title="[37]"/> his intercession for all who come to him with faith, and the sanctification of the Holy Spirit, are each and all mysteries? "Canst thou by searching find out God?" Yet the believer knows that God to be his Father and his Friend; he walks by faith and not by sight, "by faith of Jesus Christ unto all and upon all them that believe," and are "justified freely by his grace through the redemption that is in Christ Jesus." When we reflect upon the mysteries of nature, let us also think upon the mysteries of grace; let the consciousness of our own ignorance lead us to throw aside presumption, and trust to the words dictated by Him, to whom all mysteries are open, and whose wisdom and goodness have no bounds.

5. Its Dryness or Humidity.—Evaporation from the waters of the globe, from seas lakes, rivers, etc., is perpetually taking place, the ratio of evaporation being determined by the temperature of the atmosphere; that is, it increases as the temperature increases. Hence the actual quantity of water in a state of invisible vapour in the atmosphere will be the greatest when the temperature of the latter is the highest. Speaking according to our feelings, we then call the air dry, although, in fact, the quantity of water in solution in the atmosphere may balance the demands of the temperature.[5] But this invisible vapour is liable from a thousand causes to condensation, dependent upon one law, namely, that when a portion of air, saturated with invisible vapour, is cooled below the point of saturation, a portion of the vapour becomes condensed, according to the degree of cold, so as to leave a due balance of vapour in the air, according to its altered temperature. Who has not seen the window-panes of a heated room in cold weather bedewed with trickling moisture? The vapour of the room is in abundance, although invisible; but certain strata of the air thus charged come in contact with the cold panes, and lose a portion of their caloric, and with their caloric a portion of their vapour, (to which, perhaps, the respiration of a large party has contributed,) and this becomes condensed in the form of a visible fluid. If we breathe against a cold pane of glass, we soon render it dim by moisture. On a hot summer's day, we do not perceive the vapour of our breath; but on a cold, clear, frosty day, a visible vapour or little mist is exhaled.

[5] The quantity of water in solution in the atmosphere can never be greater than the quantity proper to the temperature; but it may be less.

From electric changes, from currents of air, from sudden or gradual depressions of atmospheric temperature—be the cause what it may—the condensation of invisible vapour in the atmosphere is perpetually taking place. But the extent and degree of condensation is very variable. Sometimes, or rather very commonly, the quantity of water separated is small, and condensed into such minute particles that they float in the air, higher or lower, forming wreaths of mist, or clouds of various patterns, density, and extent; sometimes they appear at a great altitude, like silvery waves, or snowy fleeces; sometimes at a lower altitude they take the shape of dense irregular masses, presenting to fancy's eye the outline of monstrous animals, of mountains, and castellated rocks; and sometimes the whole horizon is beclouded and dark; we then know that the condensation is increasing. What is the result? A fall of rain, snow, or hail, in which latter case the water is not only condensed into its ordinary form, but into pellets of ice. The water has passed through a stratum of air at the freezing point. Sudden hail-storms in summer may, we think, be classed among phenomena dependent on electric changes and operations in certain atmospheric strata.

We have here, then, two processes—one of evaporation, another of condensation; and these, by an express and admirable provision, have a constant tendency to limit each other's operations. Evaporation is increased by heat, but it produces cold in proportion to the rapidity with which it is carried on. Condensation is produced by cold, and at the same time heat is liberated.

How happens it that clouds and rain are formed in the higher regions of the air, and not closely round about us? In consequence of certain laws, the air incumbent on the earth's surface, especially in the hotter latitudes, is always under the point of saturation, the air being really dryer than it is at a great elevation. These laws depend on the difference in the tension or elastic force of vapour at different temperatures; this elasticity increases rapidly from the temperature of 32° to 212°, the increase being in a geometrical progression, while the increase of the temperature of the air from a high altitude to the earth is in arithmetical progression. Now the air of our mixed atmosphere is the ingredient which controls the whole mixture. The result is, that the quantity of vapour present in a mixed atmosphere will, at any successive diminution of height above the surface of the earth, become successively less and less than that which would be required to saturate the air. The consequences of this result are most important, and are connected with our general well-being, and the due exercise of some at least of our senses.

The following passage from Dr. Prout's Treatise is so pertinent, that we beg leave to transcribe it: "Over the greater portion of the earth, the air which, during the day at least, is warmed by contact with the earth's surface, and thus becomes lighter, has a constant tendency to rise into the higher atmosphere. Now if this air were saturated with vapour, of course, whenever by rising it became mixed with colder air, its vapour would be more or less condensed, and a cloud would be formed. Hence, if we lived in such an atmosphere, we should be always enveloped in a mist, through which the sun would not be visible. But, by the benevolent arrangement we enjoy, this consequence is so entirely prevented, that, unless under peculiar circumstances, and always for beneficial purposes, the air at the earth's surface is hardly ever saturated with moisture. The air which has been warmed by contact with the earth can, therefore, rise from the surface without any condensation of its moisture within the limits of its point of saturation. Thus, at the equator, before the air reaches the temperature of 61°, the presumed point of its saturation, it must ascend to the height of 6,000 or 7,000 feet. At this height its vapour will be condensed, and a cloud will be formed, which may either be precipitated on the spot from which its constituent vapour had risen, or may be transported by the currents of the atmosphere, similarly to refresh a distant country, or may again be dissolved in the air; while, under all these contingencies, the whole of the lower portion of the atmosphere is exempt from mist, and continues perfectly transparent. These operations are unceasing; moreover, the very clouds, by giving out their latent heat, and shielding the earth's surface from the direct influence of the sun, have a still further effect, and have a constant tendency to modify their own formation and existence."

We might here enter largely into other points of high interest connected with the science of meteorology, did the nature of our subject allow it. We do not forget that it is upon the senses that we have to write, and