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Pp. viii

(1) THE

igneous rocks. It summarises the characters of the

far as known, were originally deposited as marine chief rock-forming minerals, and of the origin and strata, and, consequently, retain some part of the classification of rocks, and is illustrated by an admir- minerals that were precipitated during sedimentation able series of photographs and diagrams showing the from the oceanic waters. The proportion of these field relations of igneous rocks. The author makes a

metals appears to be just the same, whether districts

are metalliferous or not (p. 363). useful protest against the appropriation by geologists of popular rock names in new and technical meanings.

This extract shows that the author adopts such an The terın granite, for example, is used in the stone

extreme position in regard to the genesis of ores that trade in its correct historical and etymological sense,

his book must be read with caution. which is entirely different from its use in geology. This system is as inconvenient, as Prof. Pirsson

ZOOLOGICAL PRIMERS. points out, as if botanists had re-defined the terms (1) Die Säugetiere Deutschlands. By Dr. C. Hen. bush, tree, and shrub, limiting each to a particular nings. Pp. 174. (Leipzig : Quelle und Meyer, species. Prof. Pirsson's protest is justified, and 1909.) Price 1.25 marks. though some American geologists are using the (2) Korallen und andere gesteinsbildende Tiere. By familiar terms in their popular meanings, this reform Dr. Walther May. Pp. jii+122. (Leipzig : B. G. has probably been proposed too late.

Teubner, 1909.) Price 1.25 marks. (2) Mr. Brenton Symons's “ Genesis of Metallic (3) Die Fortpflanzung der Tiere. By Dr. R. Gold. Ores and the Rocks which Enclose Them” is also schmidt. Pp. iv+124. (Leipzig : B. G. Teubner, intended to appeal to the general elementary student, 1909.) Price 1.25 marks. and is an attempt to explain the formation of ore (4) Die Stammesgeschichte unserer Haustiere. By deposits free from unnecessary technicalities. It is a Prof. Dr. T. Keller. Pp. iii+114. (Leipzig : B. G. book, however, of very different standard from Prof. Teubner, 1909.) Price 1.25 marks. Pirsson's; it is written by a practical engineer, who is (5) Biology. By Prof. R. J. Harvey Gibson. keenly interested in the theoretical study of mining +120. (London: J. M. Dent and Co., 1909.) geology, but whose knowledge of the subject is a little Price is. net. unequal.

HE most useful portion of this sketch of the The book begins with a general introduction on

mammalia of Germany lies in the synoptic geological principles, followed by a section on rock

tables placed at the head of each order; but these metamorphism; the third part of the book deals with

can hardly be considered as complete, since they do the ore deposits. Though the author avoids so far as

not include any account of the subspecies, which are possible technical scientific terms, his text is often of the greatest interest. repellent by the abundant use of such Americanisms

A complete list prefixed to this book would have, as cavations for spaces, such reformed spellings as

made comparisons with the fauna of other countries “ lentiles " for “ lenticles," and vegetal for vegetable, a much easier matter. As it is, one has to search, and mining terms of only local value. The most through the index in order to discover what forms valuable part of the book is its collection of diagrams

are included in this work. of ore occurrences; the instances drawn from Corn

(2) Dr. May is a well-known writer on the anatomy wall are the most satisfactory, for some of his of corals, and in this little work he brings together diagrams and views regarding ores in other parts of

descriptions of heterogeneous assemblage of the world are a little out of date. Mr. Symons takes,

animals, the common feature amongst which is the moreover, an extreme position as to the genesis of

property of producing a hard exoskeleton, or of conores. He has a great belief in the agency of

tributing otherwise by their remains to the formation geosynclinals, by which sediments are carried down

of strata. to depths where they are melted, and then forced to

The question inevitably arising out of this treatment re-ascend as igneous rocks into overlying strata; and is, What determines the difference between, say, a though he describes many ores as plutonic, he appears soft anemone and an encrusted coral? To this Dr. to regard the vast majority of ores as having been

