110 JOUENAL OF THE ROYAL HORTICULTXJEAL SOCIETY. 



of 42°, then we may suppose an imaginary case in which the temperature 

 from midnight to noon remained continuously at 40° ; at noon a sudden 

 rise occurs to 44°, and from noon till midnight the temperature remains 

 continuously at that point. We have therefore for twelve hours a 

 temperature 2° below the base, and for the second twelve hours a tem- 

 perature 2° above it. Now it is clear that 2° continued for twelve hours 

 is equivalent to 1° continued for twenty-four hours, and so the conditions 

 represented in this case would be expressed by saying that there was one 

 day-degree below and one day-degree above the base. 



If, we had contented ourselves with the ordinary method we should 

 have learned merely that the mean temperature was 42", and that result 

 would have given us no information whatever as to whether the tem- 

 perature had varied at all from that value throughout the day. 



If we take another case in which the supposed march of temperature 

 for the day, as shown in fig. 22, represents more nearly what might really 



Mdi 



Fig. 22. 



be expected to occur, we get a similar result. Here, again, the amount of 

 temperature above the base is balanced by a corresponding amount below, 

 and the mean is again 42°, although it is clear that for some time there 

 would have been experienced a considerable amount of effective temperature 

 as regards plant life. 



General Strachey's formula enables one to calculate these day-degree 

 values above and below 42° from the readings of the maximum and 

 minimum thermometers, and such values for a large number of places 

 throughout the British Isles — Wisley being one of them — are published 

 every week in the Weekly Weather Report published by the Government 

 Meteorological Office. 



Turning now again to Gilbert's results, he got, of course, a smaller 

 aggregate of accumulated temperature according as he moved the starting- 

 point of his calculations further into the year, and he got different results 

 also according to whether he used monthly means or daily means for 

 Greenwich ; and both these results were unlike what he got when he 

 employed the accumulated temperatures averaged for the eastern and 

 midland counties of England. 



Briefly, starting from January 1 he got by using the monthly 

 Greenwich means for twenty-seven years an average of 2,023° Fahr. 

 for the aggregate amount of temperature required to ripen wheat ; the daily 

 means for six years gave him the rather higher average of 2,189° Fahr. ; 

 and by using the Meteorological Office day-degree values for the east and 

 centre of England combined he got an average of only 1,904° Fahr. 



These values differ somewhat from those got by some other physicists 



