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XIII

THE ATMOSPHERE

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stem of the barometer shown in Fig. 95, the mercury column to its upper limit in the long tube, the air to its upper limit, which as will be seen, is a great distance from the surface of the earth. If for any reason the weight of the atmosphere becomes greater,

32 the mercury will be pushed higher to preserve the balance ; if it should become less, then similarly the amount of mercury which can be supported will be less, and so the height of the column of mercury is diminished.

The student will now understand why it is so necessary to remove all the air bubbles in Expt. 193. If this were not done, when the tube was inverted the enclosed air would rise through the mercury and take up a position in the top of the tube above the mercury. The reading would not then be thirty inches, for instead of measuring the whole pressure of the atmosphere, what we should really be measuring would be the difference between the pressure of the whole atmosphere and that of the air enclosed in the tube. In a properly constructed barometer, therefore, there is nothing above the mercury in the tube except a little mercury vapour.

An arrangement like that described constitutes a barometer, which we can define as an instrument for measuring the pressur exerted by the atmosphere.

§ Expr. 194.--Procure a thick glass tube about thirty-six inches long and closed at one end. Fill the tube with mercury; place your thumb over the open end; invert the tube; place the open Fig. 95.-To explain end in a cup of mercury and take away your

the Principle of the thumb.' A column of mercury will be supported

Barometer. in the tube by the pressure of the atmosphere. The distance between the top of the column and the surface of the mercury in the cup will be about thirty inches.

$ ExPT. 195. -Weigh the column of mercury sustained in the barometer This book has been constructed to follow the annotated syllabus of Elementary Physiography published in the Directory just issued by the Department of Science and Art. Its distinctive features are :-(1) The large number of experiments described ; (2) the liberal use of illustrations, many of them new ; (3) summaries of the chief points in each chapter ; (4) 250 questions on the subjects dealt with. The wide scope of the book will be seen from the following list of the subjects of the chapters :

Chapter I. Matter. II. Measurement of Space, Mass, and Density. III. Motion, Inertia, and Force. IV. The Mechanical Powers. V. Energy. VI. Heat and Temperature. VII. Radiation. VIII. The Chemical Composition of Matter. IX. Common Chemical Elements and Compounds. X. The Earth. XI. Rotation of the Earth, and Measurement of Time. XII. The Sea, the Earth's Revolution, and the Moon. XIII. The Atmosphere. XIV. Atmospheric Phenomena. XV. The Oceans. XVI. Currents in the Oceans. XVII. Rivers and Glaciers. XVIII. The Earth's Solid Crust, Igneous Rocks. XIX. Aqueous and Metamorphic Rocks. XX. Internal Forces acting on the Earth's Crust. XXI. Terrestrial Magnetism.

PRESS OPINIONS

Educational News—"This is really an excellent text-book on the subject. . . . The value of the work as a practical text-book on the subject cannot be overestimated."

Royal College of Science Magasine—"Mr. Simmons' book should have a large circulation, and we recommend it both to those who will use it as a text-book, or as ground work for lectures and demonstrations."

Educational Times—"Mr. Simmons has evidently written his book with extraordinary care, and to read his chapters on 'Energy' is to be convinced that he is a teacher of very exceptional ability."

Academy"It is anything but a light task to undertake the explanation of the elementary principles of mechanics to persons whose mathematical knowledge is limited to arithmetic, yet Mr. Simmons has attempted it with great success. . . . A pupil-teacher who has mastered its contents and has performed the numerous experiments described, will never lack material for an object lesson."

Nature—" It may be stated at once that the book covers the syllabus in the most complete and satisfactory manner, and we have no hesitation in saying that teachers will find it to adequately meet their requirements as a class-book. The descriptions are clear and not too long, and great pains have evidently been taken to ensure accuracy

in every section. One of the best features is the great prominence given, for the first time, we believe, to experimental illustrations of the subject, all those suggested in the syllabus having been incorporated, and others added to make a total of 216, all of which require but simple appliances."

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ASSOCIATE OF THE ROYAL COLLEGE OF SCIENCE, LONDON

AUTHOR OF PHYSIOGRAPHY FOR BEGINNERS

Adapted to the Advanced Stage of the South Kensington Syllabus

London
MACMILLAN AND CO., LIMITED

NEW YORK: THE MACMILLAN COMPANY

All rights reserved

CONTENTS

Chapter 1. Matter. 2. Work and Energy. 3. Heat and Temperature. 4. Waves in Water, Air, and the Ether. 5. The Atmosphere and Atmospheric Movements. 6. Atmospheric Phenomena in Relation to Climate. 7. Seas and Lakes. 8. Seas (continued)—The Tides. 9. The Earth's Crust -Rock-forming Minerals. 10. The Earth's Crust-Rocks and their Classi. fication. 11. The Earth's Crust-Phenomena connected with the Internal Heat of the Earth. 12. The Earth's Crust-Movements in the Earth's Crust and some of their Results. 13. The Universe-Celestial Co-ordinates, and how they are affected by the Earth's Movements. 14. The Universe -The Law of Gravity in the Solar System. 15. The Universe-Physical Features of the Sun and Moon. 16. The Universe—The Terrestrial Planets and their Moons. 17. The Universe—The Major Planets and their Moons, Comets, and Meteorites. 18. The Universe — The Stars: their Magnitudes and Proper Motions. 19. The Universe-Double Stars, Clusters, and Nebulæ. 20. The Universe-Celestial Measurements. 21. Terrestrial Magnetism. INDEX.

PRESS OPINIONS

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Scotsman,"Rich in illustrations and in references to deeper authorities, it provides a book suitable especially to students following the syllabus of the Science and Art department, but serviceable to all.”

School Guardian—"The book is an excellent example of a class-book thoroughly up to date."

Nature—“We believe the book will admirably supply the need which must have been felt by teachers and students under the new conditions created by the revised syllabus. In conjunction with the volume to which it is a supplement, it will also provide the general reader with a comprehensive view of the earth and its relation to other bodies in space.

Educational News~"No better book on the subject has hitherto come under our notice, and we recommend it with full confidence."

Educational Review "The exposition is lucid, the matter full, accurate, and up to date.

It is altogether an excellent text-book.” Pupil Teacher—"It is impossible to speak too highly of this excellent work.

We cordially recommend the work, as a continuation of Physiograpy for Beginners, as being admirably suited for advanced pupils studying for the Science and Art Department Syllabus."

Educational Times—“We can congratulate the author on his clear descriptions and on the admirable series of illustrations which he has brought together from so many sources.'

Speaker--"The essential features of the plan of the earlier book are here presented-precise instructions for simple experiments, summaries of chapters, and questions. The book will be acceptable to the general reader, who, as well as the science student, will find plenty of interesting illustrations. As the book is thorough, it is well worth a place in the science library of Messrs. Macmillan."

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XII

THE EARTH'S CRUST

271

whole melts, its valley is seen to have assumed the form of smooth undulating prominences, in appearance not unlike the back of dolphins as they appear at the surface of the water in which they are rolling. These rounded mounds are called roches moutonnées, from a similar likeness they exhibit to the backs of sheep.

The water formed from the local melting of a glacier collects on the surface and often finds its way down one of the numerous crevasses, carrying with it a considerable quantity of the moraine detritus. This water finally gets under the glacier, and in many cases, by the help of the stones it carries with it, erodes a kind of pot-hole, which is in some places spoken of as

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a giant's kettle. As was pointed out in describing the same sort of work in the case of rivers, the largest amount of erosion will be effected in those cases where the rocks are soft. It is sometimes indeed sufficiently extensive to form considerable hollows, which on the retirement of the glacier often becomes filled with water, forming tarns or lakes (p. 150).

Results of Glacial Action.—The student will readily perceive that it is quite possible to tell where glaciers have been from the permanent record they leave behind. We can summarise the occurrences, the existence of which in any country can be taken as proof of the previous existence of glaciers.

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