16 C. PIAZZI SMYTH ON 



there to find out by computations of another kind, what their instrumental 

 readings were equivalent to touching " Humidity ; " but on days then so long 

 passed by, as to have ceased to have any vivid interest for the observer. 



To bear on, or illustrate this proposition, all my graphic projections in 

 Appendix II. have been arranged so as to give, first, the raw depression of the 

 wet, below the drj r , bulb ; and second, the computed Humidity, taking account 

 of the Temperature at the time ; and it will be seen that there is no important 

 abnormal wave in the latter, that has not its crest, or hollow sufficiently, and 

 sometimes more strikingly, marked in the former. The former therefore, which 

 every observer can enter for himself at the time with a living interest in it, is 

 quite near enough to the scientific truth for all current weather discussions 

 whether for agriculturists or gardeners, sailors or country gentlemen. 



But in this particular inquiry of ours on the present occasion, we must do 

 something more. Wherefore Mr Heath, 1st Assistant Astronomer in the 

 Royal Observatory, has obligingly assisted me by both computing three forms 

 of the Hygrometric expression, and also projecting their curves, together with 

 that of the Temperature, for all the hourly observations of the three continuously 

 working Observatories alluded to. 



Now the projection of the Humidities does not diner much, as already indi- 

 cated, from the mere Depressions of the Wet^Bulb. Nor does the projection 

 of the Grains-weight of water in a cubic foot of air, except in the way of dull- 

 ing and flattening all the curves. But the map of the Grains-weight of water 

 still required to saturate each cubic foot, gives an immense result for our par- 

 ticular phenomenon of the night of April 8 ; as well as a notable insight into 

 some permanent characteristics of climate. 



These permanencies are, that in the North there is very little change 

 between clay and night either as to quantity of watery vapour in the air, or the 

 further amount required to saturate it. But in the South, teste the Royal 

 Observatory, Greenwich, there is an intensity of difference between day and 

 night, not in the amount of watery vapour contained in the air, but in the 

 further quantity which the air could take up without being saturated thereby, — 

 which is not only surprising but warning too ; as it is a condition that lies at 

 the root of many of the movements of the atmosphere, and promotes also the 

 disruptive, in place of the silent or slow, discharge of atmospheric electricity. 



Looking next to the particular phenomenon of April 8, we find it occurred 

 at 9 a.m. at Aberdeen ; and at 4 p.m. at Glasgow ; or crossed the country from 

 N. East to S. West at the rate of about 14 miles per hour, increasing in quan- 

 tity as it went ; for while it attained to only 0*80 grains at Aberdeen during 2 

 hours, it reached 180 grains at Glasgow and lasted there for nearly 8 hours. 

 That is, the increased number of grains of watery vapour 'per cubic foot, which 

 the air could take up without being saturated. 



