The book of lantern ; being a practical guide to the working of the optical (1888)

THE BOOK OF THE LANTERN. 61 For the blow-through jet only one pressure board is required, and perhaps a better form cannot be used than a couple of thin boards hinged together with a simple ledge, or shelf, above, also on hinges, for the reception of the weights. But for the mixed jet another arrangement is necessary. In this latter case the two gases must be under equal pressure. Two pairs of boards, like those just described, may be used side by side, but they will occupy a great deal of space, and a double supply of weights will become necessary. This last objection is, perhaps, more cogent than the first, for in these days of patent weighing machines the old-fashioned 56-pounders are becoming quite scarce. It is, therefore, much better to use a pair of boards, so constructed that they will embrace both bags—one set of weights being all-sufficient for the two. A further advantage of this arrangement is that both bags are under the same pressure, and additional weights put on during performance cannot affect one bag to the prejudice of the other. Dangerous accidents have before now happened when independent pressure boards have been in use, from the weights having been in- advertently shifted from one gas, while the other has been left under full pressure. To Mr. Maiden is due the credit of having first designed a pair of pressure boards which would hold both bags under one set of weights, and the arrangement which he suggested is that which is now commonly adopted by opticians, and figured in their catalogues. I myself use a modified form of these pressure boards, and as I have given much thought to the subject before