316 ME JOHN AITKEN ON THE 



illuminated by means of a spot-mirror, and viewed through a magnifying lens. The 

 instrument is held in the clouded air with the micrometer horizontal, the observer 

 watching its surface through the lens, when little drops of rain are seen falling in 

 rapid succession on the micrometer and rapidly evaporating. One small square is 

 selected, and the number of drops that fall in a measured time are counted. 

 The number falling will be found to vary greatly from time to time. If any difficulty 

 is experienced in counting the drops owing to their rapid evaporation, it will be 

 found of great assistance to cool the micrometer, either by fixing a piece of wet paper or 

 a little snow to the metal mounting ; or if this is inconvenient, then a similar effect, 

 though somewhat short-lived, may be obtained by breathing on the surface of the 

 glass. It was only after observing with this instrument that I realized the true state 

 of matters in certain clouds, and saw that though all exposed surfaces were dry, they 

 were really exposed to a continuous shower of immense numbers of fine drops of rain. 



The general result of the observations made in clouds on the Bigi is very similar to 

 that derived from the observation of fogs at low levels. The size of the particles 

 and the number falling are about the same in both cases. Of course both the 

 size and the number vary greatly at both places under different conditions. As 

 already pointed out, the number of dust particles varies greatly at different parts 

 of a cloud, and when we are in the midst of clouds and examine them closely, we 

 find that most of them vary greatly in density or thickness from time to time, 

 or, more correctly, in their different parts. At one time the passing cloud may be 

 so dense that we cannot see beyond 30 yards ; in a minute or two the limit may 

 be extended to 100 yards, when it may again close in to its original density. This 

 variability in the density of clouds is probably greatest in clouds formed of the 

 air rising from the valleys, where the mixture of the pure and impure air is 

 necessarily imperfect. Stratus and other forms of cloud probably do not have this want 

 of uniformity. All the clouds observed on this occasion varied greatly from point to 

 point, and it was also observed that the number of water particles falling varied greatly. 

 At times they showered down so quickly that it was impossible to count the number that 

 fell on one square millimetre ; but generally it was easy to count the number falling on so 

 small an area, and occasionally they were so few that they only fell at considerable 

 intervals. 



It was observed that the more dense the cloud the greater was the number of 

 drops falling, and that as the cloud thinned away the number gradually decreased. The 

 greatest number actually counted was 60 per square m.m. in 30 seconds. If they had 

 been counted for a shorter time a quicker rate would have been obtained, as they did not 

 fall so quickly during the last half of the time as during the first. Very heavy falls 

 seldom lasted more than a few seconds ; a rate of 30 drops to the minute was, 

 however, frequently observed. The maximum rate of 60 drops per square m.m. per 

 half minute gives 12,000 drops per square centimetre per minute, or 77,400 drops per 

 square inch per minute. This does seem an enormous rate of fall, yet the particles are so 



