34 J. HOPKrXSOX HEETFOltDSHIEE RAINFALL 



the mean rainfall at all the stations in Hertfordshire for which 

 monthly returns have been published in the ' Transactions of the 

 Hertfordshire Natural History Society,' varying in number from 

 t"sventy-six to thirty-six, for the fifteen years 1877 to 1891, 

 adopting this period because the number previous to the year 1877 

 is considerably less than twenty-six, and I find that the returns 

 for the whole of the stations give a mean rainfall of 27 '55 inches 

 per annum for this period, while the returns for the ten stations 

 give a mean of 27 79 inches for the same period, showing an excess 

 of 0*24 inch, being a little less than one per cent. As a further 

 test I have computed the mean annual rainfall at twenty-one 

 stations in the county for the decade 1880 to 1889, this being 

 a period which admits of comparisons being made with tables 

 compiled by Mr. G. J. Symons, F.R.S., and with conclusions 

 arrived at by him, and I find the result to be almost precisely the 

 same, the mean at these stations for this period being 26' 84 inches 

 per annum, while the mean at the ten stations for the same period 

 is 27 '13 inches, showing an excess of 0'29 inch, being a little more 

 tlian one per cent. 



It may therefore be assumed that the mean values deduced from 

 the records of these ten rainfall stations should be reduced by 

 about one per cent, to give the true mean rainfall for Hertfordshire. 

 In order to avoid repetition I shall use the term "year" for 

 the twelve months ending 31st March in all cases except w^hen 

 " calendar year " is specified. The terms " average " and " mean " 

 are used synonymously to signify the average of all the values, 

 not the mean between the extremes. The terms "ratio" and 

 "percentage" are also used synonymously, the mean in the tables 

 being considered as 100 instead of as imity in order to avoid the 

 use of decimals in the ratio columns. All measurements of rain 

 are given in inches of depth, sometimes abbreviated "ins." 



In each rainfall table the column headed " Summer" gives the 

 rainfall from 1st April to 30th September, that headed " Winter" 

 the rainfall from 1st October to 31st March, and that headed 

 " year" the rainfall from 1st April to 31st March, all these tables 

 ending 31st March, 1892. The column headed " AVinter Ratio" 

 gives the percentage in each winter to the mean winter rainfall 

 of the period included in each table, the winter rainfall only being 

 thus treated because, as will hereafter be shown, upon it depends 

 almost entirely the amount of percolation into the Chalk, and 

 consequently the permanent or dry-weather flow of our rivers and 

 the amount of water available for our water-supply. The columns 

 headed "Mean of each three successive Winters" give the mean 

 rainfall and percentage to the mean for every three winters in 

 succession because the period of three years is the one adopted 

 by engineers in investigations on water-supply as that on which 

 an estimate of probable minimum supply should be based. 



Table I, showing the mean summer, winter, and annual rainfall 

 in Hertfordshire for the half -century ending 31st March, 1892, 

 has been compiled from Tables II to Xi in the following manner. 