May has, so far as we can see, no answer. Neverderived from the destruction of Archæan rocks and

theless, his book contains a good sketch or the precipitated in the sea. He says, on p. 381, “ It has

various hypotheses accounting for the origin and been already observed that nearly all the ores that can formation of coral-reefs, and for this, if for nothing come within the reach of man have been derived from

else, it is welcome. The corals and lamellibranchs the Archean strata "; from these rocks, according to

appear to us the best parts of the work. Mr. Symons, the metals are removed in solution and (3) Dr. Goldschmidt has undertaken to compress “precipitated on the bottom of the sea by chemical

into a hundred small pages an account of the methods reactions that were principally set up by organic of animal reproduction, with especial reference to the matter." He has no doubt, for example, that the number of the young, their state on hatching, their gold in the reefs of Nova Scotia and the copper ores habits and adaptations. The work cannot be conof Mansfeld were deposited in the rocks of those

sidered as really up to date, but the treatment is mining fields during their deposition in the sea. His

interesting, and the subject is one of such importance view of the origin of crystalline rocks of most ores is that we regret more space could not have been allotted shown by the following quotation.

to it. The illustrations are better than those of any “ The presence of such minute proportions in all

other booklet of this series we have so far seen. formations is natural, since the crystalline rocks, as (4) In an earlier and larger work, published some


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! three years ago, Dr. Keller advanced his views on the the American Bell Telephone Co. has performed a origin of domesticated animals. The present little useful service in the publication of this book. To work is an abstract of the larger one, and gives only any who may still be interested in the legal aspects the most meagre outline of the evidence on this diffi- of the case the book should also prove valuable. But cult subject. The time has not yet arrived when for the general reader, even when specially interested such a work can be successfully written. We know in telephony, the verbatim report of a legal examinafar too little to establish conclusions on the origin tion and cross-examination is a very unsatisfactory of most of our familiar animals, and we can only medium for conveying information. The constant recommend this work on a most interesting subject with repetitions, the frequent insistence on what must be considerable reserve. Prof. Ewart's work on horses regarded from the broader point of view as whoilv appears to be unknown to the author. The book irrelevant details, and, above all, the clumsiness of has no index.

a dialogue devoid of literary merit, make very poor (5) The general scheme of this primer is excel- reading, and one is liable to be overcome with ennui lently devised. Beginning with a sketch of function, before any salient points have been gleaned. the author passes on to differentiation. The values,

By judicious, if comprehensive, skipping, however, transformations, and elaboration of food-stuffs are

many facts of both scientific and general interest next dealt with, and a special section is given to

may be obtained from this volume, and to many the “sensitivity." The adaptations of organisms are

detailed descriptions of the earlier struggles and briefly considered, and a short account of reproduction

difficulties leading to an invention of enormous is given. The primer concludes with a sketch of the

utility and importance will have a particular fascinatheory of natural selection. Such a concise statement

tion. It is only to be regretted that the book was of the general principles of animal and plant life

not written in consecutive narrative form, though should be of considerable use to teachers of elementary possibly some of its value as a record might have science.

been sacrificed thereby. The value of the book would have been increased

(2) That Mr. Gibson has an aptitude for the de. by better illustrations. Many of those employed (for

scription in non-technical language of the achieveexample, Nos. 8, 9, 18, 19, 37, 40, and 47) are so

ments of modern technology has been amply proved incompletely described as to lose much of their value. by his earlier books which have been reviewed in The figure of Padina (Fig. 2) is extremely vague.

these columns. The present small volume shares the The text as a whole is what we should expect from

merits of its predecessors. The very large degree to such an experienced teacher as Prof. Harvey Gibson,

which the telegraph and telephone enter into the and it has had the benefit of revision from his col daily life of the community should make this book leagues. The account of the destruction of life as illus particularly useful, and it should find a large circle trated by a dinner (p. 114) is perhaps open to criticism.

of readers. The book is more or less an amplificaThe benefits of cultivation in increasing the number

tion of the chapters dealing with this branch in and variety of edible organisms are not pointed out.

earlier more general books. The subjects covered Moreover, in contrast to wild species, the individuals

are telegraphy and telephony, both with wires and of cultivated ones have surely not remained “ fairly

without; there is a short chapter on lightning, the constant” in numbers. Demand has in this case

reason for the inclusion of which “by request created supply. So far from illustrating natural

not seem clear, and three concluding chapters of a selection, such an example seems to typify artificial

more general character on electrical units and theory. selection. The statement about green Hydra on p. 43

The volume is well printed and illustrated. goes beyond our present knowledge.

(3) This text-book was originally published in

1906, and the present is the third impression. AdSOME NEW ELECTRICAL BOOKS.

vantage has been taken of the new edition to bring

some parts of the book more up to date, but the re(1) The Bell Telephone. The Deposition of A. G. vision has not been very thorough, as reference to

Bell in the Suit brought by the United States to the chapter on lamps (in which there has been very annul the Bell Patents. Pp. iv+469. (Boston : marked progress since 1906) will show. The The American Bell Telephone Co., 1908.)

tungsten lamp is allotted seven lines of small print, (2) How Telegraphs and Telephones Work. Ex- but the osmium lamp, almost if not quite defunct.

plained in non-technical language by C. R. Gibson. remains in possession of what we presume was its Pp. vi+ 156. (London : Seeley and Co., Ltd., 1909.) original position in the main text. The whole Price is. 6d. net.

chapter on lamps seems to us poor; the drawing of (3) Technical Electricity. By H. T. Davidge and arc in Fig. 128 is purely imaginary, and the

R. W. Hutchinson. Second edition. Pp. xi+ 539. authors would do well to refer to Mrs. Ayrton's book (Cambridge: University Tutorial Press, Ltd., 1909.) before they issue their next edition; the section on Price 4s. 6d.

flame arcs and the reference to the Bremer arc lamp (1) THE 1

'HE printing of the full deposition made by lead us to the conclusion that the authors have no Mr. Bell in the suit brought by the United

correct idea of the real difference between the flame States to annul the Bell telephone patents doubtless and the ordinary arc. furnishes a valuable historical record of the experi

It is perhaps somewhat unkind to take exception ments which led to the invention of the telephone, to such errors in what is only one chapter amongst and, since the deposition was never officially printed, I four-and-twenty. But it is deplorable that a text

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book should give incorrect or misleading informa- in the discussion of the apparent suppression of heat tion; the authors' aim “at spanning the gulf which on explosion, of any reference to the increase of specific too often divides pure theory and practical engineer-heats admitted on p: 25; the error in saying (on p. 44) ing ” will not be realised if the student is obliged in an engine which is to run on kerosene, and he

that it is usual to increase the compression pressure to unlearn much that they teach him when he

should also correct the general confusion of the table becomes a practical engineer. We do not profess to on p. 167. It is difficult to understand what the author be experts in the whole subject of electrical engin- means in his description (on p. 52) of the working eering, and cannot criticise the whole book, therefore, of the gas producer by the remark :-" The limit of on the same lines as we have criticised the section

the ratio of steam to coal by weight is about i to 40." on lamps; but the authors, by writing such a book, tempted to get too much into so small a volume, it

Although, as has been 'stated, the author has atlay claim-at least so far as fundamentals are con- must be acknowledged that he has produced a book .cerned-to be such experts, and if we find them at at once interesting in treatment and clear in language. fault at one part we are led to suspect the whole. The book covers the whole electrical field; the

La materia radiante e i raggi magnetici. By Prof.

A. Righi. Pp. vii+308. (Bologna : N. Zanichelli, arrangement is that usually adopted, opening with

1909.) Price 8 lire. electrostatics and magnetism, and passing on to

In a recent number of NATURE a brief account was electric currents. The diagrams and illustrations given of Righi's “ magnetic rays," this being the are for the most part good, but the process blocks name applied to a peculiar luminosity near the (fortunately few) come out badly on the class of kathode of a vacuum tube, when the latter is placed paper used. There are numerous exercises for the

in a longitudinal magnetic field. Righi supposes student to work out at the end of each chapter.

that this luminous column is due to electrically

neutral doublets, which are not in sufficiently stable M. S.

equilibrium to be looked upon as atoms or molecules, which owe, in fact, such stability as they

possess to the action of the magnetic field. Several OUR BOOK SHELF.

papers on this subject have been published by the Gas-engine Theory and Design. By A. C. Mehrtens.

author, and the main object of the present small Pp. 1+256. (New York: John Wiley and Sons;

volume is to give a connected account of the whole London : Chapman and Hall, Ltd., 1909.) Price

research. About one-third of the book is devoted to los. 6d, net.

an extremely lucid and interesting summary of our

present knowledge concerning the corpuscular theory The writer of this book is instructor in of matter, written in a style which, as far as posmechanical engineering in the Michigan Agricul- sible, is free from technical terms. The remainder, tural College. His aim, he tells us, has been to except for three short mathematical appendices, deals prepare a book for all who are interested in gas with the evidence for and against the existence of engines, whether students, draughtsmen, engineers, or neutral doublets or magnetic rays. Here, while very engine operators.

suggestive, the experiments are not altogether conThis is an ambitious aim, and we may well doubt vincing—this is evidently the opinion of Prof. Righi the possibility of its being carried out in such a small himself-but this is du in great measure to the compass; but there can be no doubt that the cardinal difficult experimental conditions. While no one exvirtues of simplicity and conciseness of language which periment can be said to have demonstrated the any such intention must require are here presented in existence of magnetic rays, the results as a whole no usual degree. The reviewer does not remember certainly tend to support the author's view. One any book hitherto written on the gas engine which point might have been treated more fully, viz., the presents its subject with such lucidity.

conditions under which a magnetic field lowers the The chief entry to be made on the debit side of the potential difference at the terminals of the discharge account is that the extent of the field covered is far tube. Experiments are described, in some of which too great. It will be found, on perusing the volume,

an increase, in others a decrease, of potential is that it not only deals with the history and present brought about by the magnetic field, but it is not position of gas-engine invention, and with the pro- clear to what difference in the conditions this is due. perties of the gases and fuels used, but also with

R. S. W. such a big subject as the design of engine details and

Brassolidae. By Dr. H. Stichel. the dimensions of parts. Students usually learn their

(Das Tierreich, 25 physics and machine design independently of the

Lieterung.) Pp. xiv+244. (Berlin : R. Friedländer steam or gas engine, and a book on the gas engine

und Sohn, 1909.) Price 15 marks. which includes a great deal of what has already been This is a very elaborate monograph of a comparastudied separately is wasting space.

The result in tively small group of butterflies found only in .so small a book as this is that the truth and ap- Tropical America. They form a subfamily of the plicability of a great number of formulæ are taken great family Nymphalidæ, and are most nearly for granted, which may account

for the poor

allied to the great blue Morphidæ, but differ from them compliment paid to them by the author on p. 123, by their stouter bodies, darker colours, and the where he remarks :

closed cell of the hind wings, which are generally “A number of formulas will be given in the follow-ornamented with three large eye-spots on the under, ing paragraphs, but machinery cannot be designed by surface. Their fight is crepuscular, while that of formulas alone. The author has frequently found the Morphidæ (which are represented in the East that empirical, and other, formulas would sometimes Indies as well as in Tropical America), is diurnal. come within 500 per cent. of the correct result.” In 1823, Latreille and Godart, in the second part

There are also the inevitable slips of a “first of “ Papillons” in the " Encyclopédie méthodique, edition," but they are not numerous. The author were acquainted with only twenty-three species now should, however, make a point in the next edition of referred to the Brassolidæ. Of these, twenty-one correcting his description (on p. 33) of carbon mon- formed the bulk of the second section of the genus oxide as an unstable compound; his omission on p. 39, | Morpho, while the remaining two species were


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placed in Brassolis. In Kirby's Catalogue of

LETTERS TO THE EDITOR. Diurnal Lepidoptera and Supplement (1871 and 1877) we find eight genera of Brassolidæ and fifty-four | [The Editor does not hold himself responsible for opinions species, while Dr. Stichel now enumerates eleven expressed by his correspondents. Neither can he undertake genera and seventy-five species, in addition to a very

to return, or to correspond with the writers of, rejected considerable number of forms treated for the present

manuscripts intended for this or any other part of NATURE.

No notice is taken of anonymous communications.] as subspecies. Dr. Stichel describes the species at great length,

August Meteoric Shower. adding tables of the genera, species, and subspecies.

I HAVE summarised in a form which may be convenient The synonymy of the genera and species is very for comparison soine of the results of Perseid observations fully given, and the excellent text-illustrations in this year. The differences in some cases are remarkable, clude the neuration of one species of each genus, and and sufficiently prove that to arrive at definite conclusions also the markings of the wings of a large number respecting the character of a shower a large number of of species, both surfaces being usually figured. De- materials should be consulted and averaged. Weather conscriptions are also given of the eggs, larvæ and ditions are dissimilar, the places of observation are not pupæ of the insects, as far as known at present, and equally well situated (certain positions in towns are much

assected by artificial light), and there are other causes the range of each species is also indicated. On

which must introduce discordances. Though comparatively Pp. 3 and 4 we find general information on the habits

few Perseids were observed at Bristol and Meltham on of the butterflies, and should have liked more detail

August 10, they were fairly numerous at Blaina and under the various species; but we presume that there Antwerp, and on the night of August 12, when a rich was either no room, or the available information on

display of brilliant meteors was remarked at Bristol, there the subject was too meagre to be worth giving, except no striking exhibition witnessed at several other in a general manner.

W. F. K. places.

Results of Perseid Observations, 1909. The Volcanic Origin of Coal and Modern Geological


Meteors Perseids Theories : a Plea for Lessening Demands on Geo- C. B. Pennington, Notts. logical Time; and for Further Separating the Life

Mrs. H. P. Hawkins,

Brockham, Surrey...)
Histories of the Aqueous and Volcanic Formations.
Ry Col. A. T. Fraser (late R.E.). Pp.

Miss Irene Warner, Bristol 1

John Hicks, Weston(London: R. Banks and Sons, 1909.)

super-Mare Th3 old Wernerians used

Mrs. R. M. Brook, Melo) to account for volcanic

tham, Huddersfield ... ) ac-ion by the supposed combustion of coal within the

T. K. Jenkins, Blaina

948-12 19 ... earth's crust, but the author of this pamphlet turns the tables upon them by making the volcanoes pro

C. L. Brook, Meltham ... duce the coal! The way in which this feat is performed is as follows :—first by pointing out that in the sides of the active volcano Gedeh in Java the

W. F. Denning, Bristol... 11 tuffs are seen to be well stratified, and look, at a distance, like old red sandstone; then the mud deposits ejected by the eruption of Tarawera in New Zealand are also stratified. Next, we have somewhat of a leap in the advance of the argument. The Java

C. Birkenstock & another

observer, Antwerp experience showed, though coal was absent, another way in which it (coal) might originate; namely,

96 being rained down in a shower of bitumen alter

Col. E. E. Markwick,} nately with sandstones, shales, &c. In support of Ellison Hawks, Leeds

10 30-dawn this view we are told that a visit to “the quarries of Carrara and Parnassus

J.: L. Haughton and show that marble is a another, Dublin

8 30-10 30 ... volcanic rock," "ejected, accompanied by high-pres

915-10 15 ... siire steam, from a fissure and showered down." Apparently few deterininations of the radiant have been We must leave our author with the coal and marble, made, but so many values have been found for this at and not attempt to follow his leading among geo- previous returns that further estimates not much logical theories, old and new. We fear, judging needed. Photographic impressions of the trails would be from books advertised on a fly-leaf at the end of the

of essential value as giving, not only a very exact position one before us, that the author has been so much

for the radiant, but as indicating its character and the occupied with psychical research, occult powers of

extent of its diffusion.

W. F. DENNING. Eastern nations and the religions of the world, that he has not found time for even a very little ele

The Ringing of House-bells without Apparent Cause. mentary chemistry.

UNTIL I read the two letters in NATURE of July 22 and

August 12' I had no idea that the ringing of house-bells Cassell's Nature" Copies (Wild Flowers). Aids to

without apparent cause was so fascinating a subject, as Nature Study, Brushwork, and Drawing.


iny own experience of it has been rather prosaic. 'One of twelve packets. (London : Cassell and Co., Ltd.,

my bells occasionally rings when no one is in the room, n.d.) Price 6d. net per packet.

but it is entirely due to bad workmanship. The strength

of the spring which draws the wire back after it has been Each of these packets of drawing copies contains ten pulled is only about equal to the friction of the wires, examples of pictures of wild flowers executed in and the result is that, though it generally draws the wire colours on stout plate paper. Though the best plan

back immediately after it has been pulled, yet it sometimes is to have wild flowers drawn from actual specimens, hours, owing to some change in the conditions, it succeeds

fails to do so at the tiine; but after some time, it may be these copies may serve a useful purpose in town schools, where it is very difficult or impossible to

in drawing back the wire, when the bell again rings when

no one is touching it. The bell thus rings once when it procure the plants themselves; in any case they will

is pulled, and a second time when the spring succeeds in add variety to the art work, and familiarise children drawing back the wire. with the beauty of common wild flowers,

The electrical explanation of any mysterious ring

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ings seems hopeless in any conditions, save possibly the air without horizontal velocity during the down. in a thunderstorm, when remember that all the stroke of the wings, and as no means are here probells and wires are in good electric contact with each vided for restoring the wing to its primitive position other, and in more or less indifferent contact at many

the time of support is limited. The illustration places with pipes, walls, &c. Further, only the bell at the

suffices, however, to show that the work required in end of a row could be rung by electrical attraction to the

order to maintain a stationary position in the air by opposite wall, because the bells swing parallel to the wall

means of wings is equal to the work required to raise on which they are fixed, and considerable force is required to make them move in a direction at right angles to their

the total weight involved at the same rate as that at free swing:

which it would fall were no work to be expended. In the case referred to by Mr. C. L. Tweedale, it might

Of the total weight supported, namely, the animal have been worth while to see if the wire attached to the and the parachute, the animal only is a source of lively bell he mentions did not come in contact with any power. Thus, while in dynamically similar ” comother wires at any part of its length. What makes me binations the total weight varies as the cube of the suggest this is that in one of my rooms I can tell when linear dimensions, the supporting area varies as the the front door bell is rung by a sympathetic movement of square, and the living power available varies, not as the bell-pull in the room, due to the wires rubbing against the total weight, but as the total weight less the each other at some part and the wire to the door bell

weight of the supporting wing. It will be readily pulling the wire to the room. When one considers the class of workmanship put into

seen that if the animal can only deliver a certain bell-hanging, one need not be surprised at the vagaries of

amount of power per unit weight of body these conthe bells. Like plumber work, it is mostly out of sight,

ditions lead to an absolute limit to the weight of an and as the work has often to be done in very imperfect animal which can sustain itself stationary in the air. light and under cramped conditions, anything that will

For, suppose the total weight is w=wa+w. (the work is considered good enough.

JOHN AITKEN. weights, namely, of the animal and the parachute of Ardenlea, Falkirk, August 21.

areas), W, must vary as sì, and if the downward velocity is to be constant s must be proportional to w.

From this it can be shown that the greatest weight FLYING ANIMALS AND FLYING MACHINES. given speed) can have is 26°/30°, where b=w'/s' and

an animal (incapable of climbing faster than some UNTIL quite recently human flight was considered (=wist, w' and being known values of wing

so impracticable weight and wing area fulfilling the condition of fallthat "I can no more do that than fly " was a phrase ing with the required velocity when the total weight used to denote something not to be accomplished. It is w': If take wg'=w'n, the expression is no wonder, then, that the fact that several people 26°/3co becomes i w'n». (probably some dozens at the present moment) have As an example, suppose that 30 feet per minute is actually flown should appeal to the popular imagina- the limiting velocity at which an animal can continue tion, and the appeal is especially strong in such a case to climb, and that the area of the parachute which as M. Blériot's flight over the English Channel, will drop at the appropriate speed when the total although there is nothing really more formidable in a weight of parachute and load is 1 lb. is 100 square fight over water than over land. It may be of some feet, and also that the weight of the parachute alone interest to show briefly how it is that what was is lb., then it appears that no animal could mainformerly looked on as a typical impossibility has now tain itself stationary in the air by means of a parabecome a matter of everyday occurrence.

chute the weight of which exceeded :(4)(or about It will be a help to take first the case of such 10.1 lb.), and the area required for this weight would animals as have wings, and to see why it is that no be more than 1600 square feet. Thus, if no more creature the height of which approaches even favourable way of supporting a weight was available quarter that of a man has been able to fly either in than the down stroke of a wing in still air, flight present


times. In order that the would be impossible for all except the very smallest wings may support the body, their movement must animals. generate a downward current of air of which the As is well known, however, the vertical reaction on momentum per unit of time is equivalent to the down- a slightly inclined plane moving rapidly in a horizontal ward momentum which the body and wings would direction enormously exceeds that which it would acquire in the same time under the influence of experience in dropping through still air, and although gravity. This does not necessarily involve a large the proportionalities between the weights and the supexpenditure of work. For instance, when a weight porting area still remain, viz. sow and wios), the is attached to a parachute and is dropped from a actual weight which can be supported by a given area height the speed of descent soon becomes constant, increases indefinitely as the horizontal speed

inand the work done in the air by the parachute is then just equal to the product of the weight into the dis- If there were no such thing as air friction, the work tance fallen. The resistance of the parachute is pro- expended supporting a given load might also be portional to its area, and the speed of descent can reduced indefinitely, for the resistance to the horizontal be made as small as we please if the area is made motion (which, when the inclination of the plane is large enough. The work, therefore, expended in a small, may be regarded as the horizontal component of given time, that is, the power delivered to the air, is the normal force) could be diminished indefinitely by diminished in the same proportion.

decreasing the inclination. Suppose now that instead of an inanimate weight Air friction, however, fixes a limit beyond which an animal is suspended from the parachute by a long the inclination of the plane to the direction of motion rope ladder.

When the speed of descent is slow cannot be advantageously reduced. Experiments have enough, the animal will have no difficulty in climbing shown that this inclination is about 3°, and that then the ladder at such a rate that the centre of gravity the ratio of the supporting force to the resistance lies of the “

may remain stationary in the air, between 5 and 7 (depending partly on the shape of and this by an expenditure of work' which can be the plane). A knowledge of the best angle of inclinadiminished indefinitely by increasing the area of the

tion and the ratio of the resistance to the force on the parachute.

plane at right angles to its path afford means of deterThis case is analogous to the hovering of a bird in mining the possible efficiency (see " Experiments on





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