556 



Dr. R. Norris on the passage of 



[June 15, 



within their embouchures, as I first ascertained when carrying out my 

 current tests in that river ; brackish water was consequently found in the 

 lower depths, percolation of sea- water through the sandy substrata no 

 doubt then partly accounting for its brackish condition. But eddy 

 surface-currents along the sides of the river, under the influence of the 

 prevailing N.W. winds blowing directly into the mouths, no doubt also 

 assist the intermixture as far as it goes — just as the return-current down 

 the European coast is diluted and intermixed in its general and superficial 

 density from the rains and rivers of the north, and thus tends to restore 

 the lost freshness of the equatorial or trade-wind and Gulf-stream cur- 

 rents, as the tropical rivers and rains tend to restore the loss in the low 

 latitudes : thus condensation from evaporation and redilution by surface- 

 currents are throughout mainly maintaining the equilibrium. Dr. Forch- 

 hammer shows, that this lighter density or dilution of the encircling super- 

 ficial waters from the equator commences from the American rivers from 

 the parallel a little north of the Bermudas, and that it exists all along the 

 European coast and again on the African coast from the African rivers ; 

 and he has shown that the effect of the La Plata is found 900 miles from 

 its mouth. The fact I have given of the condition at the Nile's mouths 

 at certain seasons is an extreme case, quite in accordance with the great 

 undercurrent theorists' views, and I mention it as a fact of interest to them. 

 But nevertheless I believe, from my own experience, and from the facts to 

 be gathered from Dr. Forchhammer's elaborate researches into the tempe- 

 ratures and saline densities, that as it is not an appreciable and measurable 

 movement as an undercurrent at the Nile or Dardanelles, and only chemi- 

 cally testable by the tongue or hydrometer, so are there no great mechanical 

 and appreciable movements as undercurrents in the ocean as a necessary 

 result of the very slight difference in the densities of one part of the ocean 

 and another. Nevertheless a complete investigation into the phenomena 

 of ocean-currents is a most desirable operation, and can be so easily accom- 

 plished on the plan I have found so practicable and easy, and recommended 

 several years ago for adoption by all scientific captains when crossing the 

 great oceans, especially when calms detain them and favour the experiment, 

 without fear of the results being confused or mistaken ; for then only should 

 it be carried out where there are great depths and where strong surface- 

 currents exist. 



XII. " On the Physical Principles concerned in the passage of Blood- 

 corpuscles through the Walls of the Vessels.'" By Richard 

 Norris, M.D., Professor of Physiology, Queen's College, Bir- 

 mingham. Communicated by Dr. Sharpey, Sec. R.S. Re- 

 ceived June 12, 1871. 

 In the year 1846 my much-lamented teacher, Dr. Augustus Waller, 



published in the Philosophical Magazine two able papers relating to the 



