 vols, 4to, Göttingen, 1791 and 1802.-Gruithuisen, in Bode's Astron. Jahrb., 1825.

Topographie der sichtbaren Mondoberfläche, von W. G. Lohrmann; 4to, Dresden und Leipzig, 1824.

Allgemeine vergleichende Selenographie; mit besonderer Beziehung auf die von den Verfassern herausgegebene Mappa selenographica, von W. Beer und Dr. J. G. Mädler; Berlin, 1837.