FROM SELECTED DAYS DURING THE SEVEN YEARS 1898 TO 1904. 
303 
to September, when climatic influences are comparatively uniform, it may be a real 
one. If so, P tends to be loiver than usual near sun-spot minimum. The difference 
shoAvn, however, in Table I. may be accidental; it is certainly too small to rely on, 
considering the limited number of years and the many sources of uncertainty. 
—--— Mean monthly values of potential gradient. 
. (Daily range) 4- (mean monthly value). 
-Sum 24 hourly differences potential gradient. 
-Sum 24 hourly differences barometric pressure. 
Non-cyclic Effect. 
§ 5. In meteorological elements such as temperature, where there is a conspicuous 
annual variation, there is, at certain seasons of the year, a steady tendency to an 
increase, and at other seasons to a decrease, during the 24 hours. 
