January 10, 1902.] 



SCIENCE. 



67 



nearly equal amount of kinetic energy is gen- 

 erated in the smaller mass passing (with high 

 velocity) through the narrowing sectional 

 area under the edge of the gate. Here the 

 compression of the water is not considered, the 

 pressure being small; as is shown by the fact 

 that neither the modulus of elasticity (of 

 volume) nor the velocity of sound enters the 

 equation employed. It would have been well 

 to remind the student at this point that in 

 the gradual closing of a stop-gate, if the mo- 

 tion of the gate is uniform the rate of retarda- 

 tion of the water can not be uniform, and that 

 the pressure induced just behind the gate is 

 consequently variable and reaches a maximum 

 value which may be many times as great as 

 the average pressure (which average is equal 

 to the pressure produced when the gate is so 

 managed as to make the retardation uniform, 

 the whole time of closing remaining un- 

 changed). 



As to the discussion of the impact of a jet 

 upon a flat plate or vane (p. 3Y8), one cannot 

 help thinking that it would have been prefer- 

 able to substitute for this rather lengthy and 

 involved treatment (where the reader must be 

 uncertain whether the plate is furnished with 

 borders parallel to the paper or not) the sim- 

 ple and direct analysis given by Rankine in 

 Case V. of § 144 of his ' Steam Engine and 

 Other Prime Movers' (also given by Cotterill). 

 On page 500, in the theory of the turbine, 

 the term ' velocity of flow ' is used in a sense 

 entirely difi^erent from that specially defined 

 on page 498 ; and on the same page (500) ob- 

 scurity of language results from the apparent 

 statement that impulse equals momentum (in- 

 stead of change of momentum). 



The author is evidently (p. 96) of the same 

 opinion as Collignon (see ' Hydraulique,' p. 

 146) when he designates as ' gratuitous ' the 

 assumption that in the case of a flat-topped 

 weir the flow adjusts itself to such a depth on 

 the weir as to bring about a maximum dis- 

 charge. Several authors have noted that ex- 

 periment gives results not very wide of this re- 

 lation. Unwin (p. 472, article ' Hydromechan- 

 ics, Encyclop. Brit.) is rather non-committal 

 on this point, though giving the same analysis ; 

 whereas Mr. J. P. Frizell (see Engineering 



Neivs of September 29, 1892) is plainly of the 

 opinion that the flow should theoretically ad- 

 just itself to a maximum discharge. 



I. P. Church. 



Dragons of the Air, an Account of Extinct 

 Flying Reptiles. By H. G. Seeley^ Pro- 

 fessor of Geology in King's College, Lon- 

 don. London, Methuen & Co. 

 When so accomplished a student of extinct 

 life as is Professor Seeley writes in so pleas- 

 ing a way as he has of a group of animals to 

 which he has devoted many years of study, 

 the results can only be happy. Divested so 

 far as is possible of technicalities, accurate 

 in statement, lucid in presentation, and en- 

 riched by patiently gathered facts from 

 many sources, his present work upon the 

 ' Dragons of the Air ' summarizes for the 

 paleontologist, as well as for the general 

 reader, about all that is known of those 

 strange fossil reptiles called pterodactyls or 

 ornithosaurs. The book contains a discus- 

 sion of reptilian characters, the range and 

 distribution of pterodactyls, a review of the 

 known forms, and a thorough comparison of 

 them with other vertebrated animals, part by 

 part, a history of their development, infer- 

 ences as to their habits, and conclusions as to 

 their place in the animal kingdom. 



It is illustrated by many figures and 

 plates of the bones or skeletons of ptero- 

 dactyls and allied animals, and by many res- 

 torations of the creatures as the author and 

 others have conceived them. In a few words, 

 the work, while popularized, is a critical re- 

 view of this extinct order of reptiles from 

 many sides, interesting because of the 

 strangeness of the animals and valuable to 

 the student of vertebrate morphology, as well 

 as to the geologist. 



However, with the fullest respect for the 

 author's anatomical erudition and admitting 

 the force of his reasoning in many cases, the 

 present writer can not always agree with his 

 conclusions. To review them all would be 

 out of place here; the curious reader may 

 expect a wider discussion elsewhere. Many 

 of the bird-like or mammal-like characters 

 which he sees in the pterodactyl. Professor 



