A HISTORY OF HERTFORDSHIRE 



paper-mills in the valleys, and by the refuse carried into the rivers their 

 water is frequently rendered turbid and their fish have sometimes been 

 killed. This does not occur however to any great degree, and no better 

 gauge of the general purity of the rivers of Hertfordshire can be found 

 than the extent to which watercresses are cultivated. For them to 

 thrive the water must be clear, it must flow from gravel or chalk, and 

 there must be a constant gentle stream. 



The chief elements of climate are rainfall and temperature. The 

 mean temperature of a district of small area compared with that of 

 the country in which it is situated is chiefly governed by its lateral or 

 geographical position, the mean rainfall by its vertical or orographical 

 position, both elements being greatly influenced by aspect. Thus a 

 slope facing south will generally be warmer and have a greater range of 

 temperature than a northern slope, and a slope facing west or south-west 

 will generally have a greater rainfall than an eastern or north-eastern 

 slope. The general south-eastern inclination of the surface of the county 

 is perhaps too slight to affect the temperature, but the rainfall is greatly 

 affected by the form of the ground, the southern and western hills at- 

 tracting the rain, which chiefly comes from the south-west, so greatly 

 that with a mean annual rainfall for the whole of the county of about 

 26 inches, there is a difference of 3! inches between the rainfall of the 

 river-basin of the Colne on the west and that of the river-basin of the 

 Lea on the east, the former having 28 inches and the latter 24! inches. 

 (The mean of these is 26^ inches but that is reduced to 26 inches by 

 the disparity of area, the Lea basin being much larger than the Colne.) 



There have been published annually in the Transactions of the Hert- 

 fordshire Natural History Society some of the results of observations taken 

 at five meteorological stations during the twelve years 1887 to 1898. 

 It is believed that this period is a sufficiently long one, and that the 

 stations are sufficiently numerous and widely-distributed, for the results 

 of the observations to be of value in enabling us to arrive at a knowledge 

 of the chief elements of the climate of the county except the rainfall. 

 To deduce the average rainfall over an area of 630 square miles, and to 

 form an idea of the extremes, five stations are inadequate, and the 

 period of twelve years is much too short. Although therefore the 

 rainfall is tabulated from these observations, additional tables will be 

 given showing certain features of the rainfall at a much larger number of 

 stations for periods varying from thirty to sixty years. 



The five meteorological stations are Royston, Berkhamsted, St. 

 Albans, Bennington, and New Barnet, the first of these no longer exist- 

 ing. At all these stations observations have been taken in a uniform 

 manner with verified instruments similarly placed except as to the ex- 

 posure of the thermometers for ascertaining the temperature of the air 

 in the shade. At Bennington, St. Albans, and Berkhamsted the ther- 

 mometers are in ' Stevenson ' louvre-boarded screens in accordance with 

 the regulations of the Royal Meteorological Society ; at Royston and 

 New Barnet under ' Glaisher ' open screens as at most of the stations 



34 



