28 



NATURE 



{Nov. 14, 1880 J 



Thornycroft, with his turbine propeller, is able to em- 

 phasize this economy of weight still further, and, but for 

 difficulties of going astern not yet surmounted, would be 

 able to save considerable weight and space in sea-going 

 steamers with this contrivance. 



As regards their construction, turbines are divided into 

 three classes (p. 24) — the radial, axial, and mixed-flow — 

 according to the mode in which the water enters and 

 passes through the turbine ; but as regards the dynamical 

 principle on which the turbines work, they are divided 

 into two classes (p. 25), the reaction and the impulse 

 turbine. 



In the reaction or Jonval turbine, described in chap- 

 ters iii. to vi., the passages are completely filled with 

 water, and the changes of pressure play an important 

 part in the work performed. This turbine possesses the 

 advantage of being able to work when drowned by the 

 tail race, or when elevated above the tail water to a height 

 anything less than the height of the water barometer, a 

 suction tube of properly adjusted shape being fitted below 

 the turbine to carry off the water at pressure gradually 

 increasing downwards to the atmospheric pressure. 

 Against this are the disadvantages of imperfect regula- 

 tion for varying load, and that with a high fall this turbine 

 must be made so small and must run so fast as rapidly to 

 wear out, as in the Fourneyron turbines at St. Blaise 

 (p. 422) ; but this disadvantage the author professes (p. 

 263) to avoid by compounding the turbine, just as we 

 compound the steam-engine with high-pressure steam. 



The impulse or Girard turbine, on the other hand 

 (chapters vii. and viii.), derives its power entirely from 

 the change of momentum of the water without change 

 of pressure ; the buckets are freely ventilated, and 

 consequently this turbine can only work in communica- 

 tion with the surrounding air. It possesses, too, the 

 great advantage of complete regulation of power by 

 merely altering the supply of water. Girard turbines are 

 divided into outward flow (Fourneyron) turbines, and 

 inward flow (James Thomson) ; the latter, although more 

 weighty and costly, possessing the advantage of greater 

 stability of motion. 



In their difference of action we may compare the 

 Jonval turbine with the screw propeller, which works 

 entirely immersed, and derives its reaction partly from 

 the change of pressure in the water ; while the Girard 

 turbine resembles the paddle-wheel in working at the 

 surface of separation of the water and air, so that no 

 appreciable change of pressure is manifest. Against this 

 analogy, however, we find thq screw propeller far less 

 susceptible to changes of immersion than the paddle- 

 wheel, whence the manifest superiority of the screw for 

 long voyages. 



In chapters ix. to xi. the author gives a very valuable 

 collection of numerical applications of his theories to 

 actual turbines on a large scale. In designing a turbine 

 to utilize a fall, the first important measurement is that of 

 the quantity of the stream of water ; the speed of the 

 turbine is next determined from the consideration that 

 the best theoretical speed is half (or a little more than 

 half) the speed at which the turbine would run if un- 

 loaded ; and then various practical considerations inter- 

 vene in deciding whether the turbine should be reaction 

 or impulse, outward, inward, or mixed flow. 



At Holyoke, Mass., the Water-Power Company, under 

 Mr. James B. Francis, controlling the falls of the 

 Connecticut, undertake the commercial testing of 

 turbines submitted to them, and have checked to some 

 extent the wild claims of efficiency, reaching and even 

 exceeding 100 per cent., which American turbine makers 

 are said to have claimed in their advertisements. There 

 is still, however, an efficiency claimed for American 

 turbines which has not been rivalled in Europe : this 

 cannot be attributed to defect in our designs, and the 

 author thinks must be attributed to the less care bestowed 

 in America on the measurement of the quantity of water 

 consumed. It is noticeable that the American turbines 

 are generally of the reaction Jonval type, which is more 

 suitable for their unlimited supplies of water by reason of 

 its smaller weight and cost ; here in Europe, where water 

 is scarcer, the impulse Girard turbine is more in favour. 



For mining purposes, especially in California, with 

 great falls of 400 or 500 feet and small quantities of 

 water, the hurdy-gurdy or Pelton wheel (p. 419) is a 

 favourite, and in a paper by Mr. Hamilton Smith, Jun., 

 of the American Society of Civdl Engineers, the efficiency 

 of this wheel and its practical advantages are declared to 

 be very high. Similar small impulse turbines seem likely 

 to come into general domestic use. 



The author concludes (chapter xiii.) with a description 

 of the various hydraulic pressure engines and motors of 

 Armstrong, Rigg, and others. These engines act by 

 pressure only, like the steam-engine, with the disadvant- 

 age of using the same quantity of water whether working 

 at high or low power, except in the case of Mr. Rigg's 

 motor. Such motors are, however, coming into great 

 use on ships, not only for working the guns, but for 

 steering, loading, and discharging cargo. 



Although designed, and amply fulfilling its purpose, as 

 a practical treatise on hydraulic motors, this book will 

 provide the pure theorist with some of the most elegant 

 applications of relative velocity, aberration, dynamical 

 principles, and of hydromechanics ; and it is instructive 

 to notice that, as in all practical mechanical treatises, 

 gravitation units of force only are employed, even in the 

 hydrodynamical equations of Borda and Carnot, or of 

 Bernoulli, as we think they should be called. All this 

 is in direct opposition to the theoretical text-books ; 

 theorist or practical man, which is to give way ? 



A. G. G. 



PHYSIOLOGY OF EDUCATION. 



Physiological Notes on Primary Education and the 

 Study of Language. By Mary Putnam Jacobi, M.D. 

 (New York and London : G. P. Putnam's Sons, 1889.) 



THIS is a remarkable book. The authoress is an 

 original thinker who knows how to express her 

 thoughts clearly and strongly. It is worthy of being read 

 by all interested in the science of education, though few 

 perhaps even of the advocates of the present educational 

 renaissance would be prepared to receive every one of 

 her conclusions. 



The work consists of four distinct essays. The first 

 two are entitled "An Experiment in Primary Education," 

 and describe the way in which Dr. Mary Jacobi taught 



