34 



Graphic representation of the mechanical constitution 

 of the samples. 



In order to give a better general idea of the constitution 

 of the samples, a graphic representation of part of them has 

 been prepared. The different sizes of grains are taken as 

 abscissae , while the percentages are taken as ordinates. In 

 determining the units on the abscissa-axis the question of em- 

 ploying an algebraic or a geometrical progression might arise. 

 The former is inadequate for giving an exact representation of 

 the differences between the samples. As these contain ingre- 

 dients up to about bO^^^ we might for example mark off on 

 the abscissa-axis 50 units of 1™™ each, but it will easily be 

 seen that, as by far the greater number of the ingredients are 

 smaller than l^^, the curve would rise to an inconveniently 

 great height at the extreme left; and if we further consider 

 that the majority of the ingredients is smaller than 0"06"^°^ it 

 will be seen, that the curve for the first twentieth part of a mm. 

 must reach such a height, that it cannot be plotted on the same 

 sheet as will at the same time indicate the small quantities of 

 the larger ingredients present. 



By employing a geometrical progression to illustrale the 

 series of the grains, we get a representation which gives a far 

 more distinct idea of the constitution of the samples. The 

 most simple way is with 1™™ as a basis to mark off equidistant 

 divisions toward both sides , those to the right being desig- 

 nated 2, 4, 8™"i etc., while those on the left stand for 

 V2, V4, Vs™™ etc. Such a curve system is unlimited in either 

 direction, and may easily be employed for all sorts of mechan- 

 ical rocks, even the coarsest grained. 



The plotting of the curve for the ingredients greater than 

 ().5mm is easily accomplished by marking off as ordinates the 

 percentages of each size of grain. In washing, limits have been 

 employed, in the case of the finer ingredients, for the sizes of 

 grain which do not lit in with the numbers on the abscissa- 



