56 OPTICAL PROJECTION 



hinged end, and extends between the boards as shown. They 

 are often made on what is called the * skeleton ' plan, for 

 lightness in travelling ; a mere framework being covered either 

 with a trellis of thin hoop-iron, or with a sacking laced up 

 as old-fashioned bedsteads used to be. Solid boards, however, 

 need not be much heavier, are much cheaper, and for home 

 or at an institution are to be preferred ; in the latter case they 

 should be made with a flat board between. When a piece of 

 canvas is used to separate the bags, it is best to pass a buckled 

 strap round the back of the bags from top to bottom, to pre- 



FIG. 33. Double Boards 



vent them slipping backwards till the bags have flattened out 

 a little, after which there is no tendency to this. A pair with 

 a board between needs no straps. 



It is necessary to raise the front or hinged ends of the 

 boards considerably wliile the bags are full, or the top will 

 be too steep for the weights to press upon. From this arises 

 a risk, not of explosion, but of a vexatious accident which 

 has been known to occur. The middle board must be nearly 

 level with full bags, as in fig. 34, and it will be seen that in 

 this position the weights w are supported well at c within the 



