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TABLE OF TEMPERATURES.

157

TABLE Echibiting the mean temperature of various places compiled prin cipally from the observations of Baron Alez. Von Humboldt.

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Position.

Mean mal Names of Places,

temperaBands.

ture of

the Year Lat. Long. Hghi.

Oilo Feet
Melville Island ..... 74 47 110 48w. 0
Nain ...........

61 20w. +26.42 Enontekies.......

1356 26.96 Hospice de St. Gothard... | 8 23E. 6390 30.38 North Cape .....

32.00 Ulea ............

35.08 Umea ..........

33.26 St. Petersburg ...

59 56 30 19E. 38.84 Drontheim ......

39.92 Moscow ........

45 37 32E. 970 40.10 60 27 22 18. | 0 40.28

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Band from 410 to 50°.

Upsal ........
Stockholm ......
Quebec.......
Christiania......
Convent of Peissenberg
Copenhagen ........
Kendal ...,
Falkland Islands ...
Prague ........
Gottingen.,.....
Zurich .....
Edinburgh .....
Warsaw .....
Coiro....
Dublin ........
Berne .........
Geneva.............
Manheim ...........
Vienna ..............

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Band from 500 to 590,

Isother

Position.

Mean mal Names of Places.

temperaBands.

ture of

the Year. Lat. Long. Hght. LOO

Feet O Clermont ......

45 46 3 5E. 1260 50.00 Buda.......:

47 29 19 lE. 51.08 Cambridge, Mass. 42 22 71

50.36 Paris .... ........... 48 50 2

51.08 London.......

50.36 Dunkirk ....

50.54 Amsterdam,

51.62 Brussels ....

22E.

51.80 Franeker ...

6 22E. 51.80 Philadelphia....

56 75 10w. 53.42 New York ..

40 40 73 58w. 53.78 Cincinnati .

39 6 84 27w. 510 53.78 St. Malo ...

48 39 2 lw. 54.14 Nantes

47 13:1 32w. 54.68 Peking

. 39 54 116 27E. 54.86 Milan ..vevo

. 45 28 9 lle. 390 55.76 Bordeaux ....

.. 44 50 0 34w. 56.48 Marseilles........

.. 43 17 5 22E. 0 59.00 Montpellier .... . 43 36 3 52E. 0 59.36 Rome .........

41 53 12 27E. 60.44 Toulon .......

7 5 50E. 0 62.06 Nangasaki ....

45 129 55E. 0 60.80 Natchez ......

31 34 91 24w. 180 64.76 680 to Funchal .........

.. 32 37| 16 56w. 0 68.54 1720. Algiers ..........

..... 36 48 3 lE. 0 69.98 Cairo.......... ........ 302 30 18E.

72.32 Vera Cruz .....

19 11 96 lw. 0 77.72 Havana........

23 10 82 13w. 0 78.08 Cumana ........

./10 27 65 15w. 0 81.86

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THE ATMOSPHERE.

159

CHAPTER V.

Optical Phenomena.

“Why do those cliffs of shadowy tint, appear
More sweet than all the landscape smiling near ?
'Tis distance lends enchantment to the view,
And robes the mountain in its azure hue.”

Campbell

In the present chapter we shall describe and explain the general optical appearance of the sky, and some of the more striking optical phenomena connected with our present subject. When the rays of the sun strike the minute particles of air, which, accord ing to circumstances, may be more or less dense, or charged with watery vapor, they are either reflected, or transmitted ; in either case sometimes returning the most beautiful colors. It is a fact to well known to need much illustration from us, that light, whenever it is refracted by any medium, such as glass or water, is al-, ways separated into the prismatic colors, whenever the surfaces of the medium are curved, or inclined to each other. It is not however, so generally understood, that these different colored rays have different powers of penetrating through various media, and that they move with different velocities. This however, is susceptible of demonstration, and it is to this that the beautiful colors of an autumnal sunset are owing. The red, violet and orange rays have the greatest velocity, and penetrate the thick dense strata of horizontal air, with the greatest facility, giving us the rich and brilliant hues of sunset and sunrise, tinging the morning and evening clouds with glowing red, and gold ; and the sober twilight, with that purple fading into gray which is assumed when the ruddy glare of sunset is tempered by the azure of the sky. Since the red and yellow rays which compose white light, are transmitted by the air, unattended by the blue fays, it follows that these latter must be reflected, hence the beautiful

