MetJiod of Using the Dredge. 429 



(2) It is operated by weights, which work positively, and 

 does not depend upon springs or wheels, whose action is more 

 or less uncertain. 



(3) It is tight, so far as the Crustacea are concerned. The 

 coarseness of the wire mesh of our bucket does not permit us to 

 assert that it is equally tight to Peridinium and similar minute 

 creatures. It would not be difficult, however, to construct a dredge 

 on a similar pattern whose cover should be entirely secure 

 against the entrance of any organisms when the dredge is 

 closed. 



As disadvantages of the dredge may be mentioned : 



(1) The amount of the weights required lor working the cover. 

 This, however, is not much greater than is necessary to cause 

 the dredge to sink promptly and rapidly through the water. 



(2) The resistance offered by the cover in drawing up the 

 dredge, and the time required by the messengers in descending 

 to open and close it. These are, however, of comparatively 

 slight moment in water so shallow as ours. 



(3) A more important objection lies in the fact that the 

 dredge must be used as a vertical dredge only. The boat 

 from which it is worked must be practically stationary, and 

 preferably should be anchored. If the boat is drifting through 

 the water, the pressure of the water on the cover of the dredge 

 so hinders its sliding that a weight which works it perfectly 

 when the dredge is vertically suspended fails to open or close it. 

 Further, if the dredge line is not vertical, a messenger which 

 ordinarily is amply heavy for releasing the weights may fail to 

 sufficiently depress the weight-pin. 



METHOD OF USING THE DREDGE. 



The mode of working the dredge is as follows: The dredge is 

 hung to the dredge line, and the opening and closing weights 

 are hung to their respective supports. The weight-holders are 

 hung to the pins of the release, so that the weights are sup- 

 ported by the release cords. The cover is slid into place, and 

 the dredge lowered. When it has reached the proper depth the 

 smaller or opening messenger is sent down the line. It strikes 

 and depresses the short fork, releases the opening weight, which, 



