SCIOPTICON MANUAL. 



PRESSURE BOARDS. 



Instead of the ordinary iron hinges, which only allow 

 the pressure boards to open from the line of contact, 

 two long leather straps, pierced with holes, may bo per- 

 manently attached to the lower board and hitched to 

 screw-heads on the upper board, allowing it to be in a 

 plane nearly parallel with the lower board, while the bag 

 of gas is between them and the weight bears on the 

 side opposite. These straps may be hitched up, from 

 time to time, as the gas is expended. By giving sufficient 

 length to these strap-hinges, the two bags for the mixed 

 jet may be placed one upon the other and subjected to 

 the same pressure. 



The three boards hinged together in the shape of the 

 letter Z, to receive a bag in each angle, as commonly 

 recommended, are not only heavy and expensive, but a 

 measure could hardly be devised more likely to give 

 unequal pressure. A long board extending from one 

 bag to the other, with the weight upon the middle, would 

 be better. 



The necessity of exactly equal pressure to be given to 

 the two gases used with the mixed jet, is not so absolute 

 as might be inferred from the way it is usually spoken 

 of. It is surely well to see that the bags are about equally 

 weighted. When two gas cylinders are used, one nearly 

 spent need not be mated with one fully charged. If, 

 however, the pressure in each is in excess of what is 

 needed, the stop-cocks are made to regulate the flow. 



As it is inconvenient to transport heavy weights from 

 place to place, traveling exhibitors may fill a box or bag 

 with brick or stone at each place of exhibition. This 

 expedient affords a steadier weight than to seat boys 

 upon the pressure-boards, and more continuous than can 

 be effected by clamping screws. 



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