certain even yet that anaerobic action is absolutely necessary at any stage of sewage purification. Many other equally important questions might be instanced on which knowledge is still extremely limited. The outstanding result of the Royal Commission's labours which will most appeal to local authorities is the statement that adequate purification can be effected without land treatment, which, if recognised by the Local Government Board, will remove what is, in many cases, an impossible restriction. Their recommendation in regard to a central controlling and advisory authority, if resulting in the creation of a department similar to the Massachusetts Board of Health, may prevent great waste of public money. Such a board might exercise wise discretion as to the amount of purification necessary under given conditions. No central control, however, can be effective without efficient local management, and Mr. Thudicum's little book of simple methods of sewage analysis will be of great assistance to local engineers and intelligent works managers, and will help to lighten the work of the trained specialist, with whom the solution of difficulties ultimately rests. G. J. F. AN AMERICAN CONTRIBUTION TO THE ARCHEOLOGY. University of Pennsylvania: Transactions of the as any in Southern Europe," to quote her description, which is not exaggerated; she might have said "more picturesque than," with reason. Away to the left are the snowy heights of Lasithi, the hills above the skála or landing-place of Ayios Nikólas, and distant rocky Spinalonga, still the home of a peculiar race of Mohammedan fishermen-corsairs not so very long ago. To the right is the little isle of Psyrà, swimming in the blue water. One would think that the excavators on the monotonous plains of Babylonia, whose doings are chronicled by Prof. Hilprecht in the last contribution to this volume,' would have given much sometimes to have been able to transport themselves for a brief space to such goodly surroundings! Pachyammos lies a mile or so beyond, and east of, the scene of Miss Boyd's work, the low hill of Gournia, on which she has discovered the remains of a Mycenæan," or more correctly "Minoan," town, a Bronze age settlement. It is a small Pompeii. One can walk up the sinuously curving little main street and look right and left into the ruined houses of the Bronze age "Minoans." There is even a sort of court-house or "palace," to give it the stereotyped appellation, with its right-angle of low steps quite on the model of the splendid right-angled stairways of Knossos and Phaistos, which Dr. Evans considers to have been theatres, the prototypes of the stepped Greek theatres of the classical period. This "palace" must have been the official centre of the town. Formerly, judging from classical analogies, one talked of a prince or 'dynast" ruling from every one of these little palaces over his own little nodes or city-state; but it will probably eventually be found that the ruler who lived in such a "palace" as that of Gournià was no more than a mere mayor or demarch, a member of an official bureaucracy analogous to that of ancient Egypt, dependent upon the metropolitan authorities at Knossos. It becomes more and more probable that Crete in Minoan days was a homogeneous and highly organised State like Egypt, not a mere congeries of a hundred warring villages, as in classical times. The official centre was not the religious centre of the town. The cathedral of Gournià stood in the middle of the town, and was approached by a special street of its own. "Not imposing as a piece of architecture," writes Miss Boyd (p. 41), it is yet of unique importance as being the first Mycenæan or Minoan' shrine discovered intact. The worshipper ascended three steps and through a doorway 1.50 m. wide entered an enclosure, about 3 m. square, surrounded by walls half a metre thick and 50 to 60 cm. high. The floor is of beaten earth." on the shores of which she found her work. No more delightful spot for archæological exploration could be imagined. Leaving the rather arid and uninteresting Candiote shore, near which Knossos lies, dominated by the towering hill of Iuktas, on the top of which, so legend says, the god Zeus died and was buried, the traveller skirts the base of the Lasithiote mountainmass and reaches the narrow isthmus of Hierápetra (the ancient Hierapytna). Before him rises a magnificent rocky wall of mountain, Thriphte by name, behind which is the peak called the Aphendi, or Lord of, Kavoúsi, the village which lies at its foot. This wall is rent by a mighty cleft, the chasm of Thriphte, which is one of the dominating features of the landscape. Along the base of the wall runs the high-road from Kavoúsi to Hierápetra across the isthmus, which is low-lying land, forming a complete break in the mountain-backbone of Crete. On the northern shore of the isthmus is a good beach, Pachyammos ("Deep-entwined with a snake, two heads of the same type as sand") by name; in the centre of it the traveller will see a large white house. This was Miss Boyd's headquarters. All around are splendid mountains and "a coast-line as picturesque 66 The more noteworthy of its contents are a low earthen table, covered with a thin coating of plaster, which stands on three legs and possibly served as an altar, four cultus vases bearing symbols of Minoan worship, the disc, consecrated horns and double-headed axe of Zeus, a terra-cotta female idol 1 Very curiously described