BRIGHT CLOUDS ON A DARK NIGHT SKY. 15 



But before using them in the whole, I extracted their 9 a.m. and 9 p.m. 

 observations, and projected them in the same manner as had been done with 

 the 24 Scottish Stations ; when there came out a most puzzling result ; — viz. 



At Glasgow, in place of the expected evening depression, there was posi- 

 tively an elevation. And 



At Aberdeen, in place of the small morning depression of 2 0, 9, there was 

 the immense one of 6°*7 ; the average one being only 2°3. 



On using, in place of the single observations of 9 a.m. and 9 p.m., the means 

 of the whole 12 hours of observations a.m. and p.m., — the anomalies of the 

 curves were largely removed. And at length on projecting every observation, 

 and obtaining a nearly continuous history of the wet-bulb depression through 

 the whole 24 hours of each of the 5 clays— the anomalies were all most 

 abundantly and exquisitely explained. 



An enormous depression, it was thus ascertained, had really occurred at 

 Glasgow on the evening of April 8, amounting to no less than 12°"8 ; but it was 

 of short duration, and comprised so completely within the interval from 9 a.m. 

 to 9 p.m., as not to have affected either of these observations. While at 

 Aberdeen, where the Depression had occurred earlier, and to the smaller 

 extent of 6°7 only, and lasted less than two hours, — its maximum had just 

 struck on the 9 a.m. observation and made that appear excessive. 



The Greenwich observations, very much as might have been expected from 

 their great distance, were not sensibly affected by our peculiar Scottish pheno- 

 menon of April 8 ; but are extremely instructive for study, and the proof they 

 give that 9 a.m. and 9 p.m. observations anywhere, are not sufficient for arriving 

 at a knowledge of all the laws of Nature in this department of Meteorology. 



Part II. 



Thus far I have described only the simple results of rude, instantaneous 

 observation, viz., the depression of the wet, below the dry, bulb thermometer as 

 actually seen by the observer. And though that quantity by itself does not 

 give the full or exact Hygrometric state of the atmosphere, it does show forth 

 to so large an extent any variations in the same, that I could wish our Scottish 

 bi-diurnal observers were instructed to enter that quantity, viz., the depression 

 of the wet, below the dry, bulb, as they might so easily do, at the time they 

 enter the latter. 



For then, having only one column necessary to look at for instant Hygro- 

 metry, they would be far sharper in appreciating changes therein, and the final 

 results their observations might lead to, — than when their present two columns 

 of mere thermometer figures rather confuse them at the time, and are relegated 

 to the Royal Observatory, Edinburgh, at the end of the month, for some one 



