366 
Transactions of the Boijal Society of South Africa. 
GKOUP I.— GENEEAL PEESSUEE DECEEASING. 
SUB-GEOUP (A). 
If the triangle be examined it will at once be recognised that a con- 
dition like this means that a depression is advancing from the south-west, 
a typical winter one, and that if no other influence is at work the usual 
sequence of weather for such a depression will follow, and if the glass 
goes down low enough rain will come for certain. The influences which 
may upset the sequence are those brought about by a depression passing 
to the north of the triangle at the same time as the one from the south- 
west is affecting us. This can be seen by an examination of the actual 
differences of the barometrical pressure at the time ; thus, if the reading 
at L'Agulhas be the lowest of the three, there is no doubt about it being 
a winter type depression, uninfluenced in any way ; if, however, the 
reading at Clanwilliam be lowest, it is evident that there is a counter 
depression to the north which may drive away the rain and upset the 
prediction. Again, suppose the reading at the Eoyal Observatory be the 
lowest, then it is evident that there is either a summer type depression 
or a secondary to the north-west, and rain may not fall. 
DIVISION (1). 
Section (a). — During the 5 years, rain followed within 24 hours at the 
Eoyal Observatory on 79 occasions out of 110, while it did not on 31 days; 
on 61 days out of the 79 the wind was in the west, while out of the 
31 days it was westerly on 16 occasions. Again, with wind in the east 
rain followed on 13 days and dry weather on 12. We are, therefore, 
justified in considering this as a rain-bringing condition. Out of the 
31 cases only 3 were absolute failures as against the prediction for rain. 
In 16 cases rain fell, either at L'Agulhas or was delayed at the Eoyal 
Observatory or L'Agulhas until the following day. In 12 cases there 
would have been an element of doubt in issuing predictions, the con- 
ditions being such that we could only have stated that there was a 
probability of rain. In several of the successful 79 predictions a similar 
element of doubt existed, and we should have been justified in expressing 
such doubt and only predicting probable rainfall ; in most of the cases 
no doubt existed. 
Eeducing the foregoing to percentages we get — 110 days, total number. 
95 days, rain followed ; being 86 per cent. 
15 days, no rain followed ; being 14 per cent. 
Prediction. — Wind westerly ; almost certain rain. Conditions 
of doubt — Clanwilliam lower than the Eoyal Observatory — or pres- 
sure uniform at all stations. 