as "A Lecture delivered before German Court and University Circles, by H. V. Hilprecht." In it Prof. Hilprecht tells us little or nothing about the excavations at Nippur that has not already appeared in his "Explorations in Bible Lands," and the photographs pub lished are already well known to archæologists. the idol, several small clay doves and serpents' heads, all of coarse terra-cotta, and a fragment of a pithos, on which a double-axe and disc are modelled in relief." This important find has since been paralleled by Dr. Evans's discovery at Knossos of a similar shrine of the snake-goddess with fine glazed faïence figures, referred to in NATURE (vol. lxx. p. 482). But Miss Boyd was the first to discover the Minoan snake-goddess, of whose existence we had no inkling before the excavations at Gournià. Another good find, of which Miss Boyd gives a fine facsimile plate, was the head of a bull in terra-cotta, a typically "Mycenæan" object, paralleled by the famous silver bull's head found by Schliemann at Mycenae, and the Egyptian representations of golden protomae of bulls being brought as gifts to the court of Thothmes III. by the Mycenæan (or rather "Minoan") ambassadors from "Kefti" (Crete). Miss Boyd's work has contributed results to Mycenæan lore which are of the highest importance, results upon which the officers of the American Exploration Society at Philadelphia, which dispatched her expedition, are to be heartily congratulated. H. R. HALL. ELECTRICAL THEORY AND PRACTICE. Maxwell's Theory and Wireless Telegraphy. By H. Poincaré and F. K. Vreeland. Pp. xi +255. (London: A. Constable and Co., Ltd., 1904.) Price IOS. od. net. Alternating Currents. Vol. i. By A. Russell. Pp. xii+407. (Cambridge: The University Press, 1904.) Price 128. net. than he will spend wisely on the one useful article in a thousand; one is tempted still more to wish that a rigorous technical censorship might be instituted which would allow nothing to find its way into print but that which was of permanent value to the world. In this way the amount of technical literature might be brought within reasonable limits by being reduced to, say, one-tenth of its present volume. If this is true of the matter which is published in journals-which has, at least as a rule, the merit of originality-it is still more true of the matter which appears in the form of technical text-books. We imagine these books find a ready sale, else we cannot account for their publication; yet we do not know by whom they are read except the reviewers. This is exemplified by the six volumes before us, all of which have appeared within the last few months. With the exception of the first two, we would venture to say that it would have been just as well, and possibly even better, had they not been published. We do not mean thereby that they are bad books, though one of them we think, should not be left about where young electricians might see it; but they are not of merit enough to justify the expense of their publication or purchase. Take, for example, Miss Zimmern's little volume; it is tastefully bound and clearly printed on good paper— there is something in its appearance strongly suggestive of a book of minor poetry. Add to this that it is pleasantly written and that there is nothing very seriously wrong with its statements, and its merits are summed up. On the other hand, we are confident that it would fail in its object of explaining the complex theories of modern electricity to the "general reader "; he might put down the book with the feeling that his By knowledge had been increased, but it would be a mis(London: taken notion. It requires genius of a very rare kind, such as was shown by Faraday in his "Chemical History of a Candle," or by Prof. Perry in his " Spinning Tops," to write a book of this kind; we intend no disparagement to the writer of this volume by saying that such genius is not shown in it. What Do We Know Concerning Electricity? Modern Electric Practice. Vol. v. Edited by M. Electricity Control: A Treatise on Electric Switch-Gear THE Messrs. Henry and Hora's volume is of another stamp; in a preface which reads like a publisher's advertisement, the authors state that "the work will be found eminently practical, scientific, and accurate." We have found it quite the reverse, and feel sorry for the apprentice" or "artisan" who "gains a complete knowledge of the fundamental principles of electricity" from its pages. This is a book which no self-respecting electrical censor, however lenient, would have allowed to appear in print. 66 The two last books on the list are not without merit or value, but it is at best of an ephemeral kind. Of HE electrical engineer who wishes to keep pace with the development of his profession and desires to know something more than that which concerns only the particular branch in which he is engaged has a very hard task before him at the present day. He must, in the first instance, endeavour to keep an eye on the technical literature-the innumerable journals" Modern Electric Practice "we have already expressed and proceedings, the monthly magazines, and the weekly papers of at least four countries in three different languages. This is in itself a task of no mean difficulty, which is heightened rather than diminished by the various "abstracts "available. So rapid is the multiplication of journals and papers that one is tempted to think that the best advice to give a student would be to read