60 Questions Jbr Solution relating to Meteorology, 



observation are those when the vessel is nearly in her natural 

 position, for then she is precisely in the trough of the wave. 



It now remains to discover the means of determining whether 

 the visual line resting on the summit of a wave be horizontal. 



The crests of two contiguous waves are of the same height 

 above the intermediate hollow. A visual horizontal line leaving 

 the eye of the observer, when the vessel is in the trough, I sup- 

 pose to be directed to the summit of ^the approaching wave ; if 

 this line be prolonged on the opposite side, it will likewise touch 

 the summit merely of the wave already past. This last condi- 

 tion is necessary, and is sufficient to establish the horizontality 

 of the first visual line ; or with the instrument known under the 

 name of the Dip-sector, having its ordinary circles provided with 

 an additional mirror, there may be seen at the same time, in the 

 same glass, and in the same part of the field, two images, situ- 

 ate at the horizon, .one before, the other behind. The dip-sector, 

 then, will shew to the observer, as he gradually ascends the mast, 

 at what instant his eye reaches the horizontal plane touching the 

 crests of two neighbouring waves. 



We have supposed this observation to be made with all the 

 precision that nautical instruments admit of. The operation 

 will be more simple, and sometimes sufficiently exact, if the ob- 

 server merely determine, with the naked eye, to what height he 

 could ascend the mast, without ever perceiving, when the ves- 

 sel is sunk in the trough, any other wave than the nearest 

 of those approaching or receding. In this way the observa- 

 tion may be made by any one, and even during the severest tem- 

 pests, that is to say, in circumstances where the use of reflecting 

 instruments is attended with some difficulties, and when, more- 

 over, perhaps no one but a sailor could venture with impunity 

 to climb the mast. 



The transverse dimensions of waves are easily determined, by 

 comparing them with the length of the vessel as she passes 

 through them ; their rapidity is measured by well-known means. 

 We have, therefore, in concluding the article, again to recom- 

 mend these subjects of inquiry to the attention of the conjmander 

 of the Bonite. 



Visibility of Shoals. -^The bottom of the sea at a given dis- 

 tance from a vessel, is more distinctly seen in proportion as the 



