THE LIFE OF DEVILS LAKE 23 



ing from 89 to 95%, while at 8 diu. it ranged from 96 to 99% 

 depending on the time of exposure. 



In the (Trannsee) Linsbauer (1905)"*** found a reduction in 

 light between the air and a 10 m. depth of 98.6% ; while Whipple 

 (1914) found that the Secchi disk disappeared at a depth of 1.8 

 m. in the highly colored (92 on the Pt.-Co. U. S. G. S. scale) water 

 of the Chestnut Hill Reservoir near Boston, as compared with a 

 depth of 30 m. in the comparatively clear waters of Lake Tahoe in 

 California. 



In my own work on Devils Lake I have employed the apparatus, 

 which is illustrated in (fig. 3). It consists of two brass plates a 

 and b, separated by a metal strip c, around three sides, so spaced 

 as to admit a photographic plate and cover between them. The 

 opening on the fourth side is closed by a plate carrying a tongue, 

 which closely fits the opening in the box so as to render it light 

 tight. Two springs bring the plate into position against a. In a 

 are four holes placed 90 degrees apart. Upon a is a circular disk 

 (1, with a tongue on its lower surface fitting into a groove in a. d 

 also lias four holes so placed as to exactly match those in a, when 

 d is in the proper position, d is revolved by means of a spring coil, 

 s, attached to a pin, p, on d and a plug, e, which passes thru d and 

 is screwed into a. Around the edge of d are eight equally space 1 

 teeth, t,t'. Attached to a point on a is a bar, f, with a tooth at g. 

 When s is wound up, f is pressed against d by a spring, h, in such 

 a manner that g engages one of t and d is held in position covering 

 the holes in a. By means of a vertical arm, i, which swings on a 

 pivot^ j, and carrying a cog which engages one end of f, g may 

 be released and d revolved thru 45 degrees by the action of the 

 spring coil, s, thereby exposing one of the holes in a. i is con- 

 trolled by a cord in the hand of the operator. Releasing i allows g 

 to engage the next tooth, t. d is automatically prevented from re- 

 volving too far, and thereby closing the hole in a too soon, by the 

 end of the bar f opposite g which engages the tooth t' when t is 

 released. The closure of the hole is affected in the same manner 

 as its exposure by simply pulling the string attached to i. The ap- 

 paratus is supported by four cords attached at one end to each 

 corner and at the other end to a small plate, which is low- 

 ered to any desired depth by means of a marked line. The film 

 is protected from water by a glass cover sealed to the plate by 

 celloidin. The plates employed were Cramer's "isochromatic." 



For making the experiments clear, still days were chosen so far 

 as possible, a combination which is very unusual at Devils Lake, 

 occurring perhaps half a dozen days all told in any year. Some of 

 the experiments, however, were made thru the ice, when the wind 

 factor was, of course, eliminated. 



***Fide' Steuer (1910). 



