CHARACTEEISTICS AND DISTKIBUTION OF LAKES 571 
Lake " in the south by a strait, is shallow, though the strait itself is 
fairly deep. A considerable portion of the floor of Lake Titicaca lies 
at a lower level than that of Lake Poopo, the very shallow lake into 
which its waters overflow. The water of Lake Titicaca is compara- 
tively fresh, containing only O'l 1 per cent, of solid matter in solution. 
Observations of much interest were obtained regarding the 
temperature of the water in early winter, though the stay was not 
long enough to throw light on seasonal variations. The surface 
temperature was found to rise as a rule until 3 p.m., and then to fall 
again ; but the greatest variation observed in a single day was 2° '8 
Fahr., and this at a part of the lake where the depth varied greatly. 
The lowest surface temperatures observed naturally occurred in the 
shallow bays.^ The figures relating to the bottom temperature are 
striking from their great regularity, in spite of great differences of 
depth. The extreme range was only 3°'6 Fahr. (from 48°-9 at 10 feet 
to 52° -5 at 60 feet). Below 240 metres (787 feet) the temperature 
was constant at 50° '8 Fahr. (lO'''^ C.) A series of vertical tempera- 
tures near the centre of the lake showed a difference of only 1°*7 
between the surface and the bottom. A. Agassiz, in his hydrographic 
sketch of Lake Titicaca, says the usual difference between the surface 
and the bottom, even at the greatest depth (154 fathoms), was not 
more than from three to four degrees. The lowest temperature at the 
bottom (in 450 feet) was 51° Fahr. (10°-6 C), the general tempera- 
ture varying from 54"" to 55^ ; while the surface temperature ranged 
from 53° to 59° — the greater part of the time 56° to 57° (February 
1875), the temperature of the air at noon varying from 49° to 97° Fahr. 
Observations were also made on the transparency of the water, 
and on the climatic conditions, the flora and fauna, etc., of the 
surrounding country. The level of the lake rises during the summer, 
the amount being given as 5 inches ; but apart from seasonal variation, 
the level is sinking progressively. 
Lake Poopo (or Aullagas)hes about 387 feet below Lake Titicaca. 
It is irregularly oval in shape, and contains a central island (which is 
inhabited), as well as two islets near the western shore. The River 
Desaguadero enters at the north end, and has formed a delta. The 
lake is 55 miles in length by 25 miles in breadth, but is very shallow, 
two small areas in the centre alone having a depth exceeding 6 feet 
(maximum depth J 3 feet). The water is saline and muddy and the 
lake is in process of disappearing, so that at no distant date it may 
1 "W ard (Science, vol. vii. p. 28, 1898) gives two series of temperatures taken 
at the surface of Lake Titicaca on 26tli and 28th November 1897. 
^ The water contains 2"35 per cent, of solid matter in solution, about two-thirds 
of that in sea- water ; of this 1 '68 per cent, consists of sodium chloride, and the 
remainder mainly of sulphates. 
