WATERS ABOVE THE FIRMAMENT.

O ye waters that be above the Firmament, bless ye the Lord; praise Him, and magnify Him for ever.

HE word Firmament is obviously used here in the same sense in which it is employed in the 1st Chapter of Genesis. It is that space which immediately invests the earth, and which interposes between the waters which are below and those which are above it, or between seas and clouds. The Scriptures abound with imagery derived from this source. Clouds shut out the bright sun-they were therefore emblems of gloom and sorrow; at other times, they sheltered plant and cattle from his scorching rays, and they were then the symbols of tender care and protection. Of old, as now, poets turned towards the clouds for some of their grandest metaphors. The Psalmist says, "The Lord maketh the clouds his chariot;" and when the inspired writer of the Revelation exclaims, "Behold he cometh with clouds," the expression suggests the idea of grandeur and majesty.

Clouds are among the first of the objects invoked in the Hymn, and they are twice mentioned; once by themselves, as the "waters above the firmament," and again in another verse in connection with lightning.

The prominence thus given to them accords with their importance in countries like Judæa and Mesopotamia, where droughts are sometimes severely felt. Clouds, therefore, were watched for eagerly and anxiously, as signs that the parched earth was about to be blessed with refreshing rain. Unhappy the regions where "the waters never collect "above the firmament." There "the clouds drop no fatness," and the land loosens into sterile sand.

In our own country, and still more in hot climates, clouds often interpose as a friendly shield between sun and earth, to check excessive evaporation from the one, and to ward off the too scorching rays of the other. Without this protection the surface of the soil would dry up, roots would find no moisture, plants would languish or wither, and cattle might perish for want of water.

The vapour issuing from the spout of a tea-kettle supplies a favourite illustration of the theory of clouds, or they may be studied on a larger and very beautiful scale as they rise from the funnel of a locomotive. With every puff of the engine a quantity of steam is driven into the air. It will be noticed that this steam is invisible at the moment of its escape, and when it has as yet scarcely cleared the funnel; then it is quickly condensed into a white cloud; and, lastly, this cloud itself disappears. A moment's attention to these three points will unfold to us much that is interesting in cloud-philosophy. It is well known that, when water is heated to a temperature of 212° Fahrenheit, it rapidly passes into invisible steam. The steam produced by the engine boiler was, therefore, as transparent as air on first escaping into the funnel. But when steam is cooled below the temperature of 212° Fahrenheit it is condensed into vapour; hence the white cloud which the invisible steam of the locomotive formed on coming into contact

with the colder air around it. Finally, we observed that as this cloud was diffused more widely through the air it dissolved and vanished.

This last fact proves that the atmosphere has the property of absorbing or dissolving moisture, which it retains in an invisible state. Air, indeed, always contains an admixture of moisture, though the quantity is continually varying. The warmer the air, the greater is its capacity to take up water in this invisible state; on the other hand, the colder the air, the less moisture it can hold. It follows that the atmosphere of the tropics is much more loaded with vapour than that of temperate regions; while this latter, in its turn, contains more moisture than the air of polar regions. We speak of "a dry air," but the expression is only relatively correct. There is always enough of water even in the dryest air to moisten saline substances that attract it; and everybody has observed the streams condensed from unseen vapour which soon begin to trickle down the sides of a bottle of iced water brought into a room. Few people, however, would have expected to find that a cube of air measuring twenty yards each way and at a temperature of 68" Fahrenheit, is capable of taking up no less than 252 lbs. of water before it reaches the point of saturation. From this it may be imagined how enormous the quantity of water must be which is suspended invisibly in the entire atmosphere of the world.

It is out of this invisible steam pervading the atmosphere that visible vapours or clouds are manufactured. When one current of air meets another current colder than itself, they intermingle; and, if the resulting mixture be not of a temperature sufficiently high to retain in a state of invisibility the moisture diffused through it, the excess is necessarily condensed into cloud. The cloud itself is composed of particles or drops of water so ex

tremely minute that they float in air. But if the condensation be pushed farther, the minute drops coalesce into larger drops, and rain falls to the earth. On the other hand, if warm or dry currents of air happen to set in through the cloud, it will be again more or less completely dissolved, as was observed in the case of the vapour puffed out of the engine-funnel. Hence the continual changes going on in clouds their thinning, thickening, enlargement, diminution, and the other alterations of form.

The atmosphere owes its moisture to the evaporation 'going on at all temperatures both from land and water, and more especially from the great equatorial oceans of the globe. In temperate climates, like that of Europe, with a mean temperature of 5240, the annual evaporation is equal to a layer of water 37 inches thick; but within the tropics it is much greater, varying from 80 to 100 inches. The great stimulator of evaporation is the sun, and clouds check evaporation by intercepting his rays. A calm is less favourable to it than a breeze; in the former, the air resting on the water soon gets saturated, and ceases to absorb; but a breeze sweeping over the sea is continually presenting to it new and thirsty portions of air, so that the process goes on with great activity. The water thus sucked up is carried off into the atmosphere as invisible vapour or steam, which is ultimately condensed into clouds. These, therefore, may be considered as huge aerial tanks or reservoirs filled with water handed up by the ever busy air for the service of the earth. When clouds are not condensed in one place, the loaded air passes on with its burden to another; but sooner or later it is relieved, either by the vanishing of the vapour through re-absorption, or by the formation of rain.

Besides supplying all the rain and filling all the rivers

of the earth, the invisible moisture of the air is essential to the well-being both of animals and plants. Were the thirsty air not abundantly fed with water from sea and land, it would in its eager search for drink suck out the moisture from every living thing, and in spite of all precautions we should soon pass into the condition of driedup mummies. Our safety lies in the free admixture of water with the air, by which its keenness is tempered. It is astonishing to mark what care nature has taken to protect the juices of plants and animals from this desiccating action, by investing them with coverings which are more or less impermeable.

In respiration the lungs cannot support an air which is too dry. When the supply of invisible vapour in a room is deficient, unpleasant sensations arise which are relieved by softening the air with steam from hot water. While wintering beyond Smith's Sound, Doctor Kane observed that his crew suffered from the excessive dryness of the air which, in breathing, was sensibly pungent and acrid. Nor is the invisible atmospheric vapour less. necessary to the vegetable kingdom. Plants have the power of absorbing moisture not only by the roots but also through their leaves; and, in a fairly humid air, the evaporation going on from their surface is thus more or less checked or compensated. But in a too dry air this balance is upset, and the leaves droop or wither. The few plants that grow in the sandy desert are mainly dependent on the invisible moisture of the atmosphere for their supply of water, and the same may be said of those plants which live and grow when suspended in the air of a hot-house.

From the remarks just made it will be readily understood that clouds or wind coming from the north do not usually portend rain. The air, in passing southwards, has its temperature gradually elevated; and, consequently,