blue of the sky, and the bright azure which tinges the distant mountains when viewed through a considerable body of intervening air, and especially, when charged with watery vapor Perhaps this one feature, which so mellows down the distant outlines of the hills and buildings, is the most pleasing feature of the landscape. It is from strict attention to the phenomena dependent upon this principle, that the artist derives his pleasing skill in picturing objects of varying distance, introducing skillfully the color of the intervening air. How simple, and yet how beautiful are the various contrivances which administer, not to the wants merely, but to the pleasures of man. It is the same simple cause which tints the bright blue sky, and its beautiful clouds, here piled in snowy masses, and there sundered into a thousand fleecy shapes; which lights the west with a golden glow, and fringes the extended clouds that skirt the horizon with the brightest hues of red and gold; and it is owing to the peculiar nature of the red rays of the spectrum, that the sun appears a dull red globo when viewed through air highly saturated with watery vapor, or through clouds and fogs.

When the rays of the sun strike upon a cloud, they are copiously reflected, but partly absorbed by the minute suspended globules, and the quantity of light which penetrates through the nebulous medium is always much less than what traverses an equal body of air, and this gives the clouds their varying shades of color. That the color of the sky is owing to reflected light, is sufficiently evident from the fact that it becomes darker and darker, as we ascend into the higher regions of the atmosphere, through which, the blue rays find a ready passage. Were it not for the reflecting power of the atmosphere, and the clouds, we would have no softening of the day into night, as. now, by the twilight; but instantly, at sunset, darkness would veil the earth, and every cloud that obscured the sun would cause a total eclipse. The tint of the sky is deeper in the torrid zone than in high latitudes, and in the same parallel it is fainter at sea than on land, this may be attributed to the aqueous vapor continually rising towards the higher regions of the air from the surface of the soa. The presence of much moisture is also easily detected by the

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paleness of the sun at sunset, by means of which, sailors are accustomed to presage a storm.

The colored rings or halos which are often seen surrounding the sun and moon are evidently occasioned by very thin vapor diffused through the atmosphere. They are supposed chiefly to encircle the moon, but scarcely a day passes without light misty clouds, when at least portions of halos may be seen near the sun, and in order to perceive them, it is only necessary to remove the glare of light which makes the delicate colors appear white. Thus, if we examine the reflection from a smooth surface of water, we will perceive that the sun gilds the fleecy clouds with segments of beautifully colored rings. This effect is more distinctly seen, if the rays from a hazy or a mottled sky, be received upon a sheet of white paper held before a small hole in the window shutter in a dark room. But even when the sun shines from an azure firmament, circles of the richest tints may be produced by experiment, thus, holding a hot poker below, and a little before the small hole in the shutter, above mentioned, throw a few drops of water upon it, and the sun will be painted upon the paper like the glowing radiations of the passion flower.

Halos are produced by what is termed the diffraction of light, i. e. the rays of light in passing near the edges of a body appear to be bent from their rectilineal course. This diffraction may be easily observed by viewing objects through a minute hole, it will be found that the edges of straight bodies will be curved if viewed near the edge of the hole, and a line of bright white light, will appear tinged with orange on the side nearest the edge of the hole, and with blue upon the other. Halos are much more common in the northern latitudes than in warmer climates, a fact which is owing doubtless, to frozen particles of water floating in the air, though Humboldt remarks that lunar halos are much rarer in the northern than the southern countries of Europe, and seen more especially when the sky is clear and weather settled. He observes that in the torrid zone they appear almost every night, and often in the space of a few minutes disappear several times. Between the latitude of 15° N. and the equator, he has seen small halos around the planet Venus. The next figure exhibits

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