150 J. V. ELSDEN — AGRICULTURAL 



Although the excellent reports, published in our * Transactions,' 

 make it quite unnecessary for me to dwell upon this point, yet I 

 have thought it advisable to illustrate this law of the distribution 

 of rainfall by a brief summary of the rainfall of the above ten places, 

 which are arranged according to their longitudes, from west to east. 



It must not be forgotten, however, that the replenishment of our 

 springs and rivers depends rather upon percolation than upon 

 actual rainfall. Experiments with the percolation-gauge show that 

 there are extremely great variations in the amount of percolation 

 during different seasons of the year, and that in summer it is almost 

 nil.'^ Observations kept for nearly 30 years, by means of a Dalton's 

 gauge, at Apsley Mills, Hemel Hempstead, showed the average 

 annual percolation to be not more than 7"5 inches,f proving that 

 a large proportion of the rainfall is evaporated again into the air. 

 The water supplied by our springs, therefore, is derived chiefly 

 from winter rains, and their replenishment may fall short if the 

 rainfall should happen to be abnormally distributed through the 

 different seasons. Thus a very wet summer and dry winter would 

 result in a diminished supply to our springs and wells, even although 

 the total rainfall were greater than usual. J Percolation is also 

 largely influenced by the nature of the soil, upon which indeed 

 nearly all the local peculiarities of our climate ultimately depend. 



Although the somewhat heavy clay soils, which cover so large 

 an area of our county, may tend to render our climate colder and 

 wetter than would be expected in a district situated to such a large 

 extent upon the Chalk formation, yet the porous nature of the under- 

 lying rock materially assists drainage, and the superficial deposits 

 themselves in many places consist of permeable sands and gravels, 

 which contribute much to the warmth and dryness of the atmosphere. 

 To the nature of the soil, in fact, we must ascribe that salubrity for 

 which Hertfordshire has long enjoyed a reputation. Hence we 

 see that the rainfall, temperature, and physical features of Hertford- 

 shire are in accordance with its main geological character. 



III. Geological Relations and Classification of the Chief Soils. 



To the agriculturist it is of even greater importance to know the 

 nature of the superficial deposits than to understand the geological 

 structure of the rocks below ; for, where drift deposits are present 

 to any extent, the agricultural features will be completely changed. 



Soils, in fact, can be divided into two classes according to their 

 mode of origin, viz., soils of disintegration and soils of transport. 

 The character of the former depends solely upon the nature of the 

 underlying rock, from which they have been derived ; whereas the 

 latter, including drift and alluvial soils, have been carried from a 

 distance, and are in general totally distinct in character and com- 

 position from the strata upon which they lie. 



* ' Proc. Inst. Civ. En<?.,' vol. xlv, pt. iii, p. 19. 

 t ' Jourual of the Royal Aj2;ricultiiral Society,' vol. xxv, pt. ii, p. 304. 

 X For a complete account of the rainfall in Hertfordshire see 'Traus. Herts. 

 >'at. Hist. Soc.,' Vol. I, p. 151. 



