A Simple Decimal Scale for Biological Work. 65 



consideration may exceed the length chosen as a unit. 



Where that is likely to be the case, the same method can 



be applied by means of a folding rule, which offers no 

 practical difficulty in use. 



I have found that much time can be saved, and the work 

 done in a more trustworthy manner, by employing the scale 

 constructed in the following manner : — A large sheet of good 

 drawing-paper, hot pressed, is mounted upon a drawing- 

 board, which should have a clear margin of a foot on each 

 side. A iine needle is fixed vertically in the board at about 

 nine inches from the margin at the bottom left-hand corner 

 of the paper. On the right-hand side is to be fixed a metre 

 scale, with its greatest length parallel to the right-hand edge 

 of the paper. Then with a fine-edged HH pencil, and a 

 steel straight-edge kept in touch with the needle on the left 

 and with the decimal divisions on the scale to the right, 

 draw the lines across the paper. The decimetre divisions 

 can be carried right up to the left-hand edge ; but as the 

 lines are apt to be crowded if drawn at too close intervals, 

 it is well to stop the centimetre lines short of the edge. 

 The millimetres, of course, can only be drawn to a short 

 distance from the pattern scale. 



It is advantageous, in many cases, to draw the decimetre 

 divisions in some coloured ink with a fine ruling pen. These 

 are afterwards numbered at both ends of the paper, and in 

 the middle as well, so as to save trouble in identifying the 

 divisions. 



To use the scale : — Set a small straight-edge on, or below 

 and parallel to, the base line of the scale, and against that 

 work a set square to the right or the left until its edge cuts 

 the scale at the width chosen as the unit length. A fine- 

 pointed pair of dividers, set to the length whose proportion 

 is to be estimated, will, when applied to the scale along the 

 edge of the set square, show at once the relative length of 

 the distance under consideration, to two places of decimals, 

 or even to three, of the length chosen as a unit. 



I use, also, a scale divided into "vulgar fractions" for 

 VOL. XV. F 



