even so delicate that he was able to discern through them the spots on the moon. The last two classes of clouds, like the cirrus. float at a very lofty height.

248. CUMULO-STRATUS.

The variety of cloud to which this name is given, combines the characteristics of the cumulus and stratus. Its base consists of a horizontal stratum or layer of vapor, from which rise large, overhanging masses of cumulus (figure 14., f). Some

What is said respecting the height of the cirro-stratus and cirro-cumulus? Describe the cumulo-stratus.

Of what does it consist?

times contiguous cumulus clouds unite, and passing inte the state of cumulo-stratus, form groups of immense size. This cloud is seen in perfection upon the approach of a thunder-storm, when the cumulus clouds, driven together by the wind, are piled upon each other, and assume those peculiar forms commonly known by the name of thunderheads.

249. This modification also frequently arises, when the cumulus is pierced by the cirro-stratus; and it is by no means unusual to see four or five parallel bars of the cirro-stratus, one above the other, passing through the same pile of clouds, which then present successive tiers of the cumulo-stratus.

250. NIMBUS, OR RAIN-CLOUD. This cloud is so called from the Latin word, nimbus, a rainy dark cloud; it possesses no peculiarity of form, but is distinguished by its uniform gray tint and fringed edges (figure 14., e). It is usually composed of some of the preceding classes of clouds, so blended together that they cannot be distinguished, and is produced by a change in their structure, the result of an increase in density.

251. The nimbus often originates in the cumulo-stratus, which, as it increases in thickness, frequently assumes a black or bluish tint. In a short time this hue changes to gray, a circumstance which indicates that the nimbus is formed and rain descending.

When is this cloud most perfectly formed?

Under what other circumstances is the cumulo-stratus seen?
Describe the nimbus.

How is it distinguished?

Of what does it consist?

How is it caused?

In what cloud does it often originate?

What does a gray tint indicate?

5*

CHAPTER I

OF DEW.

252. DEW is the moisture spontaneously deposited upon the surfaces of bodies exposed to the atmosphere, when the latter is free from the presence of fogs ana

rain.

253. The whole subject of dew was most happily illustrated by the observations and experiments of Dr. Wells, in 1812; and the theory which he then advanced is now generally received, supported as it is by a vast assemblage of facts.

254. DEPOSITION. The deposition of dew is caused by the unequal radiation of heat from the atmosphere and the substance bedewed. During the day, the bodies, that either compose the solid crust of the earth or clothe its surface, become heated by the solar rays, and the lower stratum of that portion of the atmosphere which is directly above, is then likewise raised in temperature, and its capacity for moisture increased.

When, however, the night comes on, and even before, the earth and air, radiating their acquired heat into free space, sink in temperature; but the loss of the former is greater than that of the latter, since, during the night, as experiments show, the air a few feet above the ground, is sometimes warmer than the surface of the soil, by fif teen degrees.

It thus occurs, that the stratum of air immediately in contact with the earth is cooled down by the latter, beyond the point of saturation; and the excess of humidity is condensed, upon the substances that form its surface, in drops of dew. (Art. 65.)

255. It may therefore be assumed as a principle, that dew never begins to be deposited upon the surface of

What is the subject of chapter fourth? Define dew.

Whose theory is generally received?

How is the deposition of dew caused?

Explain the process.

How much warmer is the air sometimes than the ground?

What principle may be assumed?

any body, until it is colder than the contiguous atmosphere; and, other circumstances being the same, the greater this difference in temperature, the greater the amount of dew.

The quantity of dew deposited within any given time, depends, chiefly, upon the humidity, serenity, and tranquillity of the atmosphere; and the constitution, form, surface, and location of the bodies receiving the moisture.

INFLUENCE OF THE CONDITION OF THE ATMOSPHERE.

256. HUMIDITY. That the quantity of latent vapor in the air must regulate, in a great measure, the amount of dew, is perfectly clear, since the latter is nothing else than condensed atmospheric vapor.

257. SERENITY. Every thing that favors radiation from the earth, and consequently produces an increase of cold, contributes to the formation of dew. Thus it is copiously deposited on serene nights; for the radiation from the earth then proceeds unchecked: while, on the contrary, little or no dew is seen after a cloudy night; since the canopy of the clouds reflects back to the earth the heat that is proceeding from it, maintaining its surface and the contiguous air at nearly the same temperature.

If, however, the clouds separate only for a few moments, the heat escapes from the earth through the intervals, and dew is rapidly deposited; but if the sky is again suddenly overcast, the radiation is arrested, and the heat reflected back to the earth, raising the temperature at its surface, and speedily evaporating the dew just formed.

258. These singular changes in temperature were observed by Dr. Wells. On one night, the sky being clear, the temperature of the grass, at half past nine, was 32° Fah.; in twenty minutes afterwards, the heav ens being suddenly overcast, it rose to 39° Fah.; in

What circumstances influence the quantity of dew?
What is the effect of humidity? What of serenity?
What is the influence of clouds? Give instances.

twenty minutes more, under a serene sky, it sunk again to 32° Fah. It was also found, that a thermometer lying upon the grass, would rise several degrees, if the sky directly above it was covered by a cloud only for a few minutes. The influence of clouds upon the temperature of the air is by no means as great; for while, on one evening, when the sky was obscured for the space of forty-five minutes, a thermometer placed upon the turf rose fifteen degrees, another, suspended in the atmosphere just above, rose but three and a half degrees.

259. TRANQUILLITY. In a calm night, other circumstances being the same, more dew will be deposited than when it is windy; for the wind will not suffer any one portion of air to remain long enough in contact with the cold surface of any body to condense much of its moisture, hurrying it away before it is sufficiently cooled down for this purpose.

260. A slight agitation of the atmosphere, however, is of advantage; since, after one portion of air has deposited upon any surface its exuberant moisture, it removes it from the spot, bringing up a fresh portion to the same place, and so on successively; giving time to each to sink to the temperature of the surface bedewed. As the night advances, and the earth becomes still colder, the same volumes of air, renewing their contact with the same surface, may be again surcharged with humidity, and deposit more dew.

261. EVENING AND MORNING. Dew is often formed towards the close of the afternoon, in consequence of the earth then losing more heat by radiation than it receives from the slanting rays of the descending sun. It also frequently forms in shady places just after sunrise; for the surface of the globe, which has been gradually sinking in temperature during the night, is not

Which is most affected by clouds, the air or the ground? Illustrate.
Why does a wind lessen the amount of dew?

What is the effect of a slight agitation of the air?

Why does dew begin to form towards the close of day?

Where does it form after sunrise?