The Dredge. 423 





No well developed elevation in the bottom of the lake marks 

 the separation into two basins, of which mention has been made. 

 From the southern shore the bottom slopes off gradually to a 

 depth of from 15 to 19 meters. In passing from this depth to 

 the farther basin of the lake, a shoaling of the water amounting 

 to two or three meters is noticed in places. Whether this 

 shoaling, indicating a slight elevation, is constant throughout 

 the whole length of the lake is as yet unknown. 



About ^ mile northwest from the University buildings the 

 water is between 18 and 19 meters deep — the southern portion 

 of the deeper part of the lake. Northeast from this point a 

 depth of 22 m. is reached in about a mile, and beyond that the 

 greatest depth of the lake is probably about 24 m. The greater 

 part of the bottom of the lake, therefore, is almost a plain, and 

 a very large extent of the water is from 16 to 22 m. in depth. 

 This uniformity of depth secures uniformity of biological con- 

 ditions. The control observations made at various times, in 

 different parts of the lake have shown no noteworthy differences 

 in the crustacean life. 



The place of observation was less than \ mile from the south- 

 ern shore of the lake, and several miles from the northern 

 shore. It was about \ mile from the nearest land to 

 the west, the end of Picnic Point. In spite of this one-sided 

 location, the number of Crustacea remained substantially the 

 same, whether the wind blew off shore or toward the land. 



THE DREDGE. 



In the construction of the dredge employed in our work, it 

 was our aim to provide an apparatus which could be lowered to 

 any desired depth, opened and raised through a certain distance, 

 and then closed again. It was necessary also to devise one 

 which could be made by an ordinary tin or coppersmith. The 

 instrument consists of four parts: — the frame, the net, the re- 

 leasing apparatus, and the bucket. The general appearance of 

 the apparatus is given in Plate VII, and a full-sized section 

 through the frame and part of the cover in Plate VIII, fig. 1. 



The Frame. — The frame consists of the frame proper, bearing 

 the sliding cover of the dredge, with its supports and guides; 



