July 23, 1891] 



NA TURE 



279 



will be diminished by the examination of the preceding 

 examples. 



(2) Thai the work necessary to sustain in high velocity 

 the weights of an apparatus composed of planes and a 

 motor may be produced by motors so light as those that 

 have actually been constructed, provided that care is 

 taken to conveniently direct the apparatus in free flight ; 

 with other conclusions of an analogous character. 



I hope soon to have the honour of submitting a more 

 complete account of the experiments to the Academy. 



ON THE SOLID AND LIQUID PARTICLES 

 IN CLOUDS} 



IN this paper are given the results of some observations 

 made while on the Rigi in May last, on the solid and 

 liquid particles in clouds. It was noticed, when making 

 observations on the number of dust particles in the atmo- 

 sphere, that when the top of the mountain was in cloud, 

 the number of particles varied greatly in short intervals ; 

 while previous experience had shown that at elevated sta- 

 tions the number was fairly constant for long periods. In 

 orderto investigate the case of thiswant of uniformity in the 

 impurity of clouded air, extreme conditions were selected, 

 and the air tested in cloud and in the clear air outside of 

 it. When this was done the clouded air was found to 

 have always more dust in it than the air outside. Its 

 humidity was of course also greater. The relative amount 

 of dust in pure and in clouded air varied greatly. Some 

 parts of the cloud had only about double the number of 

 particles there were in the clear air, while in other parts 

 the proportion was much greater. The best example 

 tested occurred on the 25th of the month, when there were 

 observed 700 particles per c.c. in the clear air, while the 

 number in cloud went up to over 3000, and in one cloud 

 to 4200 particles per c.c. These observations were taken 

 on the top of the mountain while the clouds were passing 

 over it ; the readinijs being taken in the cloud and 

 again when it had passed and was replaced by clear air. 



These observations at once showed the cause of the 

 variability in the number of dust-particles in the clouds. 

 The dust acted as a kind of ear-mark, and showed that 

 the air forming the clouds was impure valley air, which 

 had forced its way up into the purer air above. This 

 impure air had become more or less mixed with the purer 

 upper air. Where little of the impure air had mixed with 

 the upper air, the number of particles was not large, and 

 the clouding slight ; but where the valley air was greatly in 

 excess, the number of particles was great, and the cloud- 

 ing dense. It should be noted here that all the clouds 

 tested were cumulus. It is quite probable that the con- 

 ditions in stratus and other clouds may be different. 



During this visit to the Rigi there were a number of 

 opportunities of investigating the water particles in clouds. 

 The apparatus used was the small instrument described 

 to the Society in May last. With this instrument the 

 water particles in clouds can be easily seen, and the 

 number falling on a given area counted. The results 

 are similar to those already communicated to the Society 

 from observations made in fogs during last winter. On 

 observing with this instrument in clouds, the water par- 

 ticleswere distinctly seen showering down, and the number 

 falling on the micrometer easily counted. The number 

 of drops falling was observed to vary greatly from time 

 to time. At times so quickly did they fall that it was 

 impossible to count the number that fell on only 

 I sq. mm. The greatest rate actually counted was 

 60 ^drops per sq. mm. in 30 seconds, but for a 



' Abstract of Paper read before the Royal Society, EdinburKh, on July 6, 

 by John Ailken, F.R.S. Cjmmunicateci by permission of the Council of the 



Soc.ety. 



NO. 1 1 34, VOL. 44] 



few seconds the rate was much quicker. Though the 

 quick falls seldom lasted long, yet 30 drops per sq. 

 mm. per minute were frequently observed for a consider- 

 able lime. The maximum rate of 60 per sq. mm. per half 

 minute gives 12,000 drops per square centimetre per 

 minute, or 77,400 drops per square inch per minute. 

 This does seem to be an enormous number of drops to 

 fall on so small an area in the time. These drops, how- 

 ever, are so extremely small they rapidly evaporate, 

 more than two or three being seldom visible at the same 

 time on one square of the micrometer. The denser the 

 cloud the quicker was the rate of fall, and as the cloud 

 thinned away the drops fell at longer intervals, and they 

 diminished in size at the same time. 



It was frequently observed when the mountain-top was 

 in clouds, particularly if they were not very dense over- 

 head, that the surfaces of all exposed objects were quite 

 dry ; not only the stones on the ground, which might 

 have received heat from the earth, but also wooden seats, 

 posts, &c., were all perfectly dry, and if wetted they soon 

 dried. While everything was dry, the fog-counter showed 

 that fine rain-drops were falling in immense numbers. 

 From the fact that the air was packed full of these small 

 drops of water, it might have been assumed that the air 

 was saturated, and tests with properly- protected wet and 

 dry bulb thermometers showed that it was saturated. A 

 few observations were therefore made to explain this 

 apparent contradiction of surfaces remaining dry while 

 exposed to a continued shower of fine rain and sur- 

 rounded by saturated air. The explanation was found to 

 be, simply, radiant heat. Though the cloud may be so 

 dense, it is impossible to see the sun or even a pre- 

 ponderance of light in one direction to indicate its 

 position ; yet, as a good deal of light penetrates under 

 these conditions, it therefore seemed possible some heat 

 might do so also. A thermometer with black bulb in 

 vacuo showed that a considerable amount of heat pene- 

 trated the clouds under the conditions, as it rose 40° to 

 50° above the temperature of the air while the observa- 

 tions were being made. This radiant heat is absorbed 

 by all exposed surfaces and heats them, while they in 

 turn heat the air in contact with them, and the fine drops 

 of water are either evaporated in this hot layer of air or 

 after they come in contact with the heated surfaces. 

 Other observations made on Pilatus pointed to the same 

 conclusion. All large objects, such as seats, posts, &c., 

 were quite dry in cloud when there was any radiation ; 

 while small objects, such as pins, fine threads, &c., were 

 covered with beads of water. The large surfaces being 

 more heated by radiation than small ones, when sur- 

 rounded by air, these surfaces evaporate the drops falling 

 on them, while the small ones, being kept cool by the 

 passing air, are unable to keep themselves dry. 



The observations made with the fog-counter point to 

 the conclusion that the density or thickness of a cloud 

 depends more on the number of water particles than 

 on the number of dust particles in it. The number 

 of the dust particles in the clouds varied too much 

 and too quickly to enable any conclusion to be drawn 

 from observations made in clouds themselves. How- 

 ever, on comparing the thickness of a cloud on the 

 Rigi and a fog at low level, when the number of water- 

 drops was about the same, it is found that the fog, though 

 thicker, was not greatly so, although there were only a few 

 thousand dust-particles per c.c. in the cloud, while there 

 were about 50,000 in the fog. 



The observations with the fog-counter show that, 

 whenever a cloud is formed, it at once begins to rain, and 

 the small drops fall into the drier air underneath, where 

 they are evaporated, the distance to which they will fall 

 depending on their size and the dryness of the air. It is 

 thought that much of the dissolving of clouds is brought 

 about in this way. 



