CHAP. XVIII. J DEEP-SEA SOUNDINGS 403 



experience in sounding is gained. To avoid heavy loss, 

 however, not more than four thermometers should be on the 

 wire. 



The temperature of the water is usually taken at intervals of 

 100 fathoms down to a depth of 1,000 fathoms, and at closer 

 intervals in the first 100 fathoms. 



Deep-sea soundings on every voyage are now a recognised Sounding 

 part of a surveying ship's routine. It is only in this way that voyages, 

 depths so useful for submarine cable, as well as for scientific 

 purposes, can be accumulated without the expenditure of 

 time involved in special sounding cruises, though those are 

 occasionally necessary. As a rough rule, a sounding after 

 daybreak, and before sunset, should be obtained daily, when 

 observationt can be got. 



Connected with deep-sea sounding, though not such a common Dredging, 

 part of a surveyor's duty, is dredging, on which a few words 

 may be useful. 



The dredge consists of a strong iron frame, the sides forming 

 lips, which are connected at each end by an iron bar, and are 

 chamfered off to fairly fine edges. These edges slightly incline 

 outwards, as seen in the sketch. On the iron bars arms are 

 fitted, and to the eye at the extremity of one of them the 

 dredging hawser is bent, the eye of the other arm being seized 

 to it to form a span of such a strength that the seizing will 

 carry away if the dredge catches. 



Attached by wire seizings to holes in the lower part of each 

 lip is a stout canvas bag, perforated with holes in its lower 

 portion to permit the water to flow through. 



A stout iron bar, to which three long swabs are secured, is 

 suspended by ropes from the iron end bars immediately below 

 the canvas bag. 



Dredges are of various dimensions, but a convenient size is 

 as follows, as illustrated by the sketch Fig. 82 : 



A and B, arms, 2| feet long. 



C, hawser. 



D E and F G, lips of the dredge, 2| feet long. 



H, holes to wliich bag is laced. 



1 1, perforations in the canvas bag. 



K K, swab bar. 



Ti, bar of dredge mouth, 6 inches. 



2G— 2 



