Meteor

in titude.

(402.) Such are the lower limits assigned to the ology. altitude of this magnificent plane, by the celebrated men whose names are recorded in the Table. If, how Oscillations ever, we stop our inquiries at this point, we shall leave many interesting portions of the problem unresolved. The plane of the perpetual snows does not maintain a constant elevation in the same latitude, but varies with the vicissitudes of the seasons, rising during the heat of summer, and sinking by the cold of winter; changing also from one summer to another, as alterations are experienced in the temperatures below.

(403.) If we first direct our attention to the Equatorial regions, we shall discover some small oscillations to exist in the altitude of the perpetual snows. To the mountaineerst of the Andes, the limit of the perpetual snows presents a phenomenon of a very constant kind; and to their uncultivated minds, the silvery line seems to run through the enormous groups of their mountains, in one continued horizontal plane: the greatness of the distance preventing them from observing those small oscillations of altitude, which the more refined inquiries of the Philosopher have disclosed. This close approximation to uniformity arises from the equality of temperature existing in the different strata of the atmosphere; and accordingly it is found that the utmost amount of the oscillation of the perpetual snows in these regions does not exceed 30 fathoms, and at the Equator itself rarely amounts to half that quantity. It is often, indeed, insensible, notwithstanding the small differences of temperature which are found to exist between the season of rain and that of great dryness. (404.) If we pass, however, from the Equatorial regions, we shall discover new conditions presented for investigation. No longer confined to the feeble oscillations we have referred to, we shall find all the attendant phenomena become more irregular. In the Mexico, mountains of Mexico, the oscillations will be found to amount to 2405 toises, attaining their maximum in January, and their minimum in September ;§ and, although we are not acquainted with the actual extent Eya of the oscillations on the mountains of Himalaya, we may infer from the observations of Captain Hodgson, that they must be considerable. The great cedar pines, those gigantic sons of the snow, says he, fringe the bare

Yntains

It may be necessary to observe, that we do not confound the phenomenon of the perpetual snows with that of glaciers. The inferior limit of ice is determined by other causes.

These people have an ancient usage of giving a snow-bath to those who, for the first time, pass from the less elevated plains, through the zone on the declivity of the Cordilleras, which may be accidentally covered with snow; the burlesque ceremony agreeing with the baptism which sailors undergo, who for the first time pass the Equator.

↑ When Humboldt published his Views of the Cordilleras, and nis Atlas of New Spain, many regarded with wonder the designs which represented the icy regions of the Cordilleras of Quito and Mexico. Accustomed to the snowy mountains of the Alps and Pyrenees, and to the vicissitudes which they undergo during the successive seasons of the year, they transferred similar ideas to the mountains of the New World, and were surprised to find those glaciers wanting, which in Europe impart so much grandeur and beauty to Alpine scenery. See Vues de Cordillères, pl. 5. 10. 16. 25. 35. 42. 51. et 61.; and Atlas de la Nouvelle Espagne, pl. 16. et 17.; and Atlas Géographique de l'Amérique Méridionalle, pl. 1.

We must not confound the perpetual snows with the snows which in winter, sometimes, fall in much lower regions. This ephemeral snow, says Humboldt, is never observed under the Equator at 12,467, or 12,795 feet; but in Mexico it is commonly seen at an elevation of 9843 feet. Snow has been seen in the streets of Mexico at 7471 feet, and at 6159 feet, in the City of Valladolid.

zone.

(405.) But much more striking vicissitudes will be Striking vifound as we advance into the temperate zone. Fields cissitudes in of snow of an enormous width are alternately melted temperate and congealed, creating in the lapse of time those mighty glaciers, which by their varied and gigantic forms impart so much sublimity to the Alps. (406.) In order that glaciers should exist on a moun- Glaciers. tain, it must rise considerably above the limit of the perpetual snows. Prodigious masses of ice are also necessary for their formation, and an enormous pressure must act from above, to force the inferior parts of the icy volumes into the valleys below. Hence the frozen masses precipitated by an avalanche, form groups below the lower limit of perpetual snow. In this position they undergo but little change during the lapse of centuries; the altera- Of the Alps. tions produced by the gradual thaw being more than compensated by other masses formed in the cold repositories above, and thence precipitated to the valleys below. Hence it is that travellers have remarked the successive invasions of the Valley of Chamouny, by the avalanches that have descended from Mont Blanc and the adjoining peaks. It is worthy, also, of remark, that the five glaciers which have forced their way into the beautiful valley just mentioned, are separated from each other, by forests, corn-fields, and meadows,-large tracts of ice mingling with the cheerful fruits of cultivation. (407.) The finest glaciers in Norway, are those pro- Of Norway. ceeding from the chain of the Justedals Eisberge. They are known to the inhabitants by the name of Jis-Bracer, and at times are much dreaded by them on account of the rapidity of their motion, which exceed that of the glaciers of Swisserland.* In Krondal, the glaciers are described by Von Buch, as a huge, dazzling, white carpet, fastened on both sides to mighty rocks; and when they have reached the valley, they still continue to push on, like the glaciers of the Rhone. Between the sixtieth and sixty-first degrees of latitude, according to Hagelstam, the perpetual glaciers have a breadth of 4600 feet, or nearly seven-eighths of a mile.t

