270 LIGHT SCIENCE FOR LEISURE HOURS. 



would expect more certain information of the existence 

 of poles of cold, since so much more of the northern 

 hemisphere can be traversed in summer than in winter, 

 we have no satisfactory evidence. In fact, the irregular 

 curve marked in near the pole in Fig. 2 is the most 

 northerly isotheral yet determined. The temperature 

 corresponding to this isotheral is 36 Fahrenheit, or 

 four degrees above freezing. From a consideration of 

 the form-variations of the isotherals as they travel 

 northwards, I have been led to the opinion that there 

 exist three poles of summer cold, and that these fall 

 not very far from the positions indicated by the small 

 dark circles in Fig. 2. 



From the direction of the isotheral line through 

 London, it is evident that along the south-eastern coast 

 of England the heat of summer is greater than in any 

 other part of the British Isles. On the other hand, 

 the northern parts of Scotland, which we have seen 

 enjoy a winter climate fully as warm as that of London, 

 have a much cooler summer climate. The south- 

 western parts of Ireland exhibit a change even more 

 remarkable For whereas the winter climate in these 

 parts is the same as that of Persia, the summer 

 climate is the same as that of the very portion of 

 Siberia in which (most probably) the greatest cold 

 ever observable in our northern hemisphere is ex- 

 perienced in winter. The summer of the Orkney 

 Islands, again, is no warmer than that of the southern 

 parts of Iceland. 



