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A. General considération of the physical feaiureû 

 of the pond and of the mèteorological data. 



Barton's pond ,1) is situated at one end of Harpenden Com- 

 mon and lies beside the main road leading from Harpenden lo 

 S'-Albans (i >. Il is situated in the stiff glacial clay (yellow clay 

 with flints), which overlies the chalk in Hertfordshire. It is of 

 small size, the maximum length being about 105 feet wiLh a 

 width of abont 39 feet at the widest part; the greatest depth (in 

 the central parts) does not normally exceed about 3 feet. 

 Roughly rectangular in shape, it is placed sothat the two ends 

 lie approximately north-east and south-west. Thèse two ends of 

 the pond are shallow and each is in connexion with a ditch, 

 whilst along the two sides there is higher ground. The north- 

 east end is particularly shallow, becoming dry early in the sum- 

 mer and forming a home for Alopecurus geniculatus. The 

 two long sides of the pond are bordered with rushes, which are 

 specially noticeable on the north-west side, where Juncusinfle- 

 xus, L., and /. congloméra tus, L.,form a close fringe and not 

 only cast shade on that side, but form important habitats for 

 many of tlie Algae, thèse being attached to the submerged fruit- 

 ing heads and stems of the previous year. On the south-east 

 side the pond is bordered by a steep bank. 



There are fréquent and prominent changes of water-level, 

 which of course stand in relation to the rainfall and tempéra- 

 ture, in summer the pond not uncommonly dries up completely 

 for a short time, the clay for the greater part being baked quite 

 hard by the sun. Such a drying up of the pond occurred in a 

 more or less marked manner in August and September, 1900; 

 September, 1907 ; August, 1908; but not in 1909. In winter the 

 water is generally frozen over for a short period, as occurred 

 in January and February, 1907; at the end of December 1907 

 and early February of 1908; January, the second hall" of 

 February, extending also in to early Mardi of 1909; and at the 

 end of Xovember, 1909. The water is sometimes muddy, at 

 other times clear, there being no apparent regularity about tins. 



(1) We are indebted to Mr. Salisbury l'or this account. 



(2) This position beside the road way is not without signiticance, since not 

 uncommonly traction engines appear to replenisli their supply of water from 

 theponil. This nu, lead to a more or less serious interférence with the normal 

 conditions. 



