318 MR JOHN AITKEN ON THE 



During this visit to Pilatus the top of the mountain was entirely surrounded by 

 cloud, and the air was thick with fog particles ; and though one might naturally conclude 

 from this that the air was saturated with moisture, yet wooden seats, walls, and all 

 exposed surfaces, were quite dry. In a previous communication, from observations 

 made on the Rigi, I have stated reasons for supposing that this dryness of exposed sur- 

 faces in a cloud does not necessarily prove that the air is not saturated. That is to say, if 

 the air is saturated, it docs not necessarily follow that all surfaces will be dripping with 

 the fog particles falling on, or .being driven against them. I therefore made on this 

 occasion as many observations of the conditions of the different surrounding objects as 

 possible while attending to the usual dust counting. The following facts were observed : 

 seats, wall tops, wooden posts, nails projecting from the posts, and stones on the ground, 

 were all quite dry. But thermometers hung up rapidly got wet, and the pins driven into 

 the wooden post for hanging the thermometers rapidly got covered with beads of water. 



It was of course natural to suspect radiation to be the cause of this difference. It is 

 true the mountain top was surrounded by a dense mass of clouds, and the sun could not be 

 seen, nor even a preponderance of light in one direction more than another to indicate its 

 position ; yet as light penetrated, it seemed possible that a perceptible amount of radiant 

 heat might do so also. A thermometer placed on a wooden seat showed that a considerable 

 amount of heat penetrated the cloud, as it rose to 60°, while one hung up registered 

 only 48°. As has been explained in a previous communication, bodies exposed to radiant 

 heat are heated in proportion to their size, the larger bodies being heated to a higher 

 temperature than the smaller ones. Now the effect of this radiant heat on objects 

 exposed in clouded air is to heat them above the temperature of the air, and if the 

 objects are of any size they are considerably heated, and become surrounded by a layer 

 of hot air, and the water particles are either evaporated in this hot layer before they 

 touch the surface, or they are evaporated after they have come in contact with it. 

 This is the reason why the seats, walls, posts, &c, were dry, though surrounded by 

 saturated cloudy air. These large bodies received so much heat by radiation that they 

 were able to evaporate the water particles falling on them, but small bodies, such as 

 thermometers, not being heated to the same degree, on account of the passing air 

 taking away more heat from them, they did not keep themselves surrounded with 

 a layer of hot air, and the cloud particles fell on and wet them. It is true that nails, 

 which were smaller than the thermometer, were quite dry. But they were driven 

 into wooden posts which were hot, and from this supply the nails drew enough 

 heat to keep them dry. The pins for hanging the thermometers did not do this, 

 though driven into the wood, on account of their small cross section compared 

 with their length not being sufficient to conduct the necessary amount of heat. 

 These few observations of different objects exposed in a cloud, showed that the air 

 was saturated, though most of the exposed surfaces were quite dry, and that if it had 

 not been for the radiant heat everything would have been dripping wet. This 

 conclusion, written out last year from observations made on Pilatus without 



