722 IOWA DEPARTMENT OP AGRICULTURE 



ance rests on the small blade of a knife. The blade must not bind and 

 the pencils must swing on its edge with the least possible friction. The 

 two pencils are firmly rubber banded together, so that the two notches 

 form an opening nearly square between the pencils. Near each end 

 of the long pencil and on its notched side make a shallow hole by boring 

 into the wood with a hard pencil having a smooth, slightly blunt point. 

 In order that the balance may operate properly the apex of the notch 

 in the short pencil — that is, the point which rests on the knife-edge — 

 must be exactly midway between the holes in the ends of the long 

 pencil. Slide the short pencil along the other till it is in proper position, 

 then with the point of the knife-blade cut a mark common to both pen- 

 cils on the line where they touch. The rubber bands should prevent the 

 pencils from slipping when in use, but the mark will show if they do 

 slip. Two similar trays are hung from the ends of the long pencil. 

 They consist of circular pieces of stiffish cardboard about 2 inches in 

 diameter suspended by means of wires curved in fishhook form, the 

 points of the hooks resting within the holes previously made in the 

 pencil ends. The points of the hooks are bluntly and smoothly pointed, 

 so as not to bind in the holes. Beneath the trays the wires are bent 

 to hold the trays in level position, and are held to the tray by pieces of 

 gummed paper. It is better to cut the trays from the edge to the center, 

 then lap the two cut edges and glue them fast, thus making a shallow 

 dish. The wire beneath the tray is then preferably bent in circular 

 form. Balancing the tray hook on the finger shows the proper bending 

 of the wire where it pierces the cardboard to make the tray hang level. 

 A triangular piece of thin board, as a cigar-box cover, serves to hold the 

 knife blade in position. A block holds the knife handle. The knife is 

 set high enough to permit the trays to hang about one-half inch above 

 the surface on which the balance rests. For the purpose of showing 

 slight movements of the balance in exact weighing, a darning needle 

 is set in the top of the short pencil directly over the knige edge, and 

 at right angles to the pencil. A pin is placed in the board directly 

 over the point where the knife point pierces it and just above the end 

 of the needle. When the device is properly balanced the end of the 

 needle will stand at rest directly under the pin. It probably will not 

 balance until a staple of wire is placed over the pencils in proper position 

 on one side of the knife blade or on the other as a counterweight. This 

 completes the construction, and when properly mounted the balance 

 should oscillate freely by the slightest touch. As the trays are likely 

 to be interchanged in use it is advisable to mark each, placing corres- 

 ponding marks on the ends of the pencil at which the trays preferably 

 belong. 



In making seed tests, we may use common BB shots (whole and frac- 

 tional) for weights. This is because we wish to know only the compar- 

 ative weights of the pure seed and of the foreign seed and other impuri- 

 ties in the sample. Thus if we test an amount of seed, balancing ten 

 shots, and find that the weed seeds it contains just balance one shot, it 

 is evident that one-tenth of the original seed, or 10 per cent, consists 



