230 Dr. A. Woeikof s Examination of 



former month, while it is known that at most places on the 

 equator the warmest and coldest months do not differ even by 

 3° F., and nowhere so much as by 5° F. 



Dr. Croll has answered the Rev. 0. Fisher*, and this 

 answer is a curious illustration of the difficulties in which he 

 has involved himself. The question as to why the annual 

 range on the equator is so small is a very simple one. The 

 quantity of heat received from the sun varies but in the 

 ratio of 100 : 115 ; and besides, the observations on which 

 our knowledge of the temperature of the equator depends 

 were mostly made on the sea-coast — two good reasons indeed 

 for a small annual range. Under the 50° N., the quantities 

 received on the days of the winter and summer solstices are 

 in the ratio of 100 : 562 ; and yet in some places in England 

 the yearly range is not above 20° F., and nowhere above 

 27° F. It is thus the small annual range in many regions of 

 the middle and high latitudes which much more needs expla- 

 nation. Dr. Croll, in his reply, expresses the opinion that the 

 northern hemisphere is the dominant one ; and as the whole 

 earth has a higher temperature in July than in January, so by 

 the operation of this cause the normal excess of the tempera- 

 ture of the equator in January is weakened and even abolished. 

 I replied to this, that the temperature on the equator was almost 

 entirely influenced by cold wdnds from cold ocean-currents 

 in some regions (the west coasts of Africa and America 

 and the adjoining parts of the Pacific and Atlantic), and in 

 these the equator was considerably colder in July than in 

 January, for example at the island of St. Thome, W. Africa, 

 by 2 0, 7. These cold winds come from the south, while winds 

 from the north seldom reach the equator, and can never have 

 a depressing influence on the temperature. In most places 

 the temperatures on the equator were influenced by the rainy 

 season; so that when it was at its height in January, this 

 month w r as cooler than July. This is the case even in Batavia, 

 7° S.; while when January is a dry month and July rainy, 

 the former is warmer not only on the equator, but even to 

 some degrees north of it (so, for example, it is warmer by 

 7°-2 at Lado, Upper Nile, 5° N., and by 3°'4 at Freetown, 

 Sierra Leone, 8J° N.)|. 



All this is, I think, conclusive enough, and proves that 

 Dr. CrolVs system of estimating temperatures breaks down when 

 tested seriously. Small errors would be quite natural in a 



* Nature, vol. xxi. p. 129. Eeprinted in ' Climate and Cosmology/ 

 chap, iv., omitting some passages relating to the Rev. O. Fisher. 



f Nature, vol. xxi. p. 249 ; reprinted also in the 'American Journal of 

 Science/ 1880. 