nothing, as if he tries to read much he will waste more time over what is of no value to him our views in writing of the previous volumes; the present one does not depart from the same high standard in production, and the three articles in section iv., dealing with boilers, engines, and auxiliary plant, are well written and well illustrated. The article on electrochemistry and electrometallurgy is less satisfactory. We must confess, however, that the inaccuracies noticed in previous volumes make us, unjustly perhaps, suspicious of the figures and data in the one be fore us. Mr. Andrews's book on "Electrical Control is a descriptive treatise on switch-gear. It possesses the same disadvantages as "Modern Electric Practice "; one cannot learn electrical practice from a book; there is only one school-the practical school-in which one can learn the principles and details of construction of apparatus in one-tenth of the time and ten times as thoroughly as by means of written descriptions. Practical men are apt to complain that text-books are valueless, as they are written by theorists; we have read a great many text-books of late written by practical men, and have come to the conclusion that it is only the theorist who should write them. He can describe the underlying principles which persist when the fashion of their application alters; the practical man describes the methods of his practice which even as he writes become antiquated. We have reserved to the last the two volumes which head our list. Messrs. Poincaré and Vreeland's book deserves a place in any electrical library on account of its remarkably simple and lucid explanation of Maxwell's theory and of the work of Hertz, Lodge, and others which led to the development of Hertzian telegraphy. This is from the pen of M. Poincaré, translated by Mr. Vreeland, and forms the first part of the book. second part, written by Mr. Vreeland, deals with the problems presented by the practice of wireless telegraphy, and the writer, by wisely confining himself to principles rather than details, has succeeded in writing a worthy sequel to M. Poincaré's work. The Mr. Russell's book is the first volume of a mathematical treatise on alternating currents. Alternating current machinery is growing so steadily in importance, and the mathematical theory in connection with it is so complex, that there is plenty of room for a thorough and comprehensive work of this kind. The present volume deals with the general theorems, and the second will be devoted to the more specific theory of alternating current machines and the transmission of power. MAURICE SOLOMON. OUR BOOK SHELF. Vegetationsbilder. Edited by Dr. G. Karsten and H. Schenck. Second series. Parts i-viii. (Jena: Gustav Fischer, 1904.) THE first series of the "Vegetationsbilder " met with well-merited success, and a second series has been appearing at intervals during the past year. Of the contributors to the first series, Drs. G. Karsten and E. Stahl have again supplied material, the former taking up a never-failing source of interest in the mangrove vegetation, whilst Dr. Stahl, in a double part, deals with the xerophytes and conifers of Mexico; amongst the latter the primeval Taxodium trees growing in the park of Chapultepec and the sombre cypresses on the road to the sacred mount of Amecameca bear the impress of historic antiquity. Another number, consisting of parts v. to vii., is devoted to the representation of mid-European forest trees, in accordance with an expressed desire for subjects taken from native sources. The photographs taken by Dr. L. Klein include typical specimens of conifers and beeches in the Schwarzwald and Switzerland, and others showing the changes wrought by browsing animals and devastating winds; many of them are excellent, notably a scene of windblown pines which have been entirely cleared of branches except to leeward, but similar subjects are accessible to most botanists, and for this reason they do not possess the interest attaching to photographs from less accessible countries. The names of several new contributors are announced, among them Mr. E. Ule, whose character sketches of epiphytes in the Amazon region of Peru appear in the first part of this series. Of the Cactaceæ, which are widely spread through South America, a number of genera include epiphytic species, and in this region Cereus is predominant. Cereus megalanthus, a species which might be called a climbing epiphyte, is shown perched on a Ficus tree. Another curious condition is that of a flourishing bromeliad, Streptocalyx angustifolius, where, according to the writer, the exuberance of vege tation is so directly traceable to ants that he compares the phenomenon with the fungus gardens described a few years ago by Dr. A. Moeller. The last part of the series contains photographs taken in the Italian colony of Eritraea by Dr. Schweinfurth. Hyphaene thebaica, the doum palm, familiar on account of its branching habit, the sycomore fig, and an arboreous Euphorbia are among the characteristic specimens chosen to illustrate different regions in the country. Author and Printer. An Attempt to Codify the best Typographical Practices of the Present Day. By F. Howard Collins. Pp. xv + 408. (London: Henry Frowde, 1905.) Price 5s, net. THE want of uniformity of spelling, capitalisation. punctuation, and use of italic type causes continual trouble to all who are responsible for the editorial supervision of scientific