CUMULO-STRATUS.—This form of cloud results from the mingling of the cumulus and cirro-stratus; it appears sometimes as a thick bank of cloud with overhanging masses. The cloud known as “_distinct_” cumulo-stratus appears as a cumulus surrounded by small fleecy clouds. 1. _Thunder may be expected_ when “distinct” cumulo-stratus appear. 2. _Sudden atmospheric changes may be expected_ when cumulo-stratus appear. [Illustration: 47. Nimbus.] NIMBUS, OR CUMULO-CIRRO-STRATUS.—The name of this cloud at once suggests that it is produced by a combination of the three primary forms of cloud. The _nimbus_ is popularly known as “the rain cloud.” It is really a system of clouds, having its origin chiefly in the tendency of the _cumulo-stratus_ to spread, overcast the sky, and settle down to a dense horizontal black or grey sheet, above which spreads the _cirrus_, and from below which rain begins to fall. 1. _A cessation of rain may be expected_ when the grey lower portion of _nimbus_ begins to break up. 2. _A thunderstorm may be expected_ when the _nimbus_ character of the cloud is very perfect. 3. _Very copious showers may be expected_ when the _cirri_ projected from the top of the rain-cloud are very numerous. AMOUNT OF CLOUDS.—Any record of the proportion of sky covered by cloud should be made on a scale of 0 to 10. A clear sky is registered 0, and a sky wholly obscured as 10, any intermediate condition being represented by 5—7, or other figures deemed appropriate by the observer. The _kind_ of cloud should be noted, as also the direction in which it is driven by the wind, whether in the upper or lower strata of the air. This operation may be assisted by an ingenious arrangement, exhibited by Mr. Goddard in 1862, and called a “cloud reflector,” obtainable at any optician’s. Observations at the Greenwich Observatory establish the facts that the least amount of cloud exists during the night, especially in May and June, and the greatest amount at midday, and in winter; also that from November to February three-fourths of the heavens are obscured by sun-repelling clouds. HEIGHT OF CLOUDS.—Great diversity of opinion exists on this point. It is asserted, on the one hand, that the region of clouds does not extend beyond five miles above sea-level, but Glaisher has attained a height of 36,960 feet, and from thence saw clouds floating at a great height above him; and it is considered probable that cirri are often ten miles above the earth.