486 



NATURE 



\April 22, 1875 



beetle trap. That it eats caterpillars is also certain : but 

 it feeds more especially on the larger insects, such as 

 may-bugs, dung-beetles, large night-flying moths, espe- 

 cially the Sphinx liloth, and various species of nocturnal 

 insects. It is a very greedy feeder, and in the autumn is 

 often very fat. The indigestible portions of the insects it 

 devours (which it swallows entire) it throws up in long 

 pellets, which may frequently be found in the places 

 where it reposes during the day. As it feeds more espe- 

 cially on those insects which are to be met with amongst 

 the dung in places where cattle have been feeding, or 

 where they are stalled, the Night-jar is often to be met 

 with in these pastures or in the immediate vicinity of 

 outlying folds ; and hence the popular delusion that it 

 sucks the goats hanging on to their udders ; and from 

 this belief has arisen the common appellation of Goat- 

 sucker. 



" This species has the claw of the middle toe furnished 

 on the side with pectinations forming a sort of close- 

 toothed comb ; and the use made of this peculiar appen- 

 dage has puzzled naturalists not a little. Some observers 

 contend that it is used to clean the bristles at the base of 

 the bill from the fragments of wings of insects which 

 may adhere to them ; but this cannot well be the case, 

 as these vibrissc-e or bristles are large, strong, and placed 

 at some distance apart, whereas the teeth of the claw are 

 thin and veiy close. Others think that as the bird invariably 

 perches along a branch in a direction parellel with it, and 

 never across the bough like almost all other birds, this 

 pectinated claw may assist it in keeping its perch more 

 firmly than it otherwise would do. Other naturalists, 

 again, contend that it is used to hold large insects with 

 greater security ; but it appears that the Night-jar almost 

 invariably takes its prey with the mouth and not with 

 the foot ; and consequently this supposition falls to the 

 ground. An anonymous writer suggests that the comb- 

 like structure of the claw may be used for disengaging 

 the hooked feet of beetles from the bill, to enable the bird 

 to swallow them ; and this may possibly be the case, as 

 the serrations are well calculated to catch the polished 

 limbs of beetles. Anyone who has attempted to confine 

 Dytisci or Scarabcri in a collecting-box, must be aware of 

 the difficulty in getting their feet free from the edge, to 

 which they hold with the greatest pertinacity, one foot 

 being no sooner pushed in than another is protruded." 



This last explanation seems the most probable one, and 

 it agrees with the observation of Gilbert White (of Sel- 

 borne), who states that he has distinctly seen the Night- 

 jar raise its foot to its mouth while hawking for insects on 

 the wing. 



The passage above quoted is a portion of seven quarto 

 pages devoted to an account of the habits and distri- 

 bution of the Night-jar. A work like the present, so 

 beautifully and artistically illustrated, and of which only 

 a limited number of copies is printed, is sure to become 

 scarce and to rise considerably in value. Lovers of 

 nature and of art may therefore be reminded, that in 

 becoming subscribers they are not only obtaining a valu- 

 able and most interesting book, but are at the same time 

 making a profitable investment. A. R. W. 



OUR BOOK SHELF 



The Monthly Journal of Education and Scholastic Ad- 

 vertiser. A medium of intercommunication for IVIasters, 

 Mistresses, and others interested in Education. Nos. i 

 to 16. (W. P. Nimmo, 1874, 1875.) 

 The original Quarterly form of this journal had been for 

 some years " slowly but steadily increasing in circula- 

 tion." The journal is now issued as a tnonthly publica- 



tion " by a number of teachers who are anxious to be of 

 service to their fellow-workers, and to all persons inte- 

 rested in education." The editor and principal contri- 

 butors to the two forms of the journal being the same, 

 as might be expected there is no great difference in the 

 earlier and later volumes, but yet there is, we believe, 

 an improvement on the side of the present series. 

 The advantage of such a frequent issue is pretty obvious, 

 but the meeting the subscription for twelve numbers 

 instead of four, is to some a serious consideration. The 

 number of subscribers, we find, is fairly satisfactory, but to 

 make it more than a barely paying matter a much larger 

 number of subscribers, the editor states, is required. 



Glancing rapidly over the articles in the numbers 

 before us, we just indicate a few which strike us as most 

 generally interesting. The first we light upon is a letter 

 from Mr. Wilson, of Rugby, to Dr. Temple, on Successive 

 V. Simultaneous Instruction : it was written in January 

 1869, and in considering the problem of education advo- 

 cates the " stratification of studies." The question is 

 naturally discussed with an eye to Rugby, but the paper 

 is, as might be supposed, deserving of careful study by 

 outsiders. Another Rugby master, Mr. Kitchener, gives 

 his views on teaching botany to junior classes ; and Mr. 

 J. Clifton Ward on natural science teaching in schools. 

 A paper on trifle blindness advocates Dr. Liebrich's 

 views. Besides, we note a reprint of a paper by Dr. 

 Hodgson, on exaggerated estimates of reading and 

 writing ; one on French accent ; and one, by Dr. Jones, 

 on Mr. Todhunter's essay on Elementary Geometry. 

 These two should be read by all who may wish to see 

 what can be said for and against Euclid as a school text- 

 book of geometry. A portion of each number is devoted 

 to correspondence, and a new feature in this new issue of 

 the journal is a Mathematical Column. What the journal 

 wants is the support and contributions of more of our 

 foremost educationalists, and then it would take a higher 

 position than it does at present. 



LETTERS TO THE EDITOR 



[Tyie Editor does not hold himself rtsponsible for opinions txpressea 



by his correspondents. Neither can he undertake to return, 



or to correspond ivith the writers of, rejected manuscripts. 



No notice is taken 0/ attonymons communications. "[ 



On the Dynamical Evidence of Molecular Constitution 



I BEG to offer the following remarks upon the extremely valu- 

 able and instructive lecture by Prof Clerk-Maxwell which 

 appeared in Nature, vol. xi. pp. 357, 374, in the hope that 

 they may tend to the further elucidation of this interesting 

 subject. 



If two bodies are attracted towards each other by a force 

 which varies inversely as the square of the distance, and R, r, 

 be the force and distance at any instant, Rr will represent the 

 sum of that portion of the energy of the two bodies which is due 

 to their mutual attraction (die mean being ^ R r] ; that is, 

 the amount which would be converted from potential to actual 

 energy while they approached each other to this point from an 

 infinite distance. 



The sum of the virials 2 2 (5 jf r), or 2 [R r), will therefore 

 represent, for a gas whose molecules are so attracted, the total 

 amount of the energy due to attraction. 



According, therefore, to the formula of Clausius, the elasticity 

 of such a gas would be the same as if those forces and a portion 

 of the kinetic energy of translation of every particle equal to the 

 energy which is due to them had no existence. 



And as the distances between the particles vary inversely as 

 the cube root of the density, if the attractive forces vary inversely 

 as the square of the distances, 2 2 (4 A' r) will vary directly as the 

 cube root of the density. The deduction from the element of 

 / K represented by | 7" will therefore vary as the cube root of 

 the density, and the value ol p V will diminish as the density 

 increases. 



If the attractive forces vary in a higher inverse ratio, this 

 effect will be further increased. 



And if this ratio be the «th power, the sum of the viiials wi 