(408.) Our limits will not permit us to pursue this Causes of chain of inquiry in all its generality, and we must alteration in therefore briefly remark, that the variations of temperature occasioned by the ordinary changes of the perpetual

*An example of their terrific power may not be uninteresting. In the year 1744, the few persons inhabiting the valleys alluded to in the text, complained that they were not able to pay their taxes, because the Jis-Braeer had rushed upon their fields, and completely covered them. This statement was not credited, and Surveyors and Excisemen (Sorenscriver und Foged) were sent as Commissioners to measure the distance of the middle of Milvirsdal from the foot of the nearest glacier, and it was ordered that the same measurement should take place every three years, to ascertain whether or not these glaciers were advancing. Three years after, the same Commissioners went to repeat their measurement, and were not a little astonished to find neither fields nor houses. The Jis-Braeer had advanced prodigiously, the inhabitants were gone, and their possessions buried under the ice. (See Thaarup's Magazin fur Statistik, 1802.)

In the Bibliothèque des Sciences et des Arts, Professor Pictet informs us, that the Glacier des Bossons has lately (1818) advanced fifty feet, much to the dismay of the neighbouring villagers.

There are many mountains in Norwegian Lapland, whose names terminate in geikna, and from all of which glaciers proceed. In Lapland, the name geikna, or jackna, is applied in the same way as the Icelandic word jockal; the Norwegian word jisbræ; the Tyrolese word ferner; and the Swiss word gletcher, (glacier.)

altitude of

snows.

Meteorology.

seasons, are not the only causes which have a tendency to produce alterations in the elevation of the plane of the perpetual snows. There must be some differences of altitude due to the diurnal changes of temperature, although the present condition of our knowledge will not permit us to estimate their value. The horizontal winds, also, which mingle the temperatures of different zones, contribute sometimes to augment the circumstances of temperature, and at other times to diminish it, in each case exercising some effect on the altitude of the perpetual snows. The enormous beds of clouds likewise, which are found sometimes to exist in the interval between the plains and the limits of constant snow, cannot but be without their effects. The production of caloric, which, says Humboldt, is the effect of the extinction of light, and which diminishes with the density of the superincumbent strata of air, may not, it is true, be appreciable by our instruments; but the masses of vesicular vapour, and the clouds which present distinct and perfect contours, and which in the tropical regions attain an elevation of 3000 toises, become heated to a sensible degree, and emit radiant caloric to great distances. On the other hand, at a less elevation on the Eastern side of the Cordilleras of Mexico, a thick stratum of clouds augments, during several months, the cold of the superior regions, by intercepting the radiant caloric of the plains. Nor must we omit the consideration, that while the directions of the great mountain chains exercise a very important influence on the figure of the plane of perpetual snows, the grouping together of mountains has, in all cases, an effect on its elevation; tending to diminish the altitude, when their summits penetrate above it. Figure also (409.) We may also add, in again alluding to the influenced figure of the plane of perpetual snows, that on account by inequa- of the inequalities of temperature which exist in the lities of tem- Northern and Southern hemispheres, some varieties, both in form and elevation, are due on this account. The elevation of the perpetual snows in the parallel of Cape Horn must differ from that of Edinburgh, since the former ranges freely over the Ocean, and the latter is subject to all the vicissitudes of temperature arising from the unequal radiating powers of large portions of the Continents of Europe, Asia, and America. Whatever circumstances, indeed, occasion diversities in the temperatures of the two hemispheres, cannot be without their effects on the altitude of the perpetual

perature of Northern

and South

ern hemispheres.

ature of lower limit

constant.

snows.

