440 



NA TURE 



[March 12, 190; 



With regard to Captain Hutton's special instance, how- 

 ever, there appear to be certain points which require con- 

 sideration before accepting the conclusion that the resem- 

 blance is merely a coincidence : — (i) The possible coexistence 

 of the two species in other localities where the resemblance 

 has a meaning ; (2) the possible change of conditions in the 

 struggle for life in the locality itself ; (3) our possibly im- 

 perfect knowledge of the struggle which is waged there 

 now. Furthermore, a careful comparison between both 

 forms and their respective allies — a comparison which takes 

 account of geographical distribution as well as of superficial 

 appearance — would certainly throw light upon the origin 

 of their present appearance, and probably upon the mean- 

 ing of the likeness which they bear to each other. 



When questions such as these have been answered so as 

 to leave no doubt about the accidental nature of the resem- 

 blance, it will be necessary to ascertain whether the 



offensive " qualities of the blow-fly are any defence against 

 insect-eating animals. If they are not, the resemblance 

 would still lack an essential characteristic of true mimetic 

 likeness. E. B. Poultos. 



Oxford, March 6. 



bardy and Tuscany, and also acted as chief engineer 

 in the army of the Pope. In addition to his executive 

 work, he thoroughly investigated the laws relating to 

 the movement of water and hydraulics generally, and 

 anticipated many of the theories for which credit is 



Area of Triangle in Terms of Sides. 



As the changes which are being introduced, in accord- 

 ance with Prof. Perry's suggestions, into geometrical teach- 

 ing are giving a stimulus to the production of text-books 

 of practical geometry and mensuration, the present is a 

 good time to point out a more direct proof of the formula 

 for the area of a triangle in terms of its sides than that 

 usually given. 



generally given to men of science who lived very many 

 years later. He appears to have grasped a knowledge of 



From the centres of the inscribed and one escribed circle, the action of gravity more especially in its relation 

 drop perpendiculars on the sides or their productions. Also t0 tr , e movement of' liquids, and states his inability 

 join these centres to the corners A, B, C. 



We have then, by similar triangles, 



whence 



■-(s-i){s-e). 



The area of the triangle ABC is equal to is, and also to 

 /;,(.<;- a) ; and therefore to ^[rr a s(s - a)\ that is to 



J\s(s-a)(s-i)(s-c)\. 

 11 Leopold Road, Ealing. J. D. Everett. 



LEONARDO DA VINCI AS A HYDRAULIC 

 ENGINEER. 



T N the December number of the Bulletin of the 

 -^ French Society for the Encouragement of National 

 Industry 1 is an article by M. M. A. Ronna on 

 Leonardo da Vinci, in his capacity of hydraulic en- 

 gineer ; with extracts from his works and several 

 reproductions of sketches of the numerous mechanical 

 contrivances he invented for saving labour, for 

 measuring the discharge of water, for lock gates and 

 other hydraulic appliances. 



Leonardo da Vinci, who was born in Italy in 1452, 

 has generally been recognised as one of the most 

 illustrious painters of the world, being classed as an 

 artist with Raphael and Michael Angelo. His most 

 celebrated work was the fresco representing the Last 

 Supper, which was painted in his middle life. He 

 was also the author of a treatise on painting, which 

 was published in several different languages. His 

 giftsas sculptor, musician and poet are less known; 

 and it may be a surprise to many to be told that 

 Leonardo da Vinci was by profession an engineer, 

 engaged principally in designing and carrying out 

 works for the construction of canals, the drain- 

 age and reclamation of marshes, and similar work 

 in Italy; and in his later life in France, to which 

 country he was invited by Francis I. to advise 

 as to hydraulic works there. He held the appoint- 

 ment of engineer and director of works in Lom- 



tin di la Socttti <T En'-ourazem ent pour VIndustrie National,-, 

 December, 1502. (Paris : Published by the Society.) 



to furnish to the human mind a scientific proof of 

 its existence, as he considered that, in common with 

 magnetism and other phenomena, it was one of the 

 secrets of Nature. Hallam, in his introduction to the 



XO. 1741, VOL. 67] 



