BRIGHT CLOUDS ON A DARK NIGHT SKY. 13 



possible reasons suggested themselves. One was, that the clouds were low, and 

 therefore nearer the lights that illuminated them. The other, that they affected 

 on that night some peculiar physical structure which enabled them to reflect 

 with far more than their wonted degree of brilliance. 



The first of these two reasons, however, did not seem very important; for 

 though cumulus clouds are never very high in the atmosphere, say about 3000 

 feet, rain-clouds are often lower and show no such light ; indeed they are 

 usually the blackest of night-clouds. This very circumstance therefore led to 

 expecting that the bright appearance of April 8, might be due to just the 

 opposite condition of the atmosphere, which moreover did then to some extent 

 prevail, in the shape of dry, Scottish spring weather and its undesirable " clouds 

 without water." That is without any to fall as rain, though there must have 

 been some retained to form the visible mass of the cloud and reflect light with 

 extra force. 



Such an idea may bring into play our reason No. 2 ; but before launching 

 on that hypothesis, it will be well to ascertain whether there was anything 

 extraordinary in the dryness, or indeed in any of the other meteorological con- 

 ditions of that one night, contrasting strongly with those of the nights imme- 

 diately before and after. 



To this end I have been kindly permitted by the Secretary of the Scottish 

 Meteorological Society to make use of the Schedules of that hard-working body. 

 And after having selected 24 of them on account of either their topographical 

 neighbourhood or high character, I have extracted their bi-diurnal observations 

 from April 6 to April 10, or for two days before, as well as two days after, the 

 phenomenal night of April 8, as will be seen in Appendix I., and have graphi- 

 cally represented their Hygrometry in Appendix II. 



One and one only of the 24 observers says there was Aurora that night ; 

 and I have requested the Secretary to communicate with him, and ascertain 

 exactly what he saw and how he knew it to be Aurora.* But otherwise there 

 was nothing remarkable in any of the returns except the Hygrometric. The 

 Barometer was high, about 30*3 inches; the Thermometer low, about 41°0; the 

 mean Depression of the Wet-bulb about 2°*5, and the Wind generally from the 

 East. 



Now so far as Barometer, Thermometer, and wind are concerned the 

 observers were, probably all of them, perfectly competent. But in the more 

 delicate and sensitive matter of the Depression of the Wet-bulb by evaporation, 

 — I have found it needful to reject three of them for reasons wherein I trust to 

 be supported. Thus in a certain case which I will only allude to as No. 14, and 

 wherein that usually most varying quantity (Depression of Wet-bulb), is made 

 to read exactly 1° degree both morning and evening and day after day through 



* See Appendix IV. 