Supposition (410.) We must now hasten to advert to another of Bouguer most interesting branch of the inquiry. It had long that temper been believed after Bouguer, that the lower limits of the plane of perpetual snow indicated everywhere a of snows is temperature corresponding to the freezing point; but Humboldt has shown, in a Memoir read to the French Shown by Institute in 1808, and, again, in his Paper on the Humboldt Inferior Limit of the Perpetual Snows on the Mounto be erro-tains of Himalaya, that such a supposition is contrary to the truth, and that the perpetual snows do not follow an isothermal plane. Their direction, indeed, follows less the traces of the isothermal lines of the planes, than the inflexions of the lines of equal summer. By uniting many good observations, Humboldt found that at the lower limit of perpetual snow, the mean temperature of

neous.

the air is as recorded in

estimate

(411.) The first theoretical attempt to trace the alti- First theotude of the plane of the perpetual snows in different retical atlatitudes, was also made by Bouguer. Mayer, although tempt to occupied with the subject of terrestrial temperature height of twenty years later, having deduced, as we have before this plane. seen, a formula to represent the same, did not venture to trace the gradations of heat in the atmosphere, and to ascend to the frozen regions above. It was sufficient, indeed, for his reputation, that, occupied with the mighty subject of the lunar Tables, he was yet able to marshal the terrestrial temperatures, and develope a law which should approximate in some tolerable degree to that of Kirwan, eager to embrace Kirwan the geographical parallels. the whole question, ventured to combine the experi- combined mental observations of Bouguer with the theoretical de- the observaductions of Mayer, and regarded the elevation of the Bouguer perpetual snows in every latitude, as proportional to the with the difference of the mean temperature and the constant deductions temperature of the freezing point.

tious of

of Mayer.

(412.) Professor Playfair, also, by means of Mayer's Playfair's formula of mean temperature, endeavoured to deduce formula for the same the altitude of the perpetual snows as follows. Since object. that formula is represented, says he, by

T5826 cos 2 L, and that at the limit of congelation T becomes 32, the equation may assume the form of

325826 cos 2 L

and from which we obtain

h = 264 (1 + cos 2 L),* and which, according to Playfair, will furnish the altitude of perpetual snow for every latitude.

(414.) Professor Leslie has likewise been led to the Leslie's i consideration of this problem. Having deduced the vestigatio

Playfair, in his Outlines of Natural Philosophy, assigned to the altitude of perpetual snow at the Equator 15,577 feet, as before given by Kirwan, and hence deduced 294 feet for the value of A. In

doing this, however, the Professor fell into an error respecting the

formula of Mayer, by assigning to the constant coefficient of the double latitude, the value of 27 instead of 26; and hence his equation for the altitude of perpetual frost was

h=7644 +7938 cos 2 L,

instead of the more convenient form given to it in the text.

temperatures of the geographical parallels by the for- Meteormula of Mayer, which not representing exactly the ology. distribution of terrestrial heat, must necessarily have some effect on the computed values for the altitude of the perpetual snows. It proceeds also on the supposition that this plane is one of an isothermal nature, which, we have before remarked, has been proved by Humboldt not to be the case. Hence the differences

actual observation.

between the results of the Table, and those recorded from actual observation in Table LXXXIX., are considerable. At the Equator, the formula is only in Differences defect 541 feet, but in the parallel of Mexico the differ- between the ence, according to one observation, amounts to 1066 Table and feet, and according to another to 1386 feet. With one of the observations made on the Himalaya chain, there is a surprising coincidence between the formula, and the point where the Gauri river emerges from the snow; but at the Nitee Ghaut, the Chárang pass, and the somewhat doubtful results relating to the mountains enclosing the dell of the Táglá river, the differences are of the most surprising kind. With the observations made on the Caucasus, the theory is in defect the enormous quantity of 2213 feet; but with the Pyrenees it is for the first and last time a small quantity in excess. With the Alps it is again considerably minus, and with the estimated range of the perpetual snows above the Carpathian chain, it is in defect upwards of 1900 feet. In Sweden, admitting the observations of Hagelstam to be correct, the difference is still greater, although a somewhat closer approximation is obtained for Folgefonden, in Norway. With the snows on the mountain of Sulitelma, given on the respectable authority of Wahlenberg, the difference amounts to 1512 feet; and at the North Cape, where the last visible traces of the perpetual snows on land are found, the defect still amounts to nearly 800 feet.

of the same.

