NA TURE 



[May 30, 1 90 1 



upon force as a cause of motion. Thus, § 138, when 

 dealing with a simple pendulum, he writes : "The idea 

 that F, t" (certain quantities appearing in his equations) 

 " on the left denote forces in the sense of causes of motion 

 and that the expression on the right are effects is a 

 fallacy." He does, however, permit himself to speak of 

 stresses as causing motion : it would surely be better to 

 avoid the idea of causation entirely ; it is doubtful if 

 anything is gained by the distinction between "stress," 

 as used by Prof. Gray, and " force." The term stress 

 does, it is true, call attention to the fact that the action it 

 denotes is a mutual one between two or more bodies, and 

 this is wanting in the term force. Still, is is difficult to 

 be consistent in the matter ; thus, § 146, in dealing with 

 Atwood's machine, we read, "putting T for the mass- 

 acceleration due to the force applied to either mass " — the 

 italics are not in the original. When once it has been 

 explained that forces are measured by mass-accelerations, 

 might we not write more simply and with equal effect the 

 words, putting T for the tension of the string ? 



Observation shows us that in many cases the mass- 

 acceleration of a particle is a constant ; if we know the 

 value of this constant from the conditions of the problem 

 we can, having given the initial conditions, determine the 

 motion ; we say, for brevity, that the particle is moving 

 under a constant force. In other cases, it has been observed 

 that the mass-acceleration is a known function of the 

 position of the particle relative to other particles. This 

 function can often be calculated without any knowlegde of 

 the velocity or acceleration of the particle ; thus, if there 

 be a second particle at a distance r from the first, each 

 will have a mass-acceleration towards the other equal 

 to min ;>■" where m and in are the masses of the two 

 particles. This is the force under which either particle 

 moves. Having given this force, by equating it to the 

 mass-acceleration, and solving the equations we can 

 determine the motion. Thus the resolution of any 

 problem of motion of a particle falls into three parts : 

 (i) We determine from the conditions the mass-accelera- 

 tion in each of three rectangular directions ; (2) We 

 equate these to the analytical expression giving these 

 mass-accelerations in terms of the coordinates of the 

 particle and their differential coefficients with respect to 

 the time ; (3) We solve the resulting equations. 



We may consistently employ the name force for the 

 quantities determined under (i), and indeed may speak 

 of them as the forces impressed on the particle without 

 implying that they are the cause of the motion. This is 

 done in the later sections of the chapter. One other 

 criticism occurs in connection with the sections of the 

 book immediately under review. Prof Gray writes, § 134, 

 " The word weight is used in two senses, in the sense of 

 the quantity of matter in a body, and sometimes, though 

 perhaps more rarely, in popular language as the down- 

 ward force of gravity on a body in certain specified 

 circumstances. It seems impossible to discard the former 

 use of the term even in scientific speech, and therefore 

 we shall use the word generally in this sense and in the 

 latter sense speak of the gravity of a body." 



Again, in the next section we find the sentence, "We 

 may take the inertia of a body as the measure of the 

 quantity of matter in the body or, as it is called, the 

 body's mass." 



NO. 1648, VOL. b\\ 



Thus weight and mass are to be used as synonymous, 

 contrary to the practice of writers on dynamics during 

 many years past. Such a change, unless the grounds for it 

 be very strong, must lead only to confusion, and the fact 

 that weight is used ambiguously in daily life is hardly a 

 satisfactory reason for the innovation which has been 

 adopted. 



We have referred at length to these few pages of the 

 work because of the importance of the fundamental ideas 

 and conceptions with which they deal. It is impossible 

 to deal with the rest in the same manner, nor, indeed, is it 

 necessary. The reader will find the book a storehouse of 

 valuable information, which is generally put clearly and 

 well ; experience alone will show whether or no it is 

 useful for students " beginning at the elements of 

 the subject." However this may be, the book should 

 be found in every physical library, and is sure to be 

 frequently consulted. 



TROPICAL CRUSTACEANS. 

 The Stalk-eyed Crustacea of British Guiana, West Indies, 

 and Bermuda. By Charles G. Young, M.A., M.D., 

 Dublin, Member of the Royal Irish ."Academy, lately of 

 the British Guiana Medical Service. 8vo. Pp. xix-f 514 ; 

 7 plates, coloured, and numerous outlines. (London: 

 Watkins, 1900.) Price \2s. bd. net. 



FROM the equator to thirty-five degrees north the 

 western Atlantic, with its neighbouring shores and 

 rivers, can supply a group of stalk-eyed Crustacea not 

 easily surpassed in interest by such a fauna from any 

 other region in the world. The descriptions relating to 

 this group lie scattered over numerous treatises. Dr. 

 C. G. Young has conceived the meritorious idea of bring- 

 ing them together under one cover. He modestly speaks 

 of his performance as a hand-list for the use of collectors. 

 Handiness and usefulness should therefore be among its 

 characteristic features. As it lays no claim to originality, 

 the virtues of accuracy, completeness and condensation 

 might have been expected. In place of these there is 

 offered to the student a volume expansively and ex- 

 pensively printed ; serious omissions are balanced by a 

 parade of unneeded trivialities ; whilst from one end to 

 the other slovenliness prevails in the use of older 

 authorities and neglect or ignorance of those that are 

 more recent. Like the curate with a questionable egg at 

 an episcopal breakfast table, one might say of this book 

 that " parts of it are good, my lord," but no one can tell 

 which parts without consulting the very authorities which 

 its publication presumes to be out of reach. 



The first page includes an old definition of the class 

 Crustacea, informing us that in these animals the body is 

 "composed of segments, in general very distinct, motile," 

 and this is followed by hundreds of pages dealing with 

 crabs, in which, as Dr. Young well knows, for many or most 

 of the component segments the distinctness is almost null 

 and the " motility " absolutely nil. The end of the story 

 is on a par with its beginning. It treats of the Squillidse, 

 and describes four species, adopting the synopsis of the 

 genera from the work of Brooks on the Challenger 

 Stomatopoda, but calmly assigning to the genus Squilla 

 two species which, according;to that very synopsis, belong 



