TRANSACTIONS OF THE SECTIONS. 2$ 



altogether overlooked. To this difficulty, and the errors likely to be produced 

 thereby on the determination of depths, it was one of the objects of the author 

 of this communication to elucidate and establish. 



He did not refer, however, to cases where the depths were not very profound, or 

 where the time occupied by the descent of the plummet was inconsiderable ; for he. 

 Dr. Scoresby, had frequently reached depths of near a mile, or even more than a 

 mile, in the Greenland Seas — at a period when such soundings were novel or 

 unprecedented — with results, owing to peculiar and favouring circumstances, he 

 believed, perfectly satisfactory. But far otherwise than satisfactory, as he expected 

 to be able to show, must be some of those extraordinary soundings of recent years, in 

 which depths of five, six, or nearly eight miles were supposed to be established. 



If the sea were a stationary body, or if its currents were uniform movements 

 of the entire mass of waters from the surface to the bottom, then the plummet 

 mi^ht be fairly expected to take a direct and perjiendicular course downward, so 

 that the length of line run out would be the accurate measure of the depth sounded. 

 But if in the place or region of sounding, strata-currents, so prevalent in the Main 

 Ocean, should be running in different directions ; or, what would have the same 

 effect, if one stratum of water, say a superficial stratum, should be at motion and the 

 main body below at rest, no correct results could be derived from the experiments 

 referred to, where the time occupied in the running out of the line extended, in some 

 of the more interesting cases, to many hours. 



Under such circumstances, during the passage of the plummet through strata- 



currents, the line, it must be obvious, would be carried away in its different portions 

 by the movements of the water, for which the tendency to assume a perpendicular 

 position below the point of surface-suspension could afford no adequate corrective. 



Thus suppose the surface-stratum, W, to be running westerly, and the lower 

 stratum, E, easterly (or at rest) with a difference of velocity of two miles an hour. 

 The descent through the first portion, where the vessel would participate in the 

 action of the current, might be quite perpendicular ; but on the entering of the 

 plummet into the lower stratum, the lead and line would be carried, or, in relation 

 to the surface position, appear to be carried eastward, at the rate of two miles in the 

 hour. Hence in the case of the experiments of Captain Denham, whore the descent 

 in the last four miles required above an hour and a half of time per mile, the 

 plummet might be carried some miles away from the perpendicular, so as to 

 occasion a very large error in the depth apparently determined. In respect to 

 Captain Denham's deep-sea soundings, indeed, the error assumed is but conjectural, 

 depending on the circumstance of the actual existence in the place of experiment — 

 latitude 36° 49' S., longitude 37° 6' W. — of strata-currents. But in some of the 

 deep soundings attempted in the Gulf-stream, where, in the difference of the 

 temperature above and below (some 46°), we have conclusive evidence of strata- 

 currents, the determinations must, it is to be believed, have been more or less 

 inaccurate, probably greatly erroneous. 



In regard to the proportion of error — being without data both as to the flow 

 of the different currents, and the measure of resistance, under the circumstances, 

 afforded by the water to the attainment of a perpendicular position by the plummet — 

 no satisfactory estimate can be offered ; but that a considerable resistance would be 

 presented against the corrective tendency of the plummet, so that the line, however 



