NATURE 



[March i, 1917 



that the approach of an attracted mass towards an 

 attracting centre is accompanied by a rise in tem- 

 perature, or, for the approach to be isothermal, heat 

 must be taken away from the approaching mass. 



Let us suppose a" mass m grams is attracted with 

 a force F dynes bv an attracting mass M grams at a 

 distance r cm. On an P\r diagram draw the Carnot 

 cycle ABCD, AB and CD being isothermals at abso- 

 lute temperatures Q and {b-W) respectively, BC 

 and DA being adiabatic changes in the distance be- 

 tween the attracting centres. Take the mass m 

 round the cycle. Let the heat supplied along AB be 

 m.SQ ergs, where 8Q is the heat required to keep the 

 temperature of i gm. constant when the distance 

 changes from r to r-^hr. 



By the second law of thermodynamics the work 



h6 

 done in the cycle is m . SQ . -5- ergs. The work is also 



given by the area ABCD, i.e. {^\ x8dx8r ergs. 



Hence - • «Q = <'^)/ S''' 



which means that 



dO /,9F /,x 



'"dr%0 ^'^ 



Let the Newtonian law hold for isothermal changes 

 in the distance between the attracting centres, then 

 dF 2GMw , , 



'dr=—^^ ^'^ 



Also, if s is the specific heat of m in ergs we have 

 for r constant 



• i- (3) 



From (3) we get 



dr.de 

 From (i) we get 



d_/6^ aF\ _a2Q ^^ 

 de\m ' do) dr.de ' 



whence Y=^m .>{r). log 6 + ^{r). 



Putting this in (2) gives 



v// \ 1 a , J.'/ , 2GM;« 

 m .fir) . log ^ + (^ (r j = - — -^ 



This requires that 



^tf \ ^ „j J,'/ \ 2GM;« 

 /(r) = o, and ^(r)= -— . 



Hence /{r) = A and cf>(r) = G^, 



which give P = G^ + A.m. log 6, 



i.e. the greater the mass the temperature of which is 

 raised, the greater the correction due to temperature. 

 If the force of attraction between two masses at 

 constant distance is Fj when one of them is at a 

 temperature of 6,, and F, when that same one is at 

 6,, then, other things being constant, we have 



F,-Fi=A.;«.log|?, 



where m is the mass the temperature of which is 

 raised. 



Calculating A from Dr. Shaw's results gives the 



approximate value of 1-4 x 10- ^^ Poyntingand Phillips 



(Proc. Roy. Soc., A 76) used 208 grams attracted by 



the earth and the temperature was varied between 



NO. 2470, VOL. 99] 



— 186° C. and 4- 100° C. Hence the change in the 

 attracting force would be 



A.m. log -2 = I -4 X io~ '■- X 208 X loge •'' -^ dynes 



^ e^ «7 



= 4X io~^", 

 or a change of i in 5x10''*, which Poynting and 

 Phillips could not possibly detect. 



George W. Todu. 

 Newcastle-upon-Tyne. 



DESTRUCTIVE WILD BIRDS. 



ONE of the evidences of the awakening in the 

 public mind to the importance of the sub- 

 ject of the status of wild birds in relation to agri- 

 culture, horticulture, forestry, and fisheries is the 

 annual newspaper correspondence. The subject 

 has been dealt with year by year in a large- 

 number of papers, from the Times to the local 

 village weekly. Unfortunately the attitude 

 assumed by the majority of the correspondents is 

 one based largely upon want of knowledge and a. 

 misconception of the subject under discussion. 

 Whilst one section of writers presupposes that 

 the majority of wild birds are distinctly injurious 

 and should be ruthlessly destroyed, the other re- 

 gards all birds as beneficial and advocates strin- 

 gent measures for their protection. Such 

 extreme views are both wrong and retard rather 

 than aid a true understanding of a most com- 

 phcated, but all-important, subject. 



At a time when it is almost imperative that the 

 land should be made to produce its maximum 

 yield, it is doubly important that any factor that 

 acts as a deterrent should be better understood 

 and receive more than passing attention. The 

 vexed question of the economic status of our wild 

 birds is indeed a matter that calls for a very 

 thorough, exhaustive, and continued inquiry. 



From the first class of writers mentioned above 

 one would conclude that little or no trustworthy 

 evidence is forthcoming, and that we possess no 

 exact knowledge of the feeding habits of any wild 

 birds, the changes in feeding habits, their relation 

 to the destruction and distribution of weeds, etc. 

 Such, however, is far from the truth. Whilst, 

 unfortunately, we have no State department or 

 organisation engaged upon an investigation of 

 the subject, tabulating records and results year by 

 year, and spreading the information thus obtained 

 amongst the people most interested, for more 

 than thirty years there has been a small but enthu- 

 siastic number of private workers whose cumu- 

 lative work has provided us with a most valuable 

 mass of facts and original observations, and, 

 thanks to these workers, it is now^ possible to 

 state definitely that at the present time there is 

 ample evidence of a far-reaching kind to prove : — 



(i.) That no quarter should be shown to the 

 wood-pigeon, which is one of the most destruc- 

 tive birds with which the agriculturist is con- 

 fronted, and that every means should be taken to 

 destroy it. 



(ii.) The results of an investigation carried out 

 by the writer in 1907-8-9 upon the feeding: 



