in the Twenty Years before 1895 79 



to observe when the motion was violent, and then kept the 

 waters for observation under more favourable circumstances. 

 I rarely found any sensible differences in the results; but the 

 labour of making the observations in bad weather is vcrv 

 great, and has an irritating effect. In the "Challenger" there 

 was no difficulty in deciding whether the observations should 

 be proceeded with or not, because there was no difficulty 

 in making them in weather that admitted of the main-deck 

 port, which lighted and ventilated the laboratory, being kept 

 open. When it was shut, the darkness put a stop to such 

 observations independently of the motion. 



It is, perhaps, not wholly unnecessary to point out that 

 to obtain good results with a method such as this the observer 

 must have a certain amount of dexterity and patience, but 

 more particularly he must approach the matter with the 

 desire to succeed. There is never any difficulty in making 

 unsuccessful experiments. 



One of the most important physical features of the ocean 

 is the motion of its waters, and as it directly affects the course 

 of a ship, it has at all times come under the notice of the mariner, 

 and it forms an essential factor in navigation. With this 

 end, it is the custom on board ship to keep two parallel series 

 of determinations of the position, which are entered in the log 

 from day to day. The one gives the position at one or more 

 times of the day as fixed by observations of the heavenly 

 bodies and of a correct time-keeper ; the other gives the position 

 by linear measurement of the distance run through the water, 

 and determination by compass of its direction. The former 

 gives the absolute position on the sphere at the moment of 

 observation ; the latter gives the position at the same moment 

 on the supposition that the water of the ocean through which 

 the ship has passed has no proper motion. The distance 

 between the two positions divided by the time from the previous 

 fixture of petition is logged as current. Tidal components of 

 the current arc eliminated by the f.i< t th it tin complete period 

 10 tidal ebb and flow i- v< -i\ n- ul\ \\\ \\- hours, and 



tin- ' orients prodt* .1 by it in th.it intcrv.il mu-t n.Mtlv kilance 

 each <tl 



