Appendix A. 



367 



note the number of the space which his search has reached, and 

 he can return to it at will, the arrow showing whether it be the 

 upper or the lower part of the space that remains to be examined. 



Diagram of ruled slate, reduced. 



ROBERTSON'S DREDGE. 



" The body of the dredge is in two pieces, which are held 

 together by the arms passing down through the dovetailing at 

 the sides, and each fixed at the bottom by a screw. The bottom 

 is made of coarse canvas, stretched over a hoop, which fits into 

 the dredge. By these arrangements the whole can be readily 

 taken asunder and packed into very small space. 



DIMENSIONS. 



Length of body ... ... .... ... ... 18 inches. 



Diameter at point of lips ... ... ... 10 ,, 



Depth of lips 



Longer diameter at lower angle of lips 



Shorter , , , , 



Longer diameter at bottom 



Shorter ,, ,, 



Circumference of arm rods 



Weight of dredge ... 



4 



7f 



12 Ibs. 



" The rope is connected to the dredge by ten feet of small 

 chain, which tends greatly to keep down the machine by 

 counteracting the buoyancy of the rope. The next thing to be 

 taken into consideration is the weight of the dredge. The 

 ordinary weight used for a small boat is about fifteen pounds ; 

 but, whatever the weight may be, it must be sufficient to take 

 with ease the rope required for its use to the bottom, and to 

 keep its place there against the ordinary speed of the boat, 

 which has a tendency to raise the rope, and with it the mouth 

 of the dredge. When the sea is found a little too rough, or the 



