162 THE DISCOVERY OF THE CHILD constant temperature (75°C), I place some of it in gradually differ- ing quantity into every vessel and then fill up the rest with cold water at 15CC. Or I prepare receptacles in equal pairs. Although the temperatures change quickly during the operation, the exercise nevertheless serves to give a certain amount of exactness. A series of substances which differ in their heat-conducting properties, like wood, felt, glass, marble, iron, is used for more delicate exercises. IMPRESSIONS OF WEIGHT For the education of the baric sense rectangular tablets 6 cms. by 8 cms. in area and | cm. thick, made of three different qualities of wood—wistaria, walnut and fir—are used; they weigh respectively, gr. 24,18,12; that is, they differ by gr. 6. They ought to be very smooth and brightly polished, so that all roughness is removed; the natural colour of the wood remains. The child, whilst observing the colour, knows that they have different weights, which gives him a check for his exercise. He takes two tablets into his hand, places them on the palm with the fingers extended, and executes an up-and-down movement in order to gauge the weight; such a movement should, little by little, be made unnotice- able. The child is advised to proceed to make comparisons with his eyes closed; so he grows accustomed to acting by himself with great interest, in order to see if he guesses. The above-mentioned methods refer to a technique which is necessary for reaching sufficient exactitude in the estimation of weights. It is absolutely necessary to place the object lightly on the skin, avoiding any feeling of temperature (hence the wood), in order to obtain a true and exact idea of the weight of the parti- cular object. Moving the hand up and down alters the weight by altering the atmospheric pressure which is bearing down on it and by making the weight more appreciable. This method of ' sub- weighing ' is instinctive* but in order to secure a more exact valuation of the weight of the object it is necessary to make these movements as small as possible.