18 



SCIENCE. 



[N. S. Vol. XX. No. 496. 



Professor Woodward described a double sus- 

 pension pendulum apparatus for determining 

 the acceleration of gravity and gave a brief 

 outline of the theory of the apparatus. The 

 latter consists of two rectangular bars of brass 

 about twenty kilograms mass each, connected 

 by two steel tapes of equal length in such a 

 way that when one bar is held rigidly hori- 

 zontal the other bar will be suspended hori- 

 zontally by the equal and parallel tapes. It 

 was shown that when the suspended bar vi- 

 brates longitudinally through small amplitudes 

 its motion is very nearly the same as that of 

 a simple pendulum whose length is equal to 

 that of the tapes. It was shown also how 

 small corrections due to the mass of the tapes 

 and to their rigidity may be applied in order 

 to get from the actual apparatus results in 

 conformity with those of a simple pendulum. 

 Measurements of the Primary Feathers of 



Recently Killed Hawks, and their Bearings 



upon the Problem of Bird Flight: C. C. 



Trowbridge. 



During the spring the author succeeded in 

 obtaining a series of measurements of the 

 primary feathers of the hawk's wings, imme- 

 diately after the death of the birds, and 

 secured additional proof of his theory that 

 certain birds of prey habitually interlock their 

 primary feathers in flight. 



It was found that when hawks are exam- 

 ined immediately after they have been killed 

 there usually appear deep depressions in the 

 edge of the posterior webs of the emarginate 

 primary feathers, where the feathers have 

 been in contact, which are caused by the inter- 

 locking of the primaries. 



The measurements consisted in determining 

 the width of these depressions at short inter- 

 vals of time immediately after the death of 

 the hawks. It was found that the depressions 

 gradually disappeared, both in cases where the 

 feathers were found locked and were then un- 

 locked, and in cases where the feathers were 

 found unlocked. Data were thus obtained 

 from which well-defined curves were con- 

 structed, showing the recovery of the web of 

 the feathers after the pressure caused by the 

 interlocking feathers was relieved. A num- 

 ber of life-size photographs were taken of the 



primary feathers immediately after the hawks 

 were killed and the photographs of the depres- 

 sions in the feathers when measured by a 

 Eepsold measuring machine, gave curves 

 which agreed very well with those obtained 

 by direct measurement. Similar curves were 

 obtained by artificially interlocking the pri- 

 maries for several hours and then measuring 

 the recovery of the web of the feathers with a 

 micrometer microscope. It was found that arti- 

 ficial locking of the feathers for ten minutes 

 produced very slight or no depressions and 

 locking them for several hours produced de- 

 pressions only about one half as deep as those 

 found when the hawks were killed. In the 

 latter case they were from 2 to 3.5 millimeters 

 deep, and required from one to five hours to 

 be reduced to twenty per cent, of the original 

 depth, the rate of change of the depth of 

 depression being most rapid at first. 



It was concluded from the measurements 

 and photographs that the jDrimary feathers 

 found with the depressions in the web had 

 been interlocked several hours or more pre- 

 vious to the death of the hawks, which were 

 killed while sailing in a strong wind, and that 

 the theory of interlocking of the primaries of 

 the wing in flight had been conclusively con- 

 firmed. 

 The Generation of Electrical Charges hy 



Radium: George B. Pegram. 



Dr. Pegram's paper related to the genera- 

 tion of electrical charges by radium, with spe- 

 cial reference to the suggestion of Soddy that 

 when the a particles, carrying their positive 

 charge, are expelled from the radium, there is 

 no corresponding negative charge left behind in 

 the mass. A few milligrams of radium bromide 

 were enclosed in a thick lead capsule, which 

 was supported on a quartz rod in an exhausted 

 vessel. Gold leaves attached to this capsule 

 gave no indication of a charge, showing either 

 that there was the usual generation of equal 

 amounts of positive and negative electricity 

 when the a particles are thrown off, if, as. has 

 been supposed, the number of a particles is 

 much greater than the number of negatively 

 charged particles, or else that the number of 

 ft particles is about equal to the number of a 

 particles. It remains to try a similar experi- 



