278 LIGHT SCIENCE FOR LEISURE HOURS. 



the Greenwich means are not the true average tem- 

 peratures. They were obtained by constructing a curve 

 from the true averages, and taking a curved line (the 

 curve of Fig. 3, in fact) in such a way as to take off 

 the most marked irregularities of the true curve of 

 averages ; or to use the words of the meteorologist who 

 constructed the Greenwich table of means, Mr. Glaisher, 

 a curved line was drawn which passed through or near 

 all the points determining the true curve of averages, 

 4 and in such a way that the area of the space above 

 the adopted line of mean temperature was equal to that 

 below the line.' Despite this process, the curve exhibits 

 no less than fourteen distinctly marked maxima of 

 elevation, and a much larger number of variations of 

 flexure. The sudden variations of temperature at the 

 beginning of February, early in April, and early in 

 May are very remarkable ; they have their counterparts 

 in the three variations which take place between the 

 latter part of November and the end of the year, only 

 these occur in much more rapid succession. The 

 nature of the curve between June and August is also 

 remarkable, as are the three convexities which are ex- 

 hibited in the September, October, and November 

 portions of the curve. 



If we follow our leading meteorologists in taking the 

 curve of Fig. 3 as representing the true annual climate 

 of London, how are we to assign physical causes for the 

 remarkable variations above indicated ? Not easily, I 

 take it. It were, indeed, as easy as inviting to specu- 

 late on cosmical causes ; to follow Ertel, for instance, 



