502 



SCIENCE. 



[Vol. IX., No. 224 



The subject is the mechanics of constrained motion, 

 and is purely kinematics. 



The book is a volume of about 650 pages, and 

 is divided into a dozen chapters. The first six 

 chapters consider purely geometric problems in 

 the elements of kinematics and mechanism, intro- 

 ducing some interesting methods of solution in- 

 volving * virtual ' rotations ; securing a means of 

 treating all mechanisms, whether of rotational or 

 of rectilinear movement, by the same sjstem ; and 

 greatly simplifying the work. In the seventh 

 chapter, accelerations and retardations are con- 

 sidered ; and in the succeeding chapter, static 

 equilibrium and work-diagrams. Then follow 

 chapters on problems in machine dynamics, and on 

 parallel and other familiar mechanisms, and various 

 trains. The last chapter considers the modifications 

 introduced by the action of friction. In these ap- 

 plications we find the motions of the steam-engine 

 and its accessories, of the fly-wheel, connecting- 

 rod, and governor, and the various sorts of gear- 

 ing. The author is one of the few writers who 

 have yet had the courage to drop the fallacious 

 and misleading so-called laws of friction, as enun- 

 ciated by earlier writers, and to introduce the re- 

 sults, even though very briefly, of recent research, 

 with correct statements of the enormously differ- 

 ing, lately discovered laws of friction of lubricated 

 surfaces. 



Professor Kennedy follows Reuleaux, in the 

 earlier part of his lectures, as far as opportunity 

 and necessity dictate, but soon gets out into a 

 field all his own, and develops his treatment in 

 his own logical and fruitful manner. 



The book is well illustrated, pictorially and by 

 examples ; the references are conscientiously in- 

 troduced throughout ; and the volume, as a whole, 

 is remarkably well adapted for use as a text-book 

 in technical schools, and will also be found very 

 useful to the practitioner. R. H. Thurston. 



A MODEL FOR AMATEUR ASTRONOMERS. 



Baron von Engelhardt has recently published, 

 in a handsomely printed and bound volume of 

 two hundred and twenty quarto pages, a series 

 of astronomical observations made at his private 

 observatory in Dresden from 1879 to 1886. The 

 observations were all made by Baron von Engel- 

 hardt himself, and they give evidence of a good 

 observer, while the reductions have been made in 

 a most thorough manner. It is rarely, indeed, 

 that we find work of this character systematically 

 carried on for so many years by an amateur ; and 

 it implies, moreover, a good deal of careful pre- 



Observations astrononiiques. Par B. D'Engelhardt. 

 Premiere partie. Dresde, 1886. 4". 



liminary training. The field chosen is not the 

 ' new astronomy,' with its many fascinations, but 

 the more prosaic ' old astronomy,' the astronomy 

 of the elder Struve and of Bessel, — painstaking 

 measurements of double stars, comets, asteroids, 

 nebulae, and clusters, observations of moon-cul- 

 minations, occultations, etc., all valuable con- 

 tributions to our knowledge of the positions and 

 motions of the heavenly bodies. Here is an excel- 

 lent example for the amateur astronomers of this 

 country. There are plenty of fine instruments 

 in the hands of amateurs, and only a moderate 

 amount of industry is called for, yet hardly one 

 of these instruments is doing any thing for 

 the advancement of science. In England there 

 are several private observatories of world-wide 

 reputation, in which the owner either carries 

 on regular observations himself, or employs a 

 competent assistant ; while here, since the death of 

 Dr. Henry Draper, the field is almost deserted. 



Baron von Engelhardt built a small observatory 

 in 1877, in which was mounted an 8-inch Grubb 

 equatorial ; but, finding this at an inconvenient 

 distance from his home, he put up a more elabo- 

 rate building connected directly with his villa on 

 the outskirts of Dresden. 



The new observatory is a three-story tower, the 

 upper story being surmounted by a cylindrical 

 'dome' containing a 12-incli Grubb equatorial. 

 The second floor connects with the transit-room, 

 in which is a ' broken-back ' transit by Bamberg 

 of 2.7 inches aperture. The observatory is also 

 thoroughly equipped with subsidiary apparatus, 

 clocks, chronometers, chronograph, etc. Upon the 

 roof of the villa isa little 'comet observatory,' where 

 were formerly two telescopes, one of 6.4 inches 

 aperture, and the other of 3.7 inches. The larger 

 instrument, which is patterned after the Strass- 

 burg comet-seeker, is of somewhat novel construc- 

 tion : the telescope is fastened by two long arras 

 to the back of a comfortable chair, so that the eye- 

 end of the telescope is just at the height of the 

 observer's eye ; the arms are pivoted to the chair- 

 back, permitting a motion in altitude, while the 

 chair turns about a vertical axis, like an ordinary 

 office-chair, so that the astronomer can examine 

 the whole sky rapidly and without fatigue. The 

 mounting for this instrument is now at the 

 University of Kiel. 



The volume before us contains a full description 

 of the instruments, illustrated by several plates. 

 The observations and reductions are given in some 

 detail, and the whole work would reflect credit 

 upon any observatory. 



Work will begin in June next on the Holstein 

 canal, to connect the Baltic with the North Sea. 



