The Climate of Great Britain. 125 



countries in the northern hemisphere should exhibit similar 

 irregularities on corresponding dates, which does not appear to 

 be the case. In fact, if there really exist effects due to cos- 

 mical causes, these are not likely to be educed from observa- 

 tions of the variation of mean diurnal temperature, since it is 

 clear that a cause of variation due to objects external to the 

 earth could affect only the temperature of certain hours of one 

 day, or of several days. A cluster of meteors between the 

 earth and the sun might diminish the mid-day heat ; one exter- 

 nal to the earth's orbit might increase the nocturnal tempe- 

 rature ; and though in either case the mean diurnal tempera- 

 ture would be affected, yet it is obvious that the effect would 

 be masked in taking the mean, or even that two or more oppos- 

 ing influences might cancel each other. If it could be shown 

 that the curve for mid-day, or for midnight heat corresponded 

 to the curve of mean heat, ErtePs theory would be overthrown 

 at once ; since, for its support it is necessary to show that 

 depressions in the mean curve are due to mid-day loss of heat, 

 and elevations to midnight gain of heat. 



There are, however, terrestrial causes to which the irregu- 

 larities of our curve (which irregularities, be it remembered, 

 represent regularly recurring irregularities of heat) may be 

 ascribed. For instance, there can be no doubt that our climate 

 is considerably affected by the changes which take place in 

 the Polar seas ; and it may not unfairly be assumed that the 

 processes by which different regions of Polar ice are succes- 

 sively set adrift (to be carried southward by the strong under- 

 current known to exist in the northern Atlantic Ocean), take 

 place at epochs which recur with tolerable regularity. And it 

 may be that the irregularity of the rising, as compared with 

 the falling half of the heat-curve is due to this cause ; since the 

 breaking-up of ice-fields, and their rapid transport southwards 

 would clearly produce sudden changes, having no counterpart 

 in the effects due to the gradual process of freezing.* 



It may well be, however, that the observations of forty- 

 three years are not sufficient to afford the true mean diurnal 

 temperature for a climate so variable as ours. Indeed, if the 

 curves given by Kaemtz for continental climates be as accu- 

 rately indicative of observed changes, as that of Fig. 3, we 

 must either accept such an hypothesis, or else assume that the 

 English climate is marked by regularly recurring variations 

 altogether wanting in continental climates; and it is to be 

 noted that the regular recurrence of changes is a peculiarity 

 wholly distinct from variability of climate, properly so termed, 

 and seems, even, inconsistent with such a characteristic. It 



* Icebergs have been seen travelling southwards against a strong northward 

 surface-current, and even forcing their way through field-ice in so travelling. 



