By means of such light-houses it would be easy to convey telegraphic messages either to vessels in distress, or for other purposes. It would simply be required to use the light itself or a subsidiary one to indicate a series of numbers corresponding to those in some known Telegraphic Dictionary. No danger of any mistake could arise during the few minutes thus employed, because any other vessel on counting the succession of obscurations would not only perceive that the light-house was telegraphing, but would also know the object of the message. A small apparatus might easily be contrived for the use of vessels, by which they might ask any questions necessary for their safety. Such means for ships sailing in company, or even for fleets, might enable them to proceed on their voyage during the night, and to communicate any orders even with greater facility than in the day. Sir David Brewster proposed in the _Edinburgh Philosophical Transactions_ a plan for distinguishing light-houses by optical means. The light transmitted through a thin film, when analysed by a prism, appears either single, or subdivided into two, three, four or more parts. Light-houses, therefore, might thus be distinguished from one another numerically. CHAPTER X. JURIES, ETC. A clear statement of the _principles_ on which each jury is to award prizes, should be placed before them. These principles ought to be well discussed, and in that discussion manufacturers should be invited to take a part. The first object of the jury should be to lay down rules by which these principles are to be carried out. Each class of the subjects to be rewarded will have its own rules. They will generally be few in number, and capable of being expressed in few words: some of these are suggested below, but merely by way of example. One of the most general rules will indicate the means by which the jury can ascertain the fact, that the material of the manufacture under consideration is truly the substance it is represented to be. For instance: some woven fabric is examined, professing to be made either entirely of wool, or wholly of flax. It may be quite true that experienced manufacturers and dealers, are able to detect any adulteration of either material by admixture with the other. But statements of facts made on authority, never possess the same weight with the public as those which are accompanied by information enabling any individual among that public to verify the fact for himself. The form of the fibre as shown by the microscope is one test. A more simple one is to burn some fibres in the flame of a candle. Every fibre which, when thus treated, produces the smell of burnt feathers, is animal matter of some kind, as wool, silk, horse-hair, &c. The burnt fibres of hemp, flax, cotton, and other vegetable matters have a totally different scent; a fact of which any one may readily assure himself by making the experiment.