192 THE MONTESSORI METHOD
ders in this set differ in height, the first being merely a little disk only a centimetre high, the others increase 5 millimetres each, the tenth one being 55 millimetres high. In the third set, the cylinders differ both in height and diameter, the first being 1 centimetre high and 1 centimetre in diameter and each succeeding one increasing 1/2 centimetre in height and diameter. With these insets, the child, working by himself, learns to differentiate objects according to thickness, according to height, and according to size.
In the schoolroom, these three sets may be played with by three children gathered about a table, an exchange of games adding variety. The child takes the cylinders out of the moulds, mixes them upon the table, and then puts each back into its corresponding opening. These objects are made of hard pine, polished and varnished.
Second. Large pieces in graded dimensions: — There are three sets of blocks which come under this head, and it is desirable to have two of each of these sets in every school.
(a) Thickness: this set consists of objects which vary from thick to thin. There are ten quadrilateral prisms, the largest of which has a base of 10 centimetres, the others decreasing by 1 centimetre. The pieces are of equal length, 20 centimetres. These prisms are stained a dark brown. The child mixes them, scattering them over the little carpet, and then puts them in order, placing one against the other according to the graduations of thickness, observing that the length shall correspond exactly. These blocks, taken from the first to the last, form a species of stair, the steps of which grow broader toward the top. The child may begin with the thinnest piece or with the thickest, as suits his pleasure. The control of the exer-