146 



SCIEXCE. 



[Vol. VII., No. 158 



sense. I should be interested to know from Professor 

 James, whose superior knowledge of this subject I of 

 course recognize, if others can do better, and if any 

 blindfolded draughtsman at his command can make 

 consecutively four such pictures as those on p. 95 

 with entire success, or can draw five lines out of six 

 through the angles of an invisible hexagon as accu- 

 rately as is clone on p. 89. If so, my remark has no 

 particular point. If not so, but if it be considered 

 that the draughtsman must have seen the picture as 

 he was drawing it, then the fact will be more valu- 

 able for what it suggests than for what it proves. It 

 will suggest the question why the committee who 

 conducted the experiments laid such stress on the 

 percipient being blindfolded when he could in fact 

 see. S. Nevvcomb. 



Sea-lcvel and ocean-currents. 



One has so little practice in differing from Profes- 

 sor Ferrel that it is difficult to know how to begin ; 

 but there are some points in his recent letter on ' Sea- 

 level and ocean-currents ' (Science, Jan. 22) that do 

 not carry conviction. The first is, that the small 

 head of water resulting from the superficial differ- 

 ence in temperature of the ocean in high and low 

 latitudes should be as effective as he claims it to be 

 in producing ocean currents, and especially in pro- 

 ducing the existing surface currents whose circuits 

 seem to be so nearly completed without descending 

 to great depths : for the supposition that there is a 

 gradual rising-up of deep water at the equator in 

 any thing like sufficient volume to feed the currents 

 that flow thence towards the poles is not warranted 

 by the known distribution of surface or deep-water 

 temperatures. Professor Ferrel ascribes the origin 

 of the southward return current from France past 

 the African islands to an elevation of the sea-level 

 on the western coast of Europe, where it is heaped 

 up by the eastward pressure of the North Atlantic 

 drift ; but the homologue of this current in the South 

 Atlantic is a well-marked stream that turns towards 

 the equator, although it finds no land-barrier to its 

 eastward passage beyond the Cape of Good Hope. 

 According to the convectional theory, it is not needed 

 at the equator, for the water that it supplies to the 

 Gulf of Guinea ought to rise there from the abysses : 

 it seems preferable to refer it to the winds, with 

 which it accords very well, provided there is reason 

 for thinking that the winds could carry it. 



The effect of the winds seems to be visible in chan- 

 ging the direction of the currents in the Indian Ocean 

 with the changes of the monsoons, and in altering 

 the area of the counter current of the equatorial At- 

 lantic as the position of the trade-winds shifts with 

 the seasons. A brief examination of Strachan's 

 charts of the 1 Currents and surface temperature of 

 the North Atlantic Ocein,' published by the British 

 meteorological committee, 1872, shows the mean 

 velocity of the return current between Portugal and 

 the Azores (latitude 37 .5 to 40 ) to be seventeen 

 miles a day in the four cold months, and only nine 

 miles for the hot months. The winter average is 

 based on forty-one determinations ; the summer 

 average, on ninety-eight. 



The sufficiency of prevailing winds to establish 

 deep currents has been discussed by Zoppritz, with 

 results that are approved so far as I have read. His 

 paper on ' Hydrodvnamic problems in reference to 

 ocean-currents' (Wiedemann's Annalen, iii., 1878, 



582) furnishes a basis for the following statements. If 

 an ocean of great depth acquire a certain velocity of 

 motion at the surface, it will take 239 years to gain 

 half this velocity at a depth of 100 metres ; at the 

 same depth, even a tenth of the surface velocity will 

 not be reached for 41 years ; at a depth of ten metres 

 the times will be 2 39 and 0.41 years. But, given 

 sufficient time, the effect of a continuous horizontal 

 surface motion will be felt to the bottom, the velocity 

 finally attainable decreasing with the increase of 

 depth. From this it appears that the effect of any 

 variations from the prevailing forces (winds) applied 

 at the surface will be propagated downwards very 

 slowly, and that below a very moderate depth the 

 motion of the greater mass of the current will de- 

 pend on the mean direction and velocity of motion 

 in the surface water. To establish the currents as 

 they now exist would r?quire something like 100,000 

 years (pp. 598, 601, 607). According to Zoppritz, 

 therefore, we should not expect to find significant 

 changes of level in Lake Ontario as a result of our 

 frequently shifting easterly and westerly winds ; nor 

 in the Atlantic, on account of the difference in the 

 velocity of the wind, winter and summer. The atti- 

 tude of the greater mass of water must be in both 

 cases adjusted to the action of the mean annual 

 winds. In view of these and other reasons, it does 

 not seem probable that the ' strongest winds have no 

 sensible effect ' on the ocean-level and the ocean- 

 currents. Doubtless both gravitative convection and 

 wind friction have a share in causing the surface 

 currents, but the latter has the larger. 



\V. M. Davis. 



Cambridge, Jan. 31. 



Association of sound and color. 



A friend who is peculiarly sensitive to music tells 

 me that in listening to an orchestra he invariably sees 

 a brilliant yellow star when the triangle is struck, 

 and a bluish green circle (hollow) at the clash of the 

 cymbals. As I understand him, these appear instan- 

 taneously, and then fade out little by little. I should 

 be glad to know whether any of the readers of Science 

 have similar experiences. Bradford Torrey. 



Boston, Feb. 9. 



Tadpoles in winter. 



In response to the inquiry of H. Iff. Hill in Science, 

 vii. No. 157, I would say that for the last ten years 

 we have been able to get tadpoles in the small streams 

 on the Ithaca flats just before they were covered with 

 ice in the autumn, and as soon as the ice had disap- 

 peared in the spring:. There has been no trouble in 

 keeping them alive in an aquarium in the laboratory 

 through the winter. Those so kept have trans- 

 formed, and have proved to be tadpoles of Kana 

 catesbiana, the common bullfrog. S. H. Gage. 



Anat. lab. Cornell university, Feb. 8. 



In the frozen marshes surrounding Fresh Pond, 

 Cambridge, I saw a large number of tadpoles under 

 the ice, and in the clear water around the edges, 

 about the last of January. The weather for a few 

 days previous had been very warm for winter, but 

 this had been preceded by very cold weather. I had 

 always supposed, as your correspondent, Mr. Hill, 

 does, that they were only found in warm weather, 

 and I was considerably puzzled. Wm. A. Ford. 



Boston, Feb. 9. 



