JEFFERY ON MECHANICAL ANALYSIS OF SOILS. 89 



p,i'aiiis ill the ;4i'(»u|i, which is mat hcniai icall v imoiicct. ^iiviii;:: an 

 averaj^v jiiohabl.N too small. 



These and other difticiilfies led Prof. Kinji', in is;»4, to iindeitake a line 

 of stnd.v which i-esulted in his method foi' detenninim: what he has 

 been })Ieased to call the ellective size of soil <;rain. 



It had already h«'en asserted that the rate of How lhrou;^h sand was 

 dependent njion the size of <>rain. I'oiseuille had demonstrated, or 

 it was thought he had, that the ll()w of jicpiids and j^ases thi-oujih 

 capillaiy lubes was i)roi»ort ional to the pressure. 



It was found, further, that the weiji,lit of a j^iven soil that could 

 be packed in a j^iven space was practically constant, and that therefore 

 for a <;iv(Mi soil the ])ore space was constant; and that fairly constant 

 results were to be had in aspirating; air throu;;h a juiven soil. 



At this time l*rof. Kin^- conceived the idea that the size of ^rain of 

 a given soil might be determined in this way. The opinion of I*rof. 

 Slichter, I'rofessoi- of A]>])lied Mathematics at the Tniveisity of Wis- 

 consin, was asked as to tlie i»raclicability of the plan, and so tlioroughly 

 did he believe in its possibility that he immediately set about develop- 

 ing a formula for making the necessary conii)utation. He was so 

 successful in this that nu^asui-ements of soil giains have been made 

 with an a])proach to accuracy that was at first hardly hoped for; ami it 

 is hojH'd that greater accuracy may still be secured as the method is 

 further studied. 



The ai)]'aratus used is tlie one before you [see plate], and is essentially 

 the same as the one first used by Prof. King.. It differs only in comijact- 

 ness of form. It consists of a soil tube (a), an aspirator of wdiich (b) 

 is the tank, and (c) the bell. The bell is lifted by the weight (d) by 

 means of a cord (e) jtassing over the jjulley (f). The tube (g) passes 

 from the soil tube into the aspirator, and by way of this, air is drawn 

 by the aspirator through the soil in the tube (a). The dial (h) performs 

 the function of air meter, and is calibrated for one litre in this case. 

 The minometer oi- ]uessure gaugf* (j) indicates the difference of }»ressui'e 

 between the ends of the soil column in the tube, or conventionally speak- 

 ing it indicates the pressure under which the air is drawn through the 

 soil. It is connected with the air chahiber (n) above (a). The height, 

 cross section, inside volume, and weight of the tube (a) are carefully 

 determined. 



Before taking up the mode of operating the apparatus, let us con- 

 sider briefly the points in Prof. Slichter's theory leading to his formula. 



He considers a hypothetical soil having apju-oximately spherical 

 grains of nearly uniform size. The least i)ore space possible in such 

 a soil occurs when the grains are so arranged that the element of volume 

 is a polyhedron with face angles of sixty or one hundred twenty 

 degrees. In this case each grain of soil is in contact with other grains 

 at twelve points, and the pore s](ace ecpnils twenty-five and ninety-five 

 hundredths per cent. When the grains touch each other at eight 

 points, the element of volume is a cube, and the pore space equals 

 forty-seven and sixty-four hundrt^lths per cent. Between these limits 

 of arrangement we have a similar range of pore space, and the face 

 angle of the element of volume will be a function of the pore space 

 and thus may be determined from it if the angle be known. The angle 

 12 "^ 



