Again and again, during the progress of these measurements, it was found that the greatest altitudes and the richest development of the higher varieties occurred towards the end of a spell of fine calm weather, when convection had had free play day after day. A slight fall of the barometer, only the hundredth part of an inch, would usually, under those circumstances, bring about abundant formation of high clouds, frequently of the undatus kind. All the cumulus clouds, by which we mean to include alto-cumulus and cirro-cumulus, are most frequent when the levels of condensation are rising, while the stratiform clouds are an indication of no vertical movement or of active descent. Pure cirrus is indicative rather of movement in a horizontal direction, and may occur when the condensation levels are stationary, or when they are rapidly changing either way. In broken weather the natural movements of the atmosphere and of its vapour are masked and disturbed by the strong eddies brought by the cyclonic systems. It not unfrequently happens that the region of disturbance does not reach up to the level of the highest cirrus, or, what is more probable, the cyclonic system leans so far forward that we may have in its rear the upper clouds floating quietly far above the comparatively shallow region of disturbance, while in front the upper part of the storm system projects above undisturbed air. The frequent appearance of cloud almost at the same time at more than one level is at first rather difficult to understand, but it will be noticed that when this occurs the barometer almost invariably falls. Now, if we suppose that the air is nearly saturated at more than one level, and that the whole is then bodily relieved of some of the superincumbent mass, so that the barometer falls, the mass of air will at once swell up, being cooled from top to bottom simultaneously, and wherever it is damp enough cloud will be formed. The converse is equally true. If we have cloud at several levels, and the whole is compressed by the addition of more air above, which is the case when the barometer rises, that compression will be accompanied by the generation of heat and the consequent disintegration and disappearance of the clouds. FOOTNOTES: [3] Ferricyanide of potassium and hyposulphite of soda. [4] From the declination of the sun corrected for variation and from the known latitude, the meridian zenith distance is calculated. From the Greenwich time, the longitude, and the equation of time, the hour angle is obtained. Now, if H be the hour angle, D the reduced declination, and M the meridian zenith distance, the sun’s altitude may be calculated by the formula-- log versin H + L cos lat. + L cos D-20 = log _n_, where _n_ is a natural number, and _n_ + vers M = covers alt. Again, to find the azimuth-- vers sup. (lat. + alt.)-vers polar dist. = _m_, where _m_ is another natural number, and log _m_ + L sec. lat. + L sec. alt.-20 = log vers azim.,