(417.) These anomalies may, however, be more clearly Graphical discerned by means of the graphical illustration denoted illustration by fig. 9, in which the horizontal line E N represents the interval between the Equator and the North Pole, divided so as to correspond with the principal latitudes contained in Table LXXXIX.; and on each vertical line raised at the latitudes referred to, is laid off the altitude of perpetual snow. according both to theory and observation. Thus, from E to B denotes the altitude of the perpetual snows at the Equator, as indicated by the Table of Leslie, and from E to G the mean of all the Equatorial observations of Humboldt. A simple extension of the same principle produces the curve of contrary flexure BCDFN, of which the one half is nearly the reverse of the other, for the general range of the perpetual snows according to theory; and the irregular line FGHIK, that which is produced by connecting together the points with which actual observation has furnished us. C is the point where theory so nearly agrees with the measurement found for the Gauri river, and D the solitary example of its being in excess of the actual observations made on the Pyrenees. The towering summit at H is worthy the most attentive consideration. The part of the horizontal line E A refers to the observations of Humboldt made South of the Equator.

(418.) Although the theory is in all cases in defect excepting one solitary instance, it would be unjust to charge upon it the whole amount of the differences that have been found, since errors in the actual observations on the perpetual snows do in all probability exist to

Meteorology.

some considerable amount. The remarkable circumstance, however, that all the differences between theory and observation, excepting one, are of a negative character, is sufficient to awaken a salutary caution respectThe results ing its results. That the conclusions of actual observaof observa- tion cannot in all cases be relied on, may be inferred tion cannot in all cases from an examination of the results of Table XC. By be relied on. computing the decrement of altitude corresponding to Proof from a single degree of Fahrenheit's thermometer, for the the decre. whole range of elevation from the Earth's surface to the plane of congelation, we shall find the results for the three zones to be of a very anomalous kind. If we adopt for the mean temperatures at the surface, the values recorded in Table XLII., and for the mean temperature of the lower limit of perpetual snow, those contained in Table XC., we shall obtain the following

ment of altitude.

the ano

malies.

results :

Causes of These anomalies may again be attributed to two sources, errors in the altitudes, or in the observed mean temperatures of the plane of perpetual snow in the latitudes referred to; and that it exists with much probability in the latter, may be inferred from the following considera

tions.

(419.) We have before found that the formula represented by (P) denotes, in a convenient manner, the relation between the depression of the thermometer, and the altitude observed; and if we suppose by way of subjecting the three observations in the preceding Table to the same test, that the altitudes are correct, Attempt to and that the question for consideration is to determine compute the the mean temperature at the altitudes recorded in the mean tem- same place, let us find from the formula last quoted, perature at that value of n, which represents the depresion of perpetual temperature due to the elevation. By an ordinary algebraical reduction, this will give

the limit of

snow.

isothermal plane.

Here the errors between the observed and computed values of the mean temperature at the plane of perpetual snow, seem to indicate that considerable anomalies exist in the observations, probably in the temperate and frigid zones. It may also be remarked Inference respecting the formula last employed, that it affords by from it that its application to the principal results contained in it is not an Table LXXXIX., a confirmation of Humboldt's remark, that the temperature of the plane of the perpetual snows is not of an isothermal kind. It may likewise be added, that the altitude at which the mean temperature of the freezing point is attained in the great range of the atmosphere, seems according to formula (P) to vanish in about latitude 66° 53′; the great isothermal plane corresponding to 32° of temperature, appearing to meet the Earth's surface in that latitude. This conclusion has been obtained by computing the altitude due to 39° by aid of the formula last quoted, and adopting the mean temperatures recorded in Table XLII.

the observe

(420.) There is another uncertainty also respecting the Probable altitudes which have been assigned to the lower limits uncertainty of the plane of perpetual snow, which must be briefly ad- respecting verted to. The perpetual snows may be said in a general altitudes of way to attain their maxima and minima of elevation at the the plane o opposite seasons, when the greatest and least tempera- perpetual tures for the year take place at the Earth's surface; and snow. the periods when travellers are most likely to approach them, will not be when they are most broadly developed, but when they are approaching their mean, or, perhaps, their least state; when the warmth of the atmosphere has caused the snowy boundary to rise considerably above the lower limit assigned to it by the minimum temperature of the year. This circumstance may afford room for supposing that the heights assigned to the lower regions of perpetual snow are above what they ought to be; and that hence by applying some correction, which the future progress of Meteorology may disclose, a closer approximation may be found between the results of observation and the deductions of

theory; particularly in the temperate and frigid zones, where the discrepancies from this cause may be supposed to be the greatest, in consequence of the greater changes that take place in the altitude of the plane of perpetual frost during the successive seasons of the year.

The whole subject is, as yet, entangled with many difficulties, and will require much laborious research to carry it to perfection. All the anomalies which accompany its progress, must, however, be fairly stated before such desired results can be obtained.*

* In the following sections (422.) ought to be numbered (421.), and so on.