literature in any form. Some authors are more German than the Germans in their use of capitals, while others underline their manuscripts as freely as ladies do their correspondence. It is frequently difficult to decide questions of orthography, and to reduce individual practice to the consistent style, which is desirable in the columns of a Mr. periodical, but is not always maintained. Collins has prepared his book to help in this end, as a standard guide for "Authors, Editors, Printers, Correctors of the Press, Compositors, and Typists. The volume contains more than twenty thousand separate entries of words arranged alphabetically. Included among these are abbreviations, disputed spellings, foreign words and phrases, divisions of words, and various rules and explanations which should prove of service to authors and editors. The proofs of the work have been read by many writers and others who can give authoritative opinions as to what is correct or customary, so that the book does not contain merely Mr. Collins's decisions, but a consensus of opinion edited by him. Highways and Byways in Derbyshire. By J. B. Firth. With illustrations by Nelly Erichsen. (London: Macmillan and Co., Ltd.) Price 6s. WITH this book as a guide, a tourist could spend many pleasant weeks in Derbyshire, and he would learn that every part of the county has literary and historical associations of great interest. But while the human side is so well represented, little notice is taken of nature, except from the aesthetic point of view. "Of natural history and geology, says the author, "there is frankly nothing in this book, of science nothing, of sport nothing." Notwithstanding this confession of what we may be permitted to describe as sins of omission, notes and descriptions of places in which scientific readers are particularly interested occur here and there. For instance, a short account is given of the stone circle of Arborlow, the Stonehenge of the Midlands. The monument consists of a circular enclosure in which are a number of blocks of limestone, all lying flat on the ground in a rude circle, while at the centre are large blocks which probably formed the central dolmen. "There are two entrances to the enclosure, a northern and a southern, and on the east side of the latter is a large detached mound. Four hundred yards west of the main enclosure is a still larger mound, known as Gib Hill, connected with it by a low rampart of earth, now nearly worn away. Buxton and Matlock lead Mr. Firth to make some quotations from Erasmus Darwin's poetical references to them in his "Botanic Garden: Economy of Vegetation," and Loves of the Plants." Dr. Darwin knew and loved the scenes he described, whatever opinion may be held as to his possession of the divine afflatus. There are a few other references to people and scenes of especial interest to the scientific world, but the book will not be valued for these so much as for its bright narrative of literary and historical centres of Derbyshire, and its fine illustrations. The Tower of Pelée. New Studies of the Great Volcano of Martinique. By Prof. Angelo Heilprin. Pp. 62+xxii plates. (Philadelphia and London: Lippincott, 1904.) PROF. HEILPRIN's latest volume on Martinique is chiefly remarkable for the beautiful photographic plates with which it is illustrated; they give an excellent idea of the features of the great tower of solid lava which for nearly three years has been the centre of interest in the crater of Pelée. One of these plates, however (No. xi), seems to have been accidentally printed upside down. In the accompanying text there is an account of the author's fourth visit to the volcano in June, 1903, and a good deal of somewhat discursive matter regarding the lessons to be learnt from the recent eruptions. The number of points which are still unsettled concerning the mechanism of the explosions and the concomitant phenomena is very large, and the author shows a wise caution in dealing with some of them. He advances the opinion that the tower of Pelée is a volcanic core of ancient consolidation, and not an extrusion of solidified new lava, as the French observers believe. We cannot believe this is at all likely to obtain general acceptance. J. S. F. Experimental Researches on the Flow of Steam Through Nozzles and Orifices. By A. Rateau. Translated by H. Boyd Brydon. Pp. iv +76. (London: Constable and Co., Ltd., 1905.) Price 4s. 6d. net. THE laws of flow of steam are of much importance in the design of turbines. A clear sketch is given of the theory, and then an account of an excellent experimental research to determine the values of the constants. Amongst previous experiments, those of Napier are English, not American as the author states. The novelty in M. Rateau's method is the use of an ejector condenser for condensing the steam. The rise of temperature, which is easily measured, gives the quantity of steam condensed. The errors of the method, especially that due to entrained water, are carefully examined. Convergent nozzles and a thin plate orifice were used. The results are compared with those by Hirn on air, and close agreement is found. In a note, the complex phenomenon of the discharge of hot water just on the point of evaporating is examined. The translation is clear. It is, however, a defect, for English readers, that the principal formulæ are left as given by the author in foreign units. The book is essentially one for practical use, and it would have