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 attracting 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 3½ 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 24½ inches. (The mean of these is 26¼ 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 Hertfordshire 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 existing. At all these stations observations have been taken in a uniform manner with verified instruments similarly placed except as to the exposure of the thermometers for ascertaining the temperature of the air in the shade. At Bennington, St. Albans, and Berkhamsted the thermometers 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 stations34
