50 THE AMERICAN MONTHLY [Feb., 



3. Draw a scale each side of the center, at CC ; be- 

 gin one incli from the center, or nearer if you wish. Be 

 sure tliat both sides are exactly alike. Make the notches 

 1-16 inch apart. Use a very sharp-pointed pair of 

 compasses to mark with, but if you have none, paste a 

 white paper over the top of your table and mark with a 

 pen and ink. Some prefer the paper. 



To use the device, place a slide upon tlie table as indi- 



cated by the dotted lines, with two of the corners upon 

 the scale, being always sure that the opposite corners are 

 equally distant from the centre. For example, if the 

 slide measures three inches from one corner to the op- 

 posite corner, place the opposite corner at li on tlu' 

 scale. Press down the clips tirmly, and you are lead)' 

 to proceed. 



Be accurate; take a little extra time at first nntil you 

 become accustomed to use it. 



