INTRODUCTION. 



added to the 28th of February. The following instructions for calcula- 

 ting the mean and the accumulated temperatures are taken from the 

 Weekly Weather Reports of the Royal Meteorological Society : 



Daily mean temperature. This is not the simple arithmetical mean 

 between the daily maximum and minimum in the shade, but is calculated 

 from these by the following method : " The approximate mean daily 

 temperature is found by multiplying the difference between the observed 

 maximum and minimum by the proper co-efficient, shown in the following 

 table, and adding the product to the observed minimum.^ 



Accumulated temperature. This is designed to give persons engaged 

 in agriculture and horticulture better means of estimating the manner in 

 which vegetation is affected by temperature than that afforded by the 

 usual readings of the thermometer. It shows for the day, and for the 

 whole period from the ist of January, the daily and progressive values 

 respectively of the combined amount and duration of the excess or defect 

 of the air temperature above the base temperature of 42° F., the tem- 

 perature at which vegetation is said to be stationary, and above which 

 growth and ripening of crops &c. is effected in European climate. 



The accumulated temperature is expressed in Day-degrees j a day- 

 degree signifying 1° F. of excess or defect of temperature above or below 

 42° F. continued for twenty-four hours, or any other number of degrees 

 for an inversely proportional number of hours. It has been ascertained 

 by calculation from a considerable series of hourly observations at various 

 places, that the accumulated temperature may be computed, with a very 

 tolerable approximation to the truth, from the observed daily maximum 

 and minimum temperatures alone. ' When the temperature during any 

 period remains either wholly above or below the base temperature, the 

 difference between the base and the mean temperature gives the correct 

 accumulated temperature. In other cases this difference gives a value 

 which does not depart greatly from the truth, the deviation depending on 

 the greater or less extent of the variations of the temperature above or 

 below the base. Further, since the mean between the maximum and the 

 minimum of any day is nearly equal to the mean temperature of the day, 

 the difference of the mean of the maximum and minimum from the base 

 (42°) will give directly a fair approximation to the accumulated tempera- 

 ture for the day. 



The following rules will supply a close approximation to the true values 

 sought, and have been adopted in the preparation of the table in the 

 Weekly Weather Report. They may be applied to any other base tern- 



