The first question has by far the greater importance, since it amounts to asking for a general explanation of the production of high clouds, especially the forms of cirro-cumulus, cirro-stratus, cirro-macula, and the corresponding alto varieties. There are, again, two divisions also to this question. How does the water vapour reach the stratum in sufficient quantity to saturate it? and when condensation takes place, why does it so frequently assume the characteristic mottled and granular forms like crowds of little cumulus clouds arranged in one level? This last sentence gives the clue. They are, in truth, little cumulus clouds, and must be formed in exactly the same way as their vastly larger prototypes of lower regions. It has been explained that low cumulus is the result of large upward moving air columns or convection currents, each one being initially caused by the heating of the vapour-laden air near the ground, and each uprising column being supplied by cooler descending air which flows down in the intervening spaces. It has also been explained that these movements result in changes of temperature, which tend to check those movements and restore the original equilibrium. Suppose this to occur, as it constantly does, without any column reaching sufficiently high to produce a cloud. There will be no visible effect, but, nevertheless, an important change has taken place. Every ascending current has lifted some water vapour with it to a higher level, and the descending drier air has come down in contact with the ground or damper air to become equally charged with moisture in its turn. The process will be repeated again and again, and at one level after another, so that the water vapour travels ever higher and higher. This process of interchange between ascending and descending air has been called by Mr. Ley inversion, but the term does not seem very suitable, and interconvection would be better. The two opposite currents pass through each other, as if the ascending air gathered itself into definite channels, and passed through holes in the descending mass like the passage of water upwards through a descending plate of perforated metal. Moreover, just as the holes in such a descending plate might have any size, so that the ascending streams might vary in breadth from the finest hair to a column of huge diameter, in exactly the same way the ascending columns of air may vary from the smallest imaginable size to the great cumulo-nimbus currents. It is the little currents which account for the constant quiver of the margins of any object which is viewed through a large telescope by day, and for the haze, so characteristic of a hot day, which makes distant objects seem ill-defined and in a state of continual tremble. The rays of light in passing through the intersecting streams are bent a little, now this way, now that, as the air currents sway to and fro.