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 



