Appendix, 349 THE PREFACE. _The difference that appears in the several processes of brewing, though executed with the same materials, by the same persons, and to the same intent, is generally acknowledged. The uneasiness this must occasion to those who are charged with the directive part of the business, cannot be small: and the more desirous they are of well executing the duty incumbent on them, the greater is their disappointment, when frustrated in their hopes. To remove this uncertainty, no method seems preferable to that of experiments, as it is by this means alone, any art whatever can be established upon a solid foundation: but these require caution, perseverance, and expence; they must be multiplied and varied both for the same and for different purposes. The operations of nature elude superficial enquiries, where we have few or no principles for our guides, many experiments are made, which tend only to confound or deceive. Effects seen, without a sufficient knowledge of their causes, often are neglected, or viewed in an improper light, seldom faithfully reported, and, for want of distinguishing the several circumstances that attend them, many times become the support of old prejudices, or the foundation of new ones._ _Whoever is attentive to the practical part of brewing, will soon be convinced that heat, or fire, is the principal agent therein, as this element, used in a greater or less degree, or differently applied, is the occasion of the greatest part of the variety we perceive. It is but a few years since the thermometer has been found to be an instrument sufficiently accurate for any purposes where the measure of heat is required. And, as it is the only one with which we are enabled to examine the processes of brewing, and to account for the difference in the effects, a theory of the art, founded on practice, must be of later date than the discovery of the instrument that guides us to the principles._