26 



SCIENCE. 



[N. S. Vol. V. No. 105. 



but this does not lessen my regret at having 

 unintentionally misrepresented the views of the 

 distinguished leader of cell-research for whose 

 splendid discoveries every investigator must 

 feel such admiration. 



Edmund B. Wilson. 

 Columbia University, 

 Nbw Yoek, December 19, 1896. 



THE VELOCITY OP A PLIGHT OF DUCKS OB- 

 TAINED BY TBIANGULATION. 



Measubements of the heights and the veloci- 

 ties of clouds are now being made at the Blue 

 Hill Meteorological Observatory by Mr. Eotch 

 as a part of an international scheme for such 

 work. The measurements are made with spe- 

 cially constructed theodolites in which a large 

 conical tube, with crossed wires at one end and 

 an eye-piece at the other, replaces the ordinary 

 telescope. 



On the morning of December 8th, while Mr. 

 S. P. Fergusson and I were engaged in measur- 

 ing clouds, a flock of ducks passed across our 

 base-line, which Is 2590.3 metres (8496 feet) in 

 length. We succeeded in getting one simultan- 

 eous set of measurements on the apex of the 

 flock from which its height was calculated, and 

 one or two independent subsequent observa- 

 tions, from which the velocity was calculated. 

 The height was 958 feet above the lower station, 

 which is situated in the valley of the Neponset 

 river, above which the ducks were flying. 



The velocity of flight calculated from this 

 measurement of height, and from the angular 

 velocity measured at one end of the base-line 

 is 47.9 miles in an hour, and from the angular 

 measurements made at the other end of the 

 base-line is 47.7 miles an hour, making a mean 

 of 47.8 miles. The wind was very light, hav- 

 ing a velocity of only two miles an hour ac- 

 cording to the automatic record made at Blue 

 Hill Observatory, 615 feet above the valley 

 station. The direction of the wind was from 

 the north, and the ducks were flying from the 

 northeast. These observations were not in our 

 program, but they may prove of interest to 

 ornithologists and students of aeronautics. 



H. Helm Clayton. 



BtuE Hill Meteorological Obseevatoey, 

 Eeadville, Mass., December 21, 1896. 



A TEST ON DIVEESITY OP OPINION. 



To the Editor op Science : It is always 

 interesting to test diversity of opinion, particu- 

 larly on questions of exact reasoning. It is 

 quite difiicult to obtain a test which is at once 

 significant and general. I should be very much 

 indebted to those of your readers who would 

 be willing to send me answers to the following 

 request. 



Here is a piece of reasoning which is certainly 

 capable of arousing criticism : 

 Granted that A is B, to prove that B is A. 



B {like everything else) is either A or -not A. 

 If B is not A, then by our first premise, 

 we have the syllogism : 

 A is B; 

 B is not A ; 

 .-.A is not A ; which is absurd. 

 Therefore, B is A. 



Is this reasoning correct or is it not ? If re- 

 garded as correct, my request is to have the 

 reasons for its correctness given as explicitly as 

 possible. If it is regarded as incorrect, I wish 

 in the same way a very explicit statement of the 

 nature of the error. Answers are requested 

 from all who are interested in the matter. I 

 am. particularly desirous of receiving replies 

 from those whose interest in thought is a phil- 

 osophical one, as well as from those who are 

 more specially devoted to scientific pursuits. 

 Joseph Jastkow. 

 Univeesity op Wisconsin, 



Madison, Wis., December 5, 1896. 



SCIENTIFIC LITEBATUBE. 

 A Geographical History of Mammals. R. Ly- 

 DEKKEE. Cambridge Geographical Series. 

 Cambridge (England) University Press. 8° 

 pp. 400, col. map and figures in text. Septem- 

 ber, 1896. For sale by The Macmillan Com- 

 pany, 66 Fifth Ave., New York City. Price, 

 $2.60. 



The subject of the geographic distribution of 

 animals is not one to be mastered in a few weeks 

 or months, and many are the pitfalls that lie in 

 wait for the author who seeks to illumine its 

 difficult problems. It is rare, indeed, that a 

 writer in his first essay on this theme suddenly 

 leaps to a position of authority, yet this is pre- 

 cisely what Mr. Lydekker has done. He has 

 approached the subject from a new direction — 



