374 



SCIENCE. 



[N. S. Vol. XI. No. 271 



errors arising from this cause. Each group 

 contains six pairs which are used directly 

 for computing the latitude variation, and of 

 which very few have zenith distances ex- 

 ceeding 20 degrees. Each group also con- 

 tains two pairs introduced for the special 

 purpose of studying the actual refraction, 

 and of which the zenith distance is about 

 60 degrees. The normal refraction at a 

 zenith distance of 60 degrees is about four 

 times that at 20 degrees ; hence the two 

 refraction pairs will furnish an effective 

 method of determining any peculiarities of 

 refraction which are sufficiently great to 

 produce any appreciable effect upon the 

 latitude pairs, provided such effect is one 

 which increases with the zenith distance. 



Any refraction effect which is analogous 

 to a displacement of the apparent zenith, 

 by a persistent barometric gradient, for ex- 

 ample, will not be put in evidence by this 

 test. To eliminate such an error, depend- 

 ence is placed upon the fact that the final 

 result is based upon observations at several 

 stations varying greatly in longitude and 

 in the surrounding climatic and local con- 

 ditions. 



It may seem at first sight that an annual 

 variation in refraction would produce an 

 apparent annual motion of the pole. This 

 would be true if the motion of the pole were 

 derived from observations at one station 

 only. It will be seen, however, on further 

 reflection that annual variations in refrac- 

 tion would tend to make all the latitudes 

 along one parallel apparently increase and 

 decrease together, and that therefore the 

 computed motion of the pole would not be 

 appreciably affected if these annual varia- 

 tions were of about the same magnitude at 

 the different stations distributed around the 

 pole. 



To sum up, the discovery of the periodic 

 motions of the pole was first made by a 

 purely inductive method. The laws govern- 

 ing those motions have been slowly and 



painfully deduced by a continual application 

 of the same method to old series of observa- 

 tions and to many new series made for the 

 special purpose. Now a new campaign of 

 observations, promising results more accu- 

 rate than any hitherto obtained, has been 

 commenced. The mean position of the 

 pole for each fortnight of the next five years 

 will probably be known within a radius of 

 five feet. There is little prospect for im- 

 provement of the observational side of this 

 question for many years. The new obser- 

 vations will furnish material for new tri- 

 umphs of the inductive method in furnish- 

 ing a still closer mathematical approxima- 

 tion to the unknown laws of motion of the 

 pole. The interesting feature of this inves- 

 tigation is now that the theorists are at sea, 

 so to speak. Will they at the end of the 

 five years be able to furnish an adequate 

 explanation for the new facts observed or, 

 indeed, for those already known ? It is on 

 this theoretical side of the investigation that 

 new energy is needed. Here is a golden 

 opportunity for some one well versed in 

 mechanics, astronomy, and geodesy. 



John Fillmore Hayfobd. 

 TJ. S. Coast and Geodetic Subvey. 



THE PLANKTON OF FRESH WATER LAKES* 

 For some years I have been interested in 

 the subject of the fauna of our fresh water 

 lakes. This interest was first aroused in 

 regard to the animals of the deeper parts of 

 the lakes. The results of the explorations 

 of the depths of the ocean were just becom- 

 ing known. I had read in the older works 

 that while the sea was densely populated 

 along shore, and had what has become to be 

 known as a ' pelagic ' fauna and flora in the 

 open sea, remote from land, the depths were 

 a barren region utterly devoid of both ani- 

 mal and vegetable life. But later it had 

 been shown that there was, even in the 



* Address of the retiring President of the Wiscon- 

 sin Academy of Sciences, Arts and Letters. 



