276 



SCIENCE. 



[Vol. XVIII. No 45S 



From a man of Dr. Leidy's industry we may expect to 

 hear of many plans entertained but subsequently abandoned, 

 of many discoveries actually his own with which his name 

 is not associated. At one time he contemplated writing a 

 woik on comparative anatomy, but was deterred from so do- 

 ing when, upon inquiry of the publishers, he learned how 

 small was the demand for writings of this kind. We cannot 

 but regret that he did not entertain the subscription plan for 

 reimbursement. For no one can doubt the fact that his ad- 

 mirers would have eagerly provided the means for publica- 

 tion had his wishes been more generally known. Respect- 

 ing his unrecorded discoveries no one can speak with 

 authority. On one subject he has himself spoken, namely, 

 that the discovery of the tactile corpuscle on the nerves of 

 the finger is his own. He occasionally referred to this as 

 an instance of tlie dangers of procrastination in not placing 

 upon record original observations the moment the facts be- 

 came clearly defined in the mind of the investigator. He 

 also frequently alluded to his having observed the amagboid 

 movement in the white corpuicles. But he interpreted them 

 to be pathological and hesitated in recording his discovery. 

 This he used to say was one of the greatest mistakes of his 

 life. But no discoveries of this kind were possible at the 

 stage of microscope technique which Lsidy commanded; 

 were our knowledge of this pi-operty of the white blood cor- 

 puscle lost to us it would be exceedingly difficult to re-estab- 

 lish it without the use of the warm stage. 



Such is a brief epitome of the labors of Joseph Leidy in 

 the anatomy of vertebrates. It is a theme for a volume. 

 But tlie man is greater than his works. A]l who knew Dr. 

 Leidy are witnesses to the impression of .strength in reserve 

 lie at all times made. It can be said of him as has been 

 said of Hallev by Francis Horner: " I never rise from an ao- 

 ccunt of such a man without a sort of thrilling palpitation 

 about me which I know not whether I should call admira- 

 tion, ambition, or despair." 



LETTERS TO THE EDITOR. 



*** Correspondents are requested to be as brief as possible. TJie mriter^a name 

 is in all cases required a^ proof of good faith. 



Onreqic^st in advance, one hundred copies of the number containing his 

 communication will be furnished free to any correspondent. 



Tlie editor itnll be glad to publish any queries consonant with the character 

 of the jc 



a body doing work ? In most text-books the student will search 

 in vain for a definite answer to this question. 



Another question is suggested by the definition of work above 

 quoted. It is clearly stated when work is done by a force and 

 when work is done against a force. But in the latter case, what 

 is it that does the work ? 



These two questions are sure to present themselves to the 

 thoughtful student. If the definition of work were so stated as to 

 furnish expHcit answers to them, the acquirement of correct no- 

 tions would be much facilitated. 



A source of confusion slightly different from that aboTe men- 

 tioned is found in certain books. Work is defined as if always done 

 by forces; while energy is defined simply as capacity for doing work. 

 The inference might naturally be drawn that energy is possessed 

 by forces. But the student who draws this logical conclusion will 

 be perplexed by finding that, in what follows, energy is always 

 referred to as belonging to bodies instead of forces. 



As an improved statement of the fundamental definitions of 

 work and energy, the following may be suggested : 



1. A force does work upon the body to which it is applietl 

 when the point of application moves (or has a component of mo- 

 tion) in the direction toward which the force acts. 



2. A body does work against a force applied to it when the 

 point of application moves (or has a component of motion) in the 

 direction opposite to that toward which the force acts. 



3. A body possesses enei-gy when its condition is such that it can 

 do work against applied forces. 



Definitions (1) and (S) are not substantially different from defi- 

 nitions commonly given. Definition (2) is usually not given ex- 

 plicitly, though always implied in the development of the theory 

 of energy. 



It is quite possible that these definitions may admit of improve- 

 ment. They must, of course, be accompanied by quantitative 

 statements as to how work and energy are to be computed. But 

 it is believed that the clear development of the subject is much 

 facilitated if explicit definitions similar to these are given at the 

 outset. 



No attempt is here made to criticise all the various methods of 

 treating the subject of work. Other forms of definition than the 

 one above considered are found in various books. In most cases, 

 however, they lead to the same difficulty above mentioned. 



A treatment practically identical with that here suggested is 

 adopted in McGregor's ''Kinematics and Dynamics" — a book 

 possessing many other admirable features — and possibly in other 

 works. It certainly is not adopted by some of the best known 

 English writers. L. M. HosKEsrs. 



Madison, Wis., Nov. 9. 



Work and Energy. 



In many of the standard text-books and treatises on mechanics 

 there is a lack of definiteness in the elementary treatment of the 

 subjects of work and energy that often proves troublesome to the 

 student. To illustrate this, let us place side by side the definitions 

 of work and energy given in the "Syllabus of Elementary Dynam- 

 ics " prepared by the Association for the Improvement of Geomet- 

 rical Teaching. 



(a) When the particle (or point of a body) to which a force is 

 applied moves in the line in which the force acts, the force is said 

 to do work, or to have work done against it, according as the 

 motion is in the sense of the force or in the opposite sense. 



(5) Energy is a general term for the capability of doing work, 

 which from any cause a mass, or different masses in their relation 

 to one another, may possess. 



These definitions are in substantial agreement with those most 

 often given, and are the only explicit statements usually found as 

 to the meaning of work and energy. 



A careful reading shows, however, that there is in definition (b) 

 an implicit suggestion of something not definitely stated, and con- 

 cerning which a definite statement is very much needed. Accord- 

 ing to the definition, energy is possessed by masses (i.e. , by bodies) ; 

 or, in other words, a body may do work. But what is meant by 



AMONG THE PUBLISHERS. 



Ever since the announcement made last winter that the au- 

 thor of "Robert Elsmere " had a new novel under way, expecta- 

 tion has been eager to know when it would appear. Mrs. Ward, 

 like George Eliot, has once more taught us that fiction, far from 

 being merely a superficial representation of passing situations and 

 emotions, may grapple with the greatest problems and teach men 

 noble truths. It is with pleasure, therefore, that we publish the 

 fact that Mrs. Ward's new book is to appear very soon from the 

 press of Messrs. Macmillan & Co., New York, and that it is to be 

 called " The History of David Grieve." It is understood that the 

 book will trace the career of a disciple of the Elsmerian doctrines 

 in his work among the poor of London. 



— There lives an Indian people on the Carribbean coasts of 

 Nicaragua and parts of Honduras, which is largely mixed with 

 African and Indian elements, foreign to them, on the littoral 

 tracts, but farther inside is of purer race. This people is iinown 

 to the whites as Sloskitos, or as they want to be called, Misskitos; 

 their language was but imperfectly studied, probably because the 

 tribes inspired their visitors with contempt on account of their 

 subserviency to English interests. Only the missionaries of the 

 Herrenhut denomination spent time enough for mastering entirely 

 the intricacies of this tropical language, and from their writings. 



