February 4, 1909] 



NA TURE 



395 



following pages will have been accomplished if they 

 are the means of inspiring somebody to further unfold 

 the subject." 



The volume is certainly a very welcome addition to 

 the literature, and can be warmly recommended to 

 (hose interested in insect life, as well as to proprietors, 

 foresters and all others interested in the growth of the 

 Jiritish oak. 



PRACTICAL ASTRONOMY. 

 Coins d'Astronomie. By H. Andoyer. Second part. 

 Astronomic Pratique. Pp. 304. (Paris : A. Herman 

 and .Sons, 1909.) Price 10 francs. 



TO provide anything likj a complete account of the 

 methods of instrumental astronomy, whilst 

 keeping the work within limits suitable for a course 

 of university lectures, is not a practicable task. The 

 secjnd part of Prof. Andoyer's " Cours d'Astronomie " 

 is much more bulky than the first part (which was 

 devoted to theoretical astronomv), yet there is every- 

 where evidence that the author has been harassed by 

 want of space, and is obliged to omit details which 

 are often of the highest practical importance. He 

 himself is keenly sensible of this limitation ; again 

 and again throughout the work he repeats that his 

 treatment must be confined to a general indication of 

 tile methods, without entering into details. 



The point of view of the w-ork is thus necessarily 

 academic, and differs somewhat from that of the 

 practical observer; nevertheless, in the descriptions of 

 instruments and accessories much interesting practi- 

 cal detail is given, which is not usually found in 

 astronomical text-books. It is clear that great care 

 has been taken that all such information should be 

 trustworthy; in fact, the precision and accuracy which 

 distinguished the first part of the course are again 

 noticeable in this part. We may, however, point out 

 one or two questionable passages; it is stated that the 

 chronographic method is only used for meridian 

 observations made at observatories (p. 63). It is 

 difficult to understand why the author should have 

 supposed that the method is thus limited; it is not 

 so in practice, .\gain, we read that in determining 

 differences of longitude of the great observatories, in 

 spite of all precautions, and in spite of the skill of 

 observers, " on est loin de pouvoir repondre du 

 dixieme de seconde de temps." Prof. Andoyer must 

 have been misled into this generalisation through 

 some exceptional discordances in one or two of the 

 classical determinations of longitude. In recent deter- 

 minations a much greater accuracy is normally 

 attained. 



The first part of the book deals with such subjects 

 as interpolation, the theory of errors, and the method 

 of least squares. Common accessory apparatus, includ- 

 ing the graduated circle, micrometer and spirit-level, 

 is ne.xt thoroughly discussed. Three instruments are 

 selected for specially detailed treatment ; these are the 

 theodolite, the equatorial, and the transit circle. The 

 theodolite is probably chosen because it is likelv to be 

 mure familiar to the • student than a more strictly 

 astroiuintical instrument. It is doubtful, however, 

 v\hether the theodolite serves as a good introduction 

 NO. 2049, VOL. 79] 



to instrumental astronomy or well exemplifies its prin- 

 ciples ; and the same may be said of the equatorial 

 when used for making absolute (as opposed to differ- 

 ential) measures. The fundamental principles of prac- 

 tical astronomy are not to be found in the development 

 of the formulas for a general type of instrument ; its 

 main problem is the design and use of specialised in- 

 struments, in which the mechanical errors are few, 

 and can be as far as possible determined and elimin- 

 ated. We feel that the treatment of the transit circle 

 has suffered somewhat from the devotion of jo much 

 space and the priority accorded to the theodolite and 

 equatorial, though it must be admitted that in his 

 account of it the author has compressed a wonderful 

 amount of matter into a concise form. Besides the 

 three chief instruments, numerous others are briefly 

 described ; these include the zenith telescope, coude 

 equatorial, heliometer,' siderostat, and coelostat. 



In most cases this short treatment appears to be 

 sufficient (though we doubt if any reader will be able 

 to picture to himself the coude equatorial from the 

 description given) ; but when the whole subject of 

 astrophotography is likewise dismissed in half a page, 

 some protest seems to be required. Surely this branch 

 of astronomy has now attained a development and 

 importance sufficient to secure for it a place in the 

 text-books. It cannot be urged that the subject is 

 unsuitable for inclusion in the university course; the 

 theory of transformation of coordinates and the 

 formulse involved should surely appeal more to the 

 mathematical student than the study of the small 

 errors of a transit instrument. 



Among the other subjects considered may be noticed 

 an excellent chapter on the fundamental constants of 

 astronomy. Although limitations of space preclude a 

 detailed discussion of the methods of avoiding error, a 

 very fair idea is given of the difficulties and uncer- 

 tainties involved in the determinations. We are glad 

 to see that in a complementary chapter an explanatioh 

 of Gauss's method of determining an orbit from three 

 observations has been included in the course. 



A. S. E. 



OUR BOOK SHELF. 

 tVatcr Hammer in Hydraulic Pipe Lines. By A. H. 



Gibson. Pp. iv + 60. (London : A. Constable and 



Co., Ltd., 1908.) Price 5.?. net. 

 The phenomenon of water hammer in pipe mains is 

 one familiar to all who have had any practical experi- 

 ence in matters of water supply, either for domestic 

 consumption or for power purposes. Indeed, it is safe 

 to say that it comes within the observation of most 

 people. There can scarcely be a householder who is 

 not aware that the abrupt closing of a tap, or valve, 

 produces a violent and perfectly audible concussion in 

 a water pipe, though perhaps he may not realise that 

 the shock, if repeated with sufficient frequency, is 

 capable, in process of time, of producing rupture, un- 

 less the pipe possess a very large margin of strength 

 to resist so considerable an excess over the normal 

 pressure, or unless a relief valve be provided. This 

 latter expedient is most generally adopted in all im- 

 portant installations, where the consequences of a 

 sudden outburst would be serious, if not disastrous. 



In the case of a phenomenon of such common 

 occurrence, it is somewhat remarkable that there is 



