SCIENCE-GOSSIP. 



179 



down the stream. What Max von dem Borne (" Das 

 Wasser," Neudamm, 1887) has done for the fish of 

 the spring-brook may be accomplished for the 

 plants, and, I may add, for aquatic insects and 

 crustaceans. At all events, springs are localities 

 of peculiar interest to the student of plant-distri- 

 bution. Montia fontanel, thriving on their oozy 

 edges, and Callitriche aquatica, in their waters, have 

 roamed the world ; and except for the accident of 

 their seeds being inclosed in mud drying on a bird 

 it is not very apparent how they have travelled 

 about. Now a spring would be a refuge for aquatic 

 plants and animals in times of drought ; and where 

 rising from considerable depths, far beyond the 

 reach of the annual wave of temperature, they 

 might remain for ages a sanctuary for the survivors 

 of a flora and fauna that, through climatic changes, 

 have long since passed away, and they would form 

 a centre of dispersal for the old amongst the new. 

 The aquatic insects and crustaceans (Nepa cinerea, 

 Gammarus pulex, Asellus aquaticus) found in mid- 

 winter by Erman ("Travels in Siberia") in the 

 valley of the Yenisei, in a spring covered by two 

 feet of snow, the temperature of the water being 

 thirty-nine degrees and that of the air two degrees 

 below zero, would have a marked advantage in the 

 race northward with a warmer climate at the pole. 

 " Thermal " springs, also, would become centres of 

 dispersal during climatic changes, especially in the 

 case of the plants flourishing in the warm ground 

 near the outlets. It is not pushing this idea too 

 far to suppose that " the watercresses, orchids, wild 

 buttercups, and other meadow flowers" that bloom 

 around the hot springs in the island of Unartok, 

 off the coast of Greenland (Koldewey's " Arctic 

 Expedition"), would be amongst the first new 

 plants to occupy the mainland with the disappear- 

 ance of its arctic climate. 



The brook, the stream, and the river of the open 

 land, beyond the influence of springs, and therefore 

 possessing an independent temperature, now claim 

 our notice. Taking the range of the air in the 

 shade during an ordinary summer day and night 

 at twenty degrees ; a brook two to three feet across 

 and three or four inches deep, would vary in the 

 twenty-four hours about fifteen degrees ; a stream 

 some twelve feet in width and eight or nine inches 

 deep, about ten degrees, and a small river forty to 

 fifty feet wide and two to three feet deep, between 

 three and five degrees ; whilst a larger river, like 

 the Thames, at Kingston, two hundred and fifty 

 feet broad and ten feet deep, would fluctuate only 

 one and a half degrees. Irrespective of size, 

 however, depth and velocity are important agencies 

 in determining the extent of the daily range; and, 

 in fact, the daily fluctuations of a river's temperature 

 will vary in different parts of its course, though the 

 localities may be in the case of a small river only a 

 few hundred yards apart ; the more rapid the current 



and the shallower the stream, the greater is the 

 daily range. It is remarkable that with rivers 

 larger than the Thames, the daily fluctuation has 

 usually but little relation to the size. This matter 

 has yet to be fully investigated, and those desirous 

 of more details may be referred to a recent paper 

 of the writer (Proc. Roy. Phys. Soc, Edin. xii.) 

 and to Dr. Forster's lately-published memoir. 



Amongst the features of river-temperature most 

 significant to the inquirer into the conditions of 

 existence of aquaticplants, is its uniformityin depth. 

 The temperatures at the surface and at the bottom 

 are much the same. This has been established by 

 Merian in the case of the Rhine, at Basle (Geogr. 

 Abhandl, Wien, v.), by Renou, in that of the Loir, 

 at Vendome (Compt. Rend. Acad. Sc. xxxiv.), by 

 Feil, in that of the Speyer (Geogr. Abhandl. Wien, 

 v.), by Marr, in the instance of the Mississippi, at 

 Memphis (Astron. Obs. Nat. Observatory, Wash- 

 ington, iii.), and by the writer in that of the Thames, 

 at Kingston (Proc. Roy. Phys. Soc, Edin., xii.). 

 The observations of Kahl in the Elbe, at Dresden 

 (Geogr. Abhandl. Wien, v.), and of Borius, in the 

 Senegal (Ann. Soc. Met. de France, xxv.) also 

 afford proof of the same fact. Whilst an impetuous 

 river like the Speyer exhibits no surface-heating, 

 and a swift river like the Rhine, at Basle, only a 

 small fraction of a degree, a sluggish river like the 

 Thames, even on a sweltering summer day, has its 

 surface-inch heated only between one and one-and- 

 a-half degrees. Very sluggish rivers, however, 

 such as those which, owing to the intervention of 

 weirs and locks, have an almost imperceptible 

 current, and may almost be compared to canals, 

 show a more marked difference between the top 

 and the bottom temperatures, and begin, in fact, to 

 display the thermal regime of a pond or lake 

 Thus the writer found in the case of the Mole, at 

 Hersham, a tributary of the Thames, when the 

 current was almost nil, a difference of about three 

 degrees. With this limitation we may accept the 

 principle in the terms stated by Dr. Forster 

 (Geogr. Abhandl. Wien, v.) : " The tempera- 

 ture of flowing water in different depths is the 

 same." 



Streams and rivers issuing from ponds and lakes, 

 and carrying off their heated surface-waters exhibit 

 at first their thermal regime. The effluent of an 

 ordinary pond will possess during fine summer 

 weather in the first part of its course a daily range 

 of some six degrees. This range rapidly increases 

 with the distance run, being about twelve degrees a 

 hundred yards away, and some sixteen or seventeen 

 degrees 300 yards from the pond, where the daily 

 fluctuations are those of a typical stream. When 

 the pond is frozen over, the effluent carries away 

 the water from under the ice at a temperature of 

 thirty-five or thirty-six degrees, and it will flow 

 some distance, according to the severity of the 



