276 LIGHT SCIENCE FOR LEISURE HOURS. 



In Fig. 3 the annual variations of mean diurnal 

 temperature are represented graphically. The figure 

 was formed in the following manner : A rectangle 

 having been drawn, each of the longer sides was 

 divided into 365 parts, and a series of parallel lines 

 joining every tenth of these divisions was pencilled in. 

 The spaces separating these lines represented successive 

 intervals of ten days throughout the year. The shorter 

 sides were divided into thirty-three parts and parallel 

 lines drawn, joining the points of division. Of these 

 longer parallels the lowest was taken to represent a 

 temperature of 32 Fahrenheit (i.e., the freezing point) 

 and the others, in order, successive degrees of heat up 

 to 65. Then, from the Greenwich tables, which have 

 been formed from the observations of forty-three years, 

 the temperature of each day was marked in, at its 

 proper level and at its proper distance from either end 

 of the rectangle. Thus 365 points were marked in, and 

 these being joined by a connected line, presented the 

 curve exhibited in Fig. 3. The lines bounding the 

 months, and the lines indicating 35, 40, &c., Fahren- 

 heit, were then inked in and the figure completed. 



The resulting curve is remarkable in many respects. 

 In the first place, it was to have been expected that a 

 curve representing the average of so many years of 

 observation would be uniform ; that is, would only 

 exhibit variations in its rate of rise and fall, not such 

 a multiplicity of alternations as are observed in Fig. 3, 

 And this irregularity will appear the more remarkable 

 when it is remembered that the temperatures used as 



