TRANSACTIONS OP THE SECTIONS. 



33 



the sense of sight ; but to ascertain the fact of a vertical motion of five or six feet in 

 the middle of a great sea, and out of sight of land, is a problem of no small difficulty, 

 and requires the exercise of many precautions to arrive at anything like true results. 

 In making an observation of this description we find two important obstacles in the way 

 of obtaining these, namely, the stream of tide and the undulating character of the sur- 

 face of the ground. Under the influence of a strong stream of tide, it is utterly im- 

 possible, exceptin very shallow water, to take a strictly correct depth from the vessel, 

 or a boat, at anchor, (and therefore a fixed point,) for the line wiiZ assume a curved form 

 in the act of descent; and after all, from the want of perpendicularity in the line, a 

 large allowance, in a depth of nearly twenty fathoms, is necessarily left to the exercise 

 of the judgment ; and both of these may amount to considerably more than the " rise 

 and fall" sought for. On the other hand, the undulations of the surface render it 

 essential that the depths should be always taken over some discoveredelevated spot. 

 The stream of tide and the undulations of the ground are therefore alternately opposed 

 to the making of observations from which correct results can be derived. I experi- 

 enced on this, as on the former occasion, considerable difficulty in overcoming these 

 obstacles ; but I soon found myself compelled to resort to the former plan, (with the 

 addition of such precautions as experience then gave me,) namely, that of mooring 

 one boat and taking the depths in another. 



The accompanying diagram will assist my account of the plan pursued. 



N.E. I E D _____ H S.W. 



ij^ 



The ship was anchored in 21| fathoms, and on searching, a convenient rise in the 

 ground A was soon found near her, over which there was exactly 18 fathoms 3 feet, 

 by a well-measured line. The second gig (of 26 feet) was then moored, " head and 

 stern," in the direction of the strength of the stream (N.E. and S.W.), so that she 

 should be as nearly as possible over the overfall A. This was accomplished thus : — 

 I prepared a coil of If inch rope, and fastened a grapnel at either end. The first 

 grapnel was let go at B, the whole of the line was then veered away, and the second 

 grapnel was let go at C ; the gig was hauled along the bight of the rope, until it was 

 found, by repeated trial, that the summit of the overfall was exactly abreast the fore- 

 mast row-lock of figure D, at about six feet from the boat, while the N.E. stream was 

 running. She was there secured. At the turn of the tide to the S.W., it was found 

 that the weight of the stream F had operated so powerfully on the bight of the N.E. 

 line, as to draw the boat from D to E, so that the summit of the overfall, which was 

 before under the foremost row-lock, was found to be eight feet on her bow. On the 

 return of the N.E. tide, its operation (G) upon the bight of the S.W. line again drew 

 the gig ahead to her former position D, and the summit of the overfall was found as 

 before under the foremost row-lock. 



It will then be evident, that at each change of tide I knew exactly where the over- 

 fall was to be found, while taking the depths ; and thus prepared, it only remained 

 to get the least and exact vertical depth over the summit of the overfall at the inter- 

 vals determined upon, and which were every half-hour. With the N.E. stream 

 running, I dropped the lead from the other gig about the point H, and exactly in 

 the stream, which I knew would drift her at the proper distance of six feet from the 

 moored boat ; the lead was constantly lifted off the ground, so that the line was 

 perfectly straight and perpendicular, and the undulations of the ground carefully 

 observed until the lead passed over the summit of the overfall, where the depths 

 were strictly noticed, and recorded in the accompanying Table. The boat on this 

 stream was allowed to drift to the point I. With the S.W. stream I began about 

 I8il. D 



