316 SEIENTIFIC RECORD FOR 1884. 



clouds, or cirro filnin; of these latter we must ilistinguisli the direction 

 of tlie motion of individual particles from the direction in which the 

 long- threads trend. This latter direction he calls the filature of the 

 threads, and is determined by the location in the horizon of the vanish- 

 ing point of two or more parallel threads. The apparent velocity is 

 expressed on a scale of 3, namely, 0, calm; 1, slow movement; 2, mod- 

 erate, and 3, a rai)id movement. There is a form of cloud which Ley 

 calls the iiseudo-cirrus, which appears at low altitudes, having an ap- 

 parent rapid movement, and is erroneously taken for true cirrus. He 

 apparently distinguishes one from the other by the rapidity of the 

 movement, and states that he has never seen a true cirrus with a rapid 

 movement. His tables of the results of over 3,000 observations, mostly 

 by himself and friends in England, allow him to conclude as follows: 



1. The movements of the cirro-filum are more frequent in the direc- 

 tion of the length than at right angles thereto. 



2. The cases in which the vanishing points lie on the right-hand side 

 of the direction from which the clouds themselves move are for north 

 and west movements I'y far the most frequent, but for south or south- 

 east movements the vanishing points more frequently lie on the left- 

 hand side of that direction. 



3. Great velocities of movement are confined to those cases in which 

 the direction of filature nearly or wholly agrees with the direction of 

 motion, and cases in which they are at right angles to each other never 

 occur with great velocities. 



The explanation of these rules he arrives at by the study of the lower 

 forms of clouds. He finds that on account of the difference in rapidity 

 and direction of the velocity of different parts of the cumulus clouds, 

 evf n the lower cumuli manifest a tendency to assume a form that reminds 

 one of streaks and threads. Especially is this the case when streaks of 

 snow falling from the summits of these clouds give rise to what he calls 

 "pseudo cirrus-pendulus;" these fall at first vertically, but subsequently 

 are deflected or bent and twisted by the movement of the air into 

 which they fall. I^ow the true cirro-filum is often found on the extreme 

 limit of a cyclone, and therefore several hundred miles from the center, 

 so that it is impossible to observe the process of formation of the 

 whole thread. It happens, however, that in otherwise clear weather a 

 local rain cloud sometimes develops a great quantity of cirri-filum, and 

 then it is possible to observe the process without difficulty. Some miles 

 from the observer on a summer day there forms a heavy cumulus; if, 

 as frequently happens, an almost perfect calm prevails in the atmosphere 

 up to an altitude of 12,000 or 14,000 feet the cumulus retains its hemi- 

 spherical form, and thus there comes to be an enormous accumulation 

 of cloud that can easily amount to a hundred cubic miles, and its dark 

 color shows that the water-drops of which it consists are very close to- 

 gether. [This statement as to color seems to be rather an assumption 

 than a well- warranted conclusion; do notthesize, form, and number of the 



