270 Scientific Proceedings^ Roijal Dublin Socieiij. 



that a considerable number of the " clear " skies were not exactly bright iu 

 the sense in wliioh a Kimberley sky can be bright. 



But it is to be remembered that dew takes time to condense, and also 

 time to evaporate. And it might therefore be that although there was 

 dew with much cloud at any hour, there might have been little or no cloud 

 earlier, and hence that the dew might still have been formed under a clear 

 sky, and not have had time to evaporate. There is, of course, the converse 

 question as to whether when dew has been observed under a clear sky there 

 might, nevertlieless, have been clouds earlier. For the purpose of considering 

 this point, Table III has been constructed. It gives the number of times dew 

 was observed at 11 p.m. compared with the state of the sky at both 8 p.m. 

 and 11 p.m. Here we have the noteworthy fact that there is as much dew 

 at 11 p.m. wlien there are clouds at both 8 p.m. and 11 p.m. as when the sky 

 is clear throughout. Also, a clear sky following cloud is more favourable 

 than a cloudy sky following clear. These results, however, are irrespective 

 of the quantity of cloud in the sky. The last column takes quantity into 

 account by considering only niglits when the amount of cloud exceeds 30 per 

 cent, of the optimum. This shows that only in the winter half of the year 

 is a clear evening more likely than a cloudy to be signalized by dew. 



So far as the Kimberley observations go, the type of cloud seems to be 

 of no great importance in determining dew-conditions. Normally cirriform 

 clouds prevail between sunrise and noon, cumulus during the hottest hours 

 of the day, and stratiform clouds after sunset. Wlien the air is damp, and 

 moisture is condensed, we find pretty much the same rule, i.e., a good lot of 

 cirrus on dewy mornings, and a good lot of stratus on dewy evenings. 



The following table sliows the distribution of cloud on the dewy 

 mornings and evenings of the four years in number of times seen : — 



High-level clouds, 

 . Low-level clouds. 



In this table the high-level clouds consist of tlie cirrus and its 

 modifications ; the low-level clouds are all others, such as cumulus, strato- 

 cumulus, &c. 



It would seem that Hersehel and his successors have gone astray in their 

 interpretation of the phenomenon of dew, first, by trusting to reason rather 

 than to observation, and, second, from a misunderstanding of the conditions 

 governing the nocturnal radiation of heat from the Earth's surface. It is 

 perfectly true that if all the clear nights of the year are compared with all 

 the cloudy nights, it will be found that radiation is on an average more 



