96 
THE YOUNG SCIENTIST. 
the same potassium salt transforms the per- 
chloride into the insoluble cyanide, well known 
under the name of Prussian blue. As the 
black lines of the engraving to be reproduced 
protect the prepared paper from the action of 
the sun, the perchloride remains unchanged 
in such places, and the drawing appears 
in the copy in sharp lines of a dark blue 
color on a white ground. The advantage of 
the process is that a positive picture is ob- 
tained immediately. 
How to Make a Drawing Board that 
will not Warp. 
The two sizes of drawing paper most com- 
monly used by engineers and architects are 
Double Elephant" and "Imperial." These 
are the most convenient sizes, and drawing 
A great many of the drawing boards kept for 
sale amongst artist's materials are made with- 
out reference to the sizes of drawing paper or 
their suitability for the T square. Most 
draughtsmen prefer to have drawing boards 
made of proper dimensions, and in a way that 
will keep them from warping. The following 
is a very excellent plan for the construction of 
a drawing board. The dimensions are given 
for a size to suit "Double Elephant" paper. 
It will be observed by the accompanying draw- 
ing that the board is made in seven pieces, 
each 4 inches wide and I4 inches thick, jointed 
together by square joints with glue only, but 
without tongueing and grooving. On the back 
are two strong cleats, 4 by 2 inches, set on 
edge near the ends of the board, and secured 
by screws from the face of the board, the heads 
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DRAWING BOARD. 
boards made to suit them admit of the use of 
other and smaller sheets of paper. A drawing 
board large enough to take "Double Ele- 
phant" paper, which in size is 40 by 26 inches, 
should be 42 by 28 inches, and this will allow 
the use of all sheets of drawing paper of sizes 
within these dimensions, as "Atlas," "Colum- 
bier," "Elephant," "Imperial," "Super 
Eoyal," "Royal," "Medium," and "Demy." 
of which must be sunk deep enough to avoid 
the points of the drawing pins. The holes 
made for countersinking the screws must be 
plugged up with wood glued in, the grain of 
which should run parallel with that of the 
board. The manner of doing this is plainly 
shown by the section, Fig. 2. The four cor- 
ners of the board and lower corners of the 
cleats should be slightly rounded, sc as to 
