472 MR ROBERT COCKBURN MOSSMAN ON 



month before the longest day, while the minimum is registered about the time of the 

 winter solstice. The following diagram (see Plate) shows the excess or defect from the 

 annual mean for each day in the year. By " mean line" is meant the average excess of 

 34°*0, which is the mean of the 365 daily values. All the values exceeding 34° are placed 

 above the mean line, while those under it are less than the mean value. In order to obtain 

 an approximation to a smooth curve, and thus eliminate irregularities due to the compara- 

 tive shortness of the period discussed, the values have been treated by Bloxam's method 

 of taking the mean of continuous five-day groups. For example, the value for January 

 3, viz. 25° "4, is not the actual departure from the normal for that day, but the mean 

 of the five-day period January 1 to 5, that of the 4th of January the mean of the five- 

 day group January 2 to 6, and so on. The effect of this method is to reduce consider- 

 ably the saw-like character of the curve, and. thus place the facts in a clear light. To 

 obtain an approximate idea of the relations subsisting between solar radiation and the 

 other climatic elements, the above curve should be compared with Plates I. and II. 

 appended, to Part I. of " The Meteorology of Edinburgh " (Trans, vol. xxxviii., partiii.). 

 Although the daily means of solar radiation now discussed are for a period of but ten 

 years, yet they will be found to offer many resemblances to the curves of other elements 

 deduced from much longer periods of observation, which are given in my previous work. 

 The following are the more important characteristics of the curve. 



In January solar radiation falls to a minimum about the 3rd of the month, and there- 

 after increases in a somewhat irregular manner till March 23, when a well marked maxi- 

 mum is reaehed. There is a decided diminution in the values of black bulb excess dur- 

 ing the first ten days of April, which is associated with a fall of pressure and a maxi- 

 mum of easterly winds. About this time the day values of shade temperature exhibit 

 a slight fall, doubtless due to the cloudy weather which accompanies winds from the 

 east. Solar radiation increases rapidly after the 4th of April, and culminates in a maxi- 

 mum about the end of the month, when pressure is high and north-west winds in excess. 

 In May there is a general fall in the values until the 14th, the minimum being associated 

 with a high barometer which accompanies the annual maximum of easterly winds. 

 After the 15th solar radiation increases rapidly till the 23rd, when the excess of the black 

 bulb over the shade temperature is greater than at any other period of the year. 

 During the first four days of June there is a decided tendency for cloudy weather, and 

 a rapid diminution in solar radiation occurs. At this time east winds are frequent. 

 From June 9 to August 2 the curve is comparatively featureless ; maxima are, how- 

 ever, shown about June 21 and July 8 and 21, and minima about July 11 and 27. 

 The period in August characterised by a great excess of rainfall known as the "Lammas 

 floods " shows a marked diminution in the intensity of solar radiation, which falls to a 

 minimum about the 9th. The effect of this heavy precipitation is evidently to greatly 

 increase the transparency and diathermancy of the air, as a rapid rise in the black bulb 

 excess follows the copious rainfall, a maximum in the radiation values being shown 

 about the 16th. From August 29 to September 7 another wet period prevails, and the 



