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of South America are still current, even among men of science.

I make no pretension to add anything of importance to our store of positive knowledge respecting the region described in this volume; I shall be content if it should be found that I have suggested trains of thought that may lead others to valuable results. I venture, indeed, to believe that the argument adduced in the sixth chapter, as to the great extent and importance of the ancient mountains of Brazil, approaches near to demonstration, and that the recognition of its validity will be found to throw fresh light on the history of organic life in that region of the globe.

In the Appendices to this volume two subjects of a somewhat technical character, not likely to interest the general reader, are separately discussed. With regard to both of them, my aim has been to show that the opinions now current amongst men of science do not rest upon adequate evidence, and that we need further knowledge of the phenomena, discoverable by observation, before we can safely arrive at positive conclusions.

In deference to the prejudices of English readers, which are unfortunately shared by many scientific writers, the ordinary British standards of measure and weight have been followed throughout the text, as well as the antiquated custom of denoting temperature by the scale of Fahrenheit's thermometer. With regard to the metrical system of measures and weights, I am fully aware of its imperfections, and if the

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question were now raised for the first time I should advocate the adoption of some considerable modifications. But seeing that no other uniform system is in existence, and that the metrical system has been adopted by nearly all civilized nations, I cannot but regret that my countrymen should retain what is practically a barrier to the free interchange of thought with the rest of the world. The defects of the metrical system are mainly those of our decimal system of numeration, which owes its existence to the fact that the human hand possesses five fingers. If in some future stage of development our race should acquire a sixth finger to each hand, it may then also acquire a more convenient system of numeration, to which the scale of measures would naturally be adapted. In the mean time the advantages of a uniform system far outweigh its attendant defects.

The adherence to the Fahrenheit scale for the thermometer is even less defensible. It belongs to a primitive epoch of science, when a knowledge of the facts of physics was in a rudimentary stage, and its survival at the present day is a matter of marvel to the student of progress.

I should not conclude these prefatory words without expressing my obligations to many scientific friends whom I have from time to time consulted with advantage; and I must especially record my obligation to Mr. Robert Scott, F.R.S., who has on many occasions been my guide to the valuable materials available in the library of the Meteorological Office.

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