274 PHYSICAL FACTORS [CH. 



water, depth and velocity are important factors in determining 

 the extent of the variation ; the more rapid the current and the 

 shallower the stream, the greater is the daily range. 



Besides the changes from hour to hour, the different tem- 

 peratures, which occur simultaneously at different depths in 

 the same body of water, must be noted. The heat received by a 

 water surface is said to be absorbed almost completely (94 per 

 cent.) by the topmost millimetre of liquid, warmth being con- 

 veyed to lower layers by means of currents only 1 . This explains 

 a curious fact, to which attention is drawn by Guppy 2 . He 

 points out that, in a river about I o feet deep, the temperature 

 at the surface and bottom are much the same, but that ponds 

 and ditches differ from rivers in their liability to surface 

 heating; this becomes especially marked where the water is 

 crowded with plants, so that even the slight currents, that occur 

 in stagnant pools, are checked by the mass of vegetation. A 

 ditch full of plants, on a sultry afternoon, may exhibit a differ- 

 ence in temperature of 5 C. in nine inches, while a large pond, 

 4 or 5 feet deep, may be 6 C. to 7 C. warmer at the surface 

 than the bottom. The result is that, on sunny days, the tempera- 

 ture of the ponds in the neighbourhood of a river generally 

 stands some degrees above that of the river itself, and, in the 

 height of summer, the variation may be nearly 7 C. As 

 Guppy 3 remarks, "Everything in plant-life is behindhand in 

 a river in comparison with a pond." This difference may pos- 

 sibly explain certain apparent anomalies in the distribution of 

 aquatic plants in a single neighbourhood. 



Guppy's observations relate only to comparatively shallow 

 waters; in deep water the currents appear to be, as a rule, 

 unable to convey the daily heat of the sun to a greater depth 

 than about 10 metres. Beneath this level 1 the temperature 

 sinks, until, at about 100 metres, it becomes constant at 4 or 

 5 C. Temperature is undoubtedly one of the principal factors 



1 Magnin, A. (1893). 



2 Guppy, H. B. (1894!), (18948) and (1896). 



3 Guppy, H. B. (1896). 



