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motion, and the leading principles of the science of energy, by which Heat becomes related to other forms of motion, are discussed.

It may be well to state the basis on which the reasoning of this part has been founded. It has appeared to the Author that the foundation which involves the smallest amount of assumption is that adopted by Professor W. Thomson, namely the denial of the possibility of a perpetual motion of any kind, and it will be shewn in the sequel that the denial of one form of perpetual motion involves the principle of the conservation of energy,' while the denial of another form involves the principle of the dissipation of energy.'

In our ignorance of the ultimate constitution of matter it would thus appear that both the 'conservation of energy and the degradation or dissipation of energy' should be viewed as principles having very strong claims to recognition, and as increasing these claims every day by the new facts which their employment as instruments of research is constantly bringing to light.

A few words with regard to the mode in which the subject of temperature is viewed. Some eminent philosophers are of opinion that our methods of subdividing a range of temperature are to a great extent arbitrary, so that provided we always adhere to the same method we shall not be led into error.

Be this as it may, there can be no question that some methods of doing this are much more convenient than others; nay, even that one method, that by the air ther

mometer, enjoys such a pre-eminence of convenience that it may with propriety be termed the proper method of subdividing a range of temperature.

Starting with the assumption that there is a proper method of measuring temperature, it is shewn near the beginning of this work that even if we are ignorant of this proper method there is yet an advantage in employing an air thermometer. This advantage consists in the fact that we may use any permanent gas we choose for our air thermometer, and yet obtain results as nearly as may be identical with one another if all our instruments are read on the same principle; while, on the other hand, the indications of two thermometers filled with different liquids, and both graduated on the same principle, are not strictly comparable with each other. If we determine to prefer an air thermometer we still have to decide on what principle it ought to be read, and, while this principle is indicated at the commencement of the work, the reasons in favour of its adoption are not fully discussed until the end.

The author cannot omit the opportunity of acknowledging his obligations to several scientific friends for the suggestions and advice they have kindly given him ; more especially is he indebted to the organizing Secretary of this Series of Works at Oxford for much valuable assistance throughout the book.

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