176 REPORT— 1870. 



In order that tlie proportion which the altitudinal deficiency bears to the 

 total rainfall may be readily seen, the annual diagram represents, by light 

 shading, the actual fall at various heights above the ground ; the amount col- 

 lected by the "pit-gauge" is shown by the horizontal line of 1-000, and the 

 deficiency in the iipper gauges by the breadth of darker shading. 



In the monthly diagrams, the variable breadth of this darker shading shows 

 the variation of this altitude correction with the seasons. 



Probably the most I'emarkable features in the annual curve are : — (1) its 

 rapid curvature, ?'. e. rapid increase in the amount of rain collected within 

 1 foot of the surface of the ground ; (2) the very shght alteration from about 

 7 feet to 20 feet. 



This would seem to show that the rainfall actually reaching the ground is 

 greater than what has hitherto been supposed, inasmuch as the majority of 

 gauges hitherto erected have their orifice about 1 foot above the ground, and 

 therefore collect, according to the results now before lis, about 5 per cent. 

 Zsss than the real rainfall reaching the ground. This result ajjpears so start- 

 ling that further experiments will be conducted on the subject. 



The difference between the curves of decrease in the various months is 

 very considerable, and sufficiently proves that the annual mean values are 

 not at all applicable to single months. 



Tables A'lII., IX., and X. seem to show that the variation is in no way 

 dependent on the humidity of the air, but either on temperature or wind- 

 force, or possibly on radiation ; the present investigation does not reveal 

 which, inasmuch as the cold days, with rain, are usually windy ones. 



Our duty in connexion with this subject is discharged by Tables VI. and 

 VII., which give (for 8-inch gauges) the means of deducing the fall, either 

 at the surface of the ground or at 1 foot above it, from observations of 

 gauges on pedestals or i)illars at any height less than 20 feet. 



Perhaps it may be well to give an example. At Cockcrmouth there arc 

 two gauges, one G inches and the other 6 feet G inches above the ground ; in 

 the year 1SG9 they collected 4G-31 and 44-48 respectively: the corrections 

 from Table VII. are -9 GO and -920 ; then we have : — 



Gauge inches above ground=4G-31-^-9G9 = 47-79=ground fall. 

 „ 6 feet inches „ =44-48 -=--920 =48-34 = 



Or if we require the 1-foot fall, then the corrections from Table VI. arc 

 1014 and -900 ; then we have : — 



Gauge 6 inches above ground = 4G-31-^l-014=45-07=faU at 1 foot. 

 „ 6 feet 6 inches „ =44-48-r- -900 = 40-33= „ 



This example was taken quite by chance, and the agreement being in both 

 cases within 1 h per cent., seems to sliow that the corrections are generally 

 applicable. But on this point we have at present no evidence. 



The inquiry we have just been pursuing naturally leads to the considera- 

 tion of the best height ibr rain-gauges ; and, althougli a novel plan, we are 

 inclined to consider that, theoretically, the " pit-gango" is the best. Eut 

 there are several strong grounds of objection to it and to all gauges whose 

 orifices are nearly level with the soil. We may mention a few : — 



1. They become more or less filled with leaves during the autumn months, 

 and their accuracy is thereljy vitiated. 



2. They become buried under snow, and the melting snow runs unfairly 

 into them. 



