28 REPORT OF NATIONAL MUSEUM, 1893. 



iiess, wliifli are necessary for use in connection witli tbe device for diist- 

 proofing-. 



The device for diist-i)rooflng is dependent upon a double door and a 

 double system of rubber tubing. The system by which the double 

 doors are made is shown in the accompanying- sketch (Fig. 2). These 

 doors are separated by an air space of 2 inches. The inner one is of 

 soft wood, paneled, and lined with zinc. The outer one is of hard wood, 

 paneled. The pressure against the rubber tubing, which is necessary 

 for absolute tightness, is secured by three sets of stubs and plates, at 

 the bottom (Fig. .'>), and by two bolts, one in each corner above. These 

 are so shaped that, when pressed, they have the effect of wedges (Fig. 4). 



The outer door has the same system of stubs and plates, and a rod 

 lock of the ordinary type, fitted with a Yale key for the greater security 

 of the precious contents. 



This case has been used for some months and has proved thoroughly 

 satisfactory, being practically air tight, while its construction is such 

 that it will doubtless be as good fifty years hence as it is now. 



Improvements have also been eftected in the construction of the unit 

 drawers. At first these were joined at the corners by dovetailing. 

 This proved unsatisfactory, and the device of " fingering '' was substi- 

 tuted. (PI. 8, tig. 2.) The fingered corners have been secured in 

 two ways, one by wooden dowels, the other by the ordinary process of 

 gluing. The doweled trays were exceedingly strong and satisfactory, 

 but it has been more convenient to use the other method and this is 

 now exclusively employed. 



The bottoms, which are inserted in grooves about a (piarter of an inch 

 from the bottom of the tray, are of three kinds: 



(1) Of pineK)r of poi)lar, seven-eighths of an inch in thickness, for the 

 minerals and heavy specimens and three eighths of an inch for light 

 specimens. 



(2) Of " three-ply veneer,'* such as is used for the seats of chairs and 

 for the lining of cars and in other kinds of cabinetwork. These are 

 made of three layers of very thin, straight-grained wood glued together, 

 the central layer being of pine, the outer layers of ash, maple, or other 

 hard wood. These layers are so adjusted that the grain of the 

 center layer runs at riglit angles with that of the two outer layers. 

 They are solidly glued together under heavy pressure, the thickness 

 of the whole not exceeding a quarter of an inch. Although sonieM'hat 

 more ex])ensive than the plain wooden bottoms, they are stronger and 

 very nnich lighter and have the positive advantage of never cracking 

 or shrinking. The i)lain bcttoms, it has been found, often shrink away 

 from their attachments to the sides of the drawer, even when thoroughly 

 kiln-dried lumber is used. INIany thousands of "three ply" bottoms 

 are in use, and they have satisfactorily stood the test of hard use for 

 ten years or more. 



