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The history of Astronomy furnishes another illustration equally beautiful and instructive,—The Astronomer observed certain oscillations in the motions of Saturn and Jupiter; by Analogy he conjectured that this phenomenon was produced by the influence of a planet still more remote: a supposition which was happily confirmed by a telescopic experiment, in the discovery of Uranus, by Herschel.

But it is clear that such principles of research, and combination of methods, can rarely be applied in the investigation of remedies, for every problem which involves the phenomena of life is unavoidably embarrassed by circumstances, so complicated in their nature, and fluctuating in their operation, as to set at defiance every attempt to appreciate their influence; thus an observation or experiment upon the effects of a medicine is liable to a thousand fallacies, unless it be carefully repeated under the various circumstances of health and disease, in different climates, and on different constitutions. We all know how very differently opium, or mercury, will act upon different individuals, or even upon the same individual, at different times, or under different circumstances; the effect of a stimulant upon the living body is not in the ratio of the intensity of its impulse, but in proportion to the degree of excitement, or vital susceptibility of the individual, to whom it is applied. This is illustrated in a clear and familiar manner, by the very different sensations of heat which the same temperature will produce under different circumstances. In the road over the Andes, at about half way between the foot and the summit, there is a cottage in which the ascending and descending travellers meet; the former, who have just quitted the sultry vallies at the base, are so relaxed, that the sudden diminution of temperature produces in them the feeling of intense cold, whilst the latter, who have left the frozen summits of the mountain, are overcome by the distressing sensation of extreme heat.