reckoned from the south. Hence the position of the sun is ascertained for both negatives. By actual measurements on the plates and reference to a previously constructed scale the position of the cloud as seen from each camera is next determined, and the angle subtended by the base line at a point X vertically beneath the cloud is calculated. If A and B are the stations, and _a_ and _b_ the angles from them respectively, the distance AX is given thus-- log AX = L sin _b_-L sin AXB + log AB, and the height _h_ of the cloud above X is given by-- log _h_ = log AX + L tan alt.-10. CHAPTER X CLOUD NOMENCLATURE SINCE a considerable number of new terms have been suggested in the foregoing pages, it may be convenient to collect them and tabulate them, so as to show their relation to those already recognized by the International system. In the atlas put forward by the committee, sixteen varieties are recognized by distinct names, and these are drawn up in tabular form with appropriate abbreviations for use in making records. The names are-- Cirrus. Ci. Cirro-stratus. Ci. S. Cirro-cumulus. Ci. Cu. Alto-cumulus. A. Cu. Alto-stratus. A. S. Strato-cumulus. S. Cu. Nimbus. N. Cumulus. Cu. Cumulo-nimbus. Cu. N. Stratus. S. Fracto-cumulus. Fr. Cu. Fracto-nimbus. Fr. N. Fracto-stratus. Fr. S. Stratus-cumuliformis. S. Cf. Nimbus-cumuliformis. N. Cf. Mammato-cumulus. M. Cu. During our survey of these groups we have found that some of them include clouds of many shapes, which must be due to very diverse conditions. It follows that if observations are to be made on the occurrence of these special kinds, with a view to arriving at a thorough understanding of the circumstances to which they owe their forms, it becomes necessary to devise a code of names and symbols whereby an interchange of ideas and records may be rendered possible. Specific names have been proposed as each form was considered, and it only remains to sum them up concisely. Subsequent observation, particularly in other climates, may show that further additions should be made; but if the principle of specific names be once admitted, it will be easy to fill any omission. GROUP CIRRUS. Under the general head of cirrus we have found nine distinct forms-- 1. _Cirro-nebula_ (Ley) (Plates 2 and 3). Cirrus veil. Characterized by comparative absence of structure and by the formation of halo. Ci. Na. 2. _Cirro-filum_ (Ley) (Plate 7). Thread cirrus. Built up of fine long threads, straight, curved, or crossing, but free from hazy curling or flocculent structures. Ci. F. 3. _Cirrus excelsus_ (Plate 5). High cirrus. Characterized by great altitude, thinness, irregular branching structure. Ci. Ex. 4. _Cirrus ventosus_ (Plate 6). Windy cirrus.