44 MR JOHN AITKEN ON DEW. 



following morning. During the whole of the 15th the air remained calm and 

 frosty, and the cold intensified during the night. On the morning of the 16th 

 the minimum thermometer indicated a temperature of 19°. On this occasion 

 we had a hoar or white frost. Grass, fences, shrubs &c, were all white, and 

 the trees even to their top branches. The air on this occasion had evidently got 

 cooled to near its dew-point, and moisture had condensed on almost every ex- 

 posed surface, causing nature to present a remarkable contrast to its appearance 

 on the previous morning, though both mornings were frosty. 



We shall now refer in more detail to some of the points most worth 

 noticing on these mornings. It was observed that the distribution of the hoar- 

 frost on the grass on the morning of the 16th was the reverse of what it was 

 the previous morning, the high blades on this occasion having rather a thicker 

 coating of hoar-frost than those lower down. The reason for this was that 

 the higher blades were exposed to the passing air, which on this occasion was 

 saturated; whereas those on the hollows had to depend for their supply on 

 what rose from the ground, which could not be much under the conditions, as 

 the bottom of the grass and the top of the soil were cooled below the freezing- 

 point, and most of the rising vapour would be trapped before it reached the 

 surface. We may also note here that the test surfaces on the soil were quite 

 dry, and that the slate and iron weight resting on the grass were free from 

 deposit ; while the elevated slate and weight, the grass, and almost everything 

 else had a coat of hoar-frost, showing that the ground kept hot enough to give 

 off moisture. An examination of the bare soil also showed that at most parts 

 of its surface vapour was being given off. Wherever the contact with the 

 ground underneath was good, no hoar-frost formed, and it was deposited only 

 on the small clods that were lying on the surface; of course, there was plenty 

 of hoar-frost on the under sides of the large clods, but none on their upper 

 surfaces. 



The slates and weights resting on the grass were frequently examined while 

 the frost lasted, which it did till the morning of the 19th. During all this time 

 the radiation was great, and the temperature very low. The minimum on 

 different nights was as low as 12 0, 5, 17°5, and 19°. During all that time, though 

 the ground received no heat direct from the sun, and but little in any way 

 from above, yet the supply from beneath was sufficient to keep the tempera- 

 ture of the surface above the dew-point, and the slate and weight in contact 

 with the ground remained black amidst the surrounding whiteness. 



Another peculiarity of mornings such as this — which arc of frequent occur- 

 rence during winter — is the deposition of moisture on trees. During my observa- 

 tions in summer I never saw shrubs dewed to a height of more than a few feet 

 from the ground, while in winter tall trees frequently have vapour deposited on 

 them to the top branches. But as my observations in warm weather are very 



