"Who can number the clouds in wisdom? or who can stay the bottles of heaven?"-JOB XXXVIII.

150. Why do clouds generally gather and obscure the sky before a considerable fall of rain?

Because when the vapour of the air is small in amount, upon reaching the higher regions of the atmosphere it produces the light cirriform clouds; but as the vapour augments, and its condensation advances in the upper regions, its weight, relative to that of the atmosphere, increases, and it subsides to a lower station, where, met by a denser atmosphere, it floats as a cloud of the cirro-cumulus or cumulo-stratus form.

151. Why do clouds frequently gather and disperse several days in succession before a fall of rain?

Because when the clouds obscure the sky, they arrest some of the heat of the sun's rays, and they also receive the heat that is radiated from the earth. Consequently, when they gather, they themselves produce the cause of their dispersion, and unless the air is fully saturated with moisture, the clouds may pass away and gather again several times before a fall of rain.

152. Does water evaporate at all temperatures?

Yes, even ice and snow evaporate. Millions of tons of water are evaporated from the surface of the earth every day. By this evaporation the earth is refreshed, temperatures are equalised, rivers are supplied, and the depth and extent of oceans are considerably influenced and regulated.

153. Why do clouds of dust indicate the coming of rain? Because evaporation takes place much more rapidly during the prevalence of wind than when the air is still. Clouds of dust not only indicate the dryness of the earth, through the absence of rain, but they also show that winds are prevailing, and that evaporation by those winds is actively going on; therefore the air will the sooner be saturated with moisture.

154. Why does not evaporation proceed so rapidly in a calm as in a breeze?

Because when air is still the same portions of it remain in contact with the watery surfaces; but when breezes prevail fresh and dry

"Who giveth rain upon the earth, and sendeth waters upon the fields.”—JOB V.

volumes of air are repeatedly brought in contact with the evaporating surfaces. It is for this reason that wet linen dries more rapidly in a wind than in calm air.

155. Why do the different phases of the moon affect the quantity of rain?

Because the influence of the moon's attraction alters the density of our atmosphere. From observations made during nineteen years, it has been found that the average number of rainy days for each of the moon's phases is as follows: new moon, 77; first quarter, 82; full moon, 79; last quarter, 60; moon in perigee, 93; moon in apogee, 78.

Perigee-the point at which the moon is least distant from the earth. Apogee -her most distant point from the earth.

CHAPTER XI.

156. What is dew?

Dew is condensed vapour.

157. What causes the condensation of the vapour which forms dew?

When the sun sets, the earth, and the various objects upon its surface, begin to cool. They throw off, by radiation, some of the heat which they received during the day, and, becoming cooler than the surrounding air, they condense the vapour of that portion of the air which comes in contact with them.

158. Why do some bodies become wetter with dew than others?

Because substances differ in their powers of radiation. Those which radiate the least heat remain comparatively warm, and do not favour the formation of dew upon their surfaces; while those which radiate most heat become cold, and cause the moisture of the air to settle upon them.

159. Why is there more dew formed during some nights

than on others?

Because of the variations in temperature, and the manner in

C

"And there are diversities of operations, but it is the same God which worketh in all."-CORINTHIANS XII.

which those variations affect the relative heat of the earth, and the air in which the vapour is suspended.

160. Is the formation of dew the cause, or the effect, of cold?

It is the effect of cold. The coldness of the bodies with which the air comes in contact causes the vapour to condense, and form dew-drops.

161. Does the formation of dew tend to cool or warm the air?

It tends to warm the air, by lessening the quantity of watery vapour floating in it, and by giving to the air the latent heat which the vapour contained.

162. Upon what substances does dew form most freely?

Upon woolly, rough, and fibrous substances, such as the wool of animals, the leaves of plants, blades of grass, &c.

163. Upon what substances does dew form least freely ? Upon smooth surfaces, such as those of metal, stones, and glass.

164. Why does dew form upon rough surfaces more freely than upon smooth ones?

Because rough surfaces radiate heat freely, and cause their substances to become cooled; while smooth surfaces radiate less heat, and, therefore, their temperature does not fall in the same degree,

165. In what way does the radiation of heat from the earth by night illustrate the Divine wisdom?

If the heat received by day were not radiated back by night, the earth, acquiring an addition of heat every day, would soon become so hot as to be unendurable by the vegetable and animal kingdoms now existing upon it.

166. Why does the radiation of heat from rough surfaces exhibit the Divine wisdom?

Because it causes the deposition of dew upon grass, plants, and

"Thou visitest the earth, and waterest it: thou greatly enrichest it with the river of God, which is full of water: thou preparest them corn,

when thou hast so provided for it."-PSALM lxv.

trees; but upon barren rocks, gravel walks, and bare and hard soils, where dew would be of no benefit, it does not form.

167. Why does little dew form at sea?

Because of the low radiating power of the smooth surface of water; and also of the tendency of the water that has become cooled to sink, and allow warmer portions to rise, by which the temperature of the surface of the sea is seldom greatly or suddenly reduced.

168. Why is little dew formed on windy nights ?

Because the air, being kept in rapid motion, does not remain long enough in contact with cooling surfaces to deposit its vapour.

169. Why does a gentle breeze favour the formation of dew? Because, while it allows each stratum of air to remain in contact with the cold surface of the earth long enough to deposit its moisture, it brings fresh strata to the earth, by the movements of the and as these fresh strata bring additional moisture, there results an increased formation of dew.

air;

170. Why is a very calm night less favourable to the formation of dew than when a gentle breeze prevails?

Because, through the stillness of the air, the same portions of the atmosphere remain in contact with the earth's surface; consequently, there is not so much vapour presented to the cooling bodies as when the air is in gentle motion.

171. Why are clear and starlight nights favourable to the formation of dew?

Because the radiation of heat from the surface of the earth is not checked by the intervention of the clouds.

172. Why does the intervention of the clouds check the radiation of heat from the earth?

Because the clouds act as secondary radiators; they receive the radiated heat of the earth, and return it, by which the degree of cold that would otherwise occur upon the surface of the earth is considerably modified.

"The Lord is my shepherd, I shall not want. He maketh me to lie down in green pastures."-PSALM XXXIII.

If plates of glass be laid over grass-beds, as in the engraving, no dew wil be deposited on the grass underneath the glass plates, although all around the grass will be completely wetted. The explanation is that the glasses, being radiators of heat, act in the same manner as the clouds, returning the heat to the bodies underneath them, and preventing the formation of dew thereon. It has been observed that sheep that have lain on the grass, during the formation of dew, have their backs completely saturated with it, but that underneath the line where their bodies turn to the earth, their coats are dry. In the same manner glass globes suspended in the air, on dew-forming nights, will be found loaded with globules of dew upon the top, but there will be no appearance of moisture underneath.

CHAPTER XII.

173. Why is there little or no dew formed when the clouds are dense and low?

Because then the heat radiated by the clouds to the earth is considerable, and prevents the cooling of the surfaces upon which dew usually forms.

174. Why is there an increase of dew when the clouds are very high in the atmosphere?

Because the radiating effect of the clouds diminishes with their distance from the earth; and when they are far away, it is comparatively feeble.

175. Why are islands, and places close to the sea, frequently warmer in winter than places removed far from the

sea?

Because, being subject to cloudy skies, formed by the evaporation