added much to the convenience of engineers if other formulæ than the one on p. 6 had been given in English units. Introductory Mathematics. IN Mr. Morgan's Introductory Mathematics the view of the author is that as soon as a boy knows decimal and vulgar fractions he should begin a mixed course of elementary practical mathematics comprising algebra, geometry, and squared-paper work, developed as a whole in mutual dependence, leading up through the manipulation of formulæ to the solution of problems involving simultaneous simple equations and giving a knowledge of the fundamental facts of geometry with a training in practical applications such as the plotting of graphs and of figures to scale, and the finding of simple areas and volumes. This scheme, ignoring the old water-tight compartment system, is a good one. The chapters on algebra and geometry usually alternate, and the work progresses on natural and easy lines, with illustrations of every-day interest. The author might with advantage have carried the idea still further and have brought in computations from quantitative experimental work in the laboratory, involving the use of the balance and measuring flask, and perhaps an investigation of the action of forces at a point. There are some minor defects, such as an occasional lack of precision in a statement, bad perspective in several of the figures, the use of a graph to give a forecast of population fifty years hence, &c. But the treatment of the subject as a whole is very satisfactory; there is a good collection of exercises, and the book is well suited to its purpose. LETTERS TO THE EDITOR. [The Editor does not hold himself responsible for opinions expressed by his correspondents. Neither can he undertake to return, or to correspond with the writers of, rejected manuscripts intended for this or any other part of NATURE. No notice is taken of anonymous communications.] The Dynamical Theory of Gases and of Radiation. LORD RAYLEIGH, in a letter which appears in NATURE of May 18, opens up the general question of the applicability of the theorem of equipartition to the energy of the ether. As the discussion has arisen out of my Theory of Gases," may I, by way of personal explanation, say that although I was fully alive to the questions referred to in this letter when writing my book, yet it seemed to me better not to drag the whole subject of radiation into a book on gases, but to reserve it for subsequent discussion? Since then I have written two papers in which questions similar to those raised by Lord Rayleigh are discussed from different aspects, but as neither of these papers is yet in print, I ask for space for a short reply explaining how my contentions bear on the special points raised by Lord Rayleigh's letter. May I, in the first place, suggest that the slowness with which energy is transferred to the quicker modes of ethervibration is a matter of calculation, and not of speculation? If the average time of collision of two molecules in a gas is a great multiple N of the period of a vibration, whether of matter or of ether, then the average transfer of energy to the vibration per collision can be shown to contain a factor of the order of smallness of e-N. The calculations will be found in $$ 236-244 of my book. It is on these that I base my position, not on a mere speculation that the rate of transfer may be slow. Lord Rayleigh's example of a stretched string, say a piano wire, will illustrate the physical principle involved. If a piano hammer is heavily felted, the impact is of long duration compared with the shortest periods of vibration, so that the quickest vibrations are left with very little energy after the impact, and the higher harmonics are not heard. If the felting is worn away, the impact is of shorter duration, the higher harmonics are sounded, and the tone of the wire is "metallic." The factor e-N is so small for most of the ethervibrations as to be negligible. There is no sharp line of demarcation between those vibrations which acquire energy very slowly and those for which the rate is appreciable; but as e-N varies rapidly with N when N is large, there will be but few vibrations near the border, so that it seems legitimate, for purposes of a general discussion, to divide the vibrations into the two distinct classes, quick and slow, relatively to the scale of time provided by molecular collisions. When the material bodies are solid, the physical principle is the same, the relatively slow motions of the atoms affecting the " quick" vibrations of the ether only by raising a sort of " equilibrium tide." The number of "slow" vibrations of the ether in any finite enclosure is finite. These quickly receive the energy allotted to them by the theorem of equipartition. Thus they form the medium of transfer of radiant energy between two bodies at different temperatures. After a moderate time the slow vibrations have each, on the average, energy equal to that of two degrees of translational freedom of one molecule; the quick vibrations have no appreciable energy, while the intermediate vibrations possess some energy, but not their full share. It is easily seen that the number of slow vibrations is approximately proportional to the volume of the enclosure, so that roughly the energy of ether must be measured per unit volume in order to be independent of the size of the enclosure. For air under normal conditions, I find as the result of a brief calculation that this value is of the order of 5×10- times that of the matter. The law of distribution of this energy will be After these values of X are passed, the formula must be modified by the introduction of a multiplying factor which falls off very rapidly as a decreases, and which involves the time during which the gas has been shut up. It is easily found (cf. "The Dynamical Theory of Gases,' S247) that at o° C. the spectrum of radiant energy is entirely in the infra-red; at 28,000° C. it certainly extends to the ultra-violet, and probably does SO at lower temperatures. Finally, Lord Rayleigh asks : "Does the postulated slowness of transformation really obtain? Red light falling upon the blackened face of a thermopile is absorbed, and the instrument rapidly indicates a rise of temperature. Vibrational energy is readily converted into translational energy. Why, then, does the thermopile itself not shine in the dark? Before trying to answer this, I wish to emphasise that my position does not require the forces of interaction between matter and ether to be small. Considering a gas for simplicity, the transfer of energy per collision to vibration of frequency p is found to be proportional to the square of the modulus of an integral of the form (cf. "The Dynamical Theory of Gases," § 237) a where f(t) is a generalised force between matter and ether. The integral may be very small either through the smallness of f(t) or the largeness of p. I rely entirely on the largeness of p, because calculation shows this to be adequate. The thermopile experiment gives evidence as to the magnitude of f(t), but this does not alter the fact that the integral is small for large values of p. This being so, I am afraid I do not very clearly understand why the thermopile should be expected to shine in the dark. If the red light is a plane monochromatic wave, its energy represents only two coordinates of the ether, and has to be shared between the great number of coordinates, six for each atom, which belong to the thermopile. If the red light comes from a large mass of redhot matter inside the same enclosure as the thermopile, then the thermopile will soon be raised to the temperature of this mass, and may shine in the dark. If the hot mass consists of iron, say at 600° C., the atomic motions in the iron must be sufficiently rapid to excite the red Fictitious Problems in Mathematics. I HAVE to thank your reviewer for so readily supplying (NATURE, May 18, p. 56) the example to prove his contention and which appears (to me) to disprove it. The man who set that example did so in order to test (inter alia) whether the pupil knew that, for any friction to arise, both the surfaces must be rough; your reviewer originally wrote:-"What the average college don forgets is that roughness or smoothness are matters which concern two surfaces not one body." The italics are your reviewer's; and this is the statement which I called (and still call) in question. It is no part of my book to uphold the verbiage in which the example is couched; by chance, in my former letter, I explained in anticipation the terms used in it. I do not see, however, why your reviewer applies the favourite word inaccurate to these terms. Perfect smoothness may not occur in nature; still, in considering the pendulum, I probably begin by assuming no friction on the axis of suspension, and, if I try afterwards to apply a correction for this friction, I probably make an assumption which is inaccurate. Friction = pressure x a constant is inaccurate, statically and dynamically. C. B. CLARKE. As I take it, the mathematician's perfectly rough body means a body which never by any chance slips on any other body with which it is placed in contact, similarly the " perfectly smooth body" is supposed never to offer any tangential resistance to any other body which it touches. The inconsistency of this nomenclature is evident when we imagine the two bodies placed in contact with each other, as in the case of the perfectly rough plank resting on the smooth horizontal plane. The subsequent course of events cannot at the same time be compatible with the assumed perfect roughness of the one body and the assumed perfect smoothness of the other. The coefficient of friction between two bodies depends essentially on the nature of the parts of the surfaces of both bodies which are in contact as well as on their lubrication, and neither body can be said to have a coefficient of friction apart from the other. It is equally incorrect to speak of perfect smoothness or perfect roughness as attributes of a single body. Moreover, this misleading language is quite unnecessary; it is very easy to frame questions in a way that is free from objection. For instance, A man walks without slipping along a plank which can slip without friction on a horizontal table." Or again, A sphere is placed in perfectly rough contact with the slanting face of a wedge whose base rests in perfectly smooth contact with a horizontal plane." G. H. BRYAN. A New Slide Rule. IN the article which appeared on p. 45 of NATURE, May 11, describing the Jackson-Davis double slide rule, you notice one little fault in the rule sent for examination. We desire to exonerate the designer of the instrument, Mr. C. S. Jackson, from responsibility for the very obvious fault to which you allude, viz. that the scale on the feather edge is divided into inches and sixteenths, and that the continuation scale which is read below the ordinary slide |