A child notices that his bedroom is smaller than the family parlor, that to-day is warmer than yesterday was, that iron is much heavier than wood and less easily marked by a blow. The child becomes a well grown boy before he paces the length and breadth of rooms so as to compare their areas and add to his mensuration lesson an example from home. If instead of pacing he were to use a foot-rule, or a tape-line, so much the better. About this time he may begin to observe the thermometer, noting that within five hours, let us say, it has fallen eight degrees. As a child he took account of bigness or smallness, lightness or heaviness, warmth or cold; now he passes to measuring their amount. In so doing he spans in a few years what has required for mankind ages of history. When corn and peltries are bartered, or axes and calumets are bought and sold, a shrewd guess at sizes and weights is enough for the parties to the bargain. But when gold or gems change owners a balance of delicacy must be set up, and the moral code resounds with imprecations on all who tamper with its weights or beam. Perhaps the balance was suggested by the children’s teeter, that primitive means of sport which crosses one prone tree with another, playmates rising and falling at the ends of the upper, moving trunk. In essence the most refined balance of to-day is a teeter still. Its successive improvements register the transition from merely considering what a thing is, whether stone, wood, oil or what not, to ascertaining just how much there is of it; or, in formal phrase, to make and use an accurate balance means passing from the qualitative to the quantitative stage of inquiry. Before Lavoisier’s day it was thought that any part of a substance which disappeared in burning was annihilated. Lavoisier carefully gathered all the products of combustion, and with scales of precision showed that they weighed just