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© Cambridge University Press 2000

This edition © Cambridge University Press (Virtual Publishing) 2003 First published in printed format 2000

A catalogue record for the original printed book is available from the British Library and from the Library of Congress Original ISBN 0 521 77218 4 hardback

ISBN 0 511 00963 1 virtual (netLibrary Edition)

To my wife Monique

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There are epochs in the history of every great operation and in the course of every undertaking, to which the co-operations of successive generations of men have contributed (...), when it becomes desirable to pause for a while, and, as it were, to take stock; to review the progress made, and estimate the amount of work done: not so much for complacency, as for the purpose of forming a judgement of the efficiency of the methods resorted to, to do it; and to lead us to inquire how they may yet be improved, if such improvement be possible, to accelerate the furtherance of the object, or to ensure the ultimate perfection of its attainments. In scientific, no less than in material and social undertakings, such pauses and résumés are eminently useful, and are sometimes forced on our considerations by a conjuncture of circumstances which almost of necessity obliges us to take a coup d'oeil of the whole subject, and make up our minds, not only as to the validity of what is done, but of the manner in which it has been done, the methods employed, and the direction in which we are henceforth to proceed, and probability of further progress.*

Sir John Herschel (1792-1871)

* Quoted in Hatton Turnor, Astra Castra—Experiments and Adventures in the Atmosphere, p. v, London: Chapman and Hall, 1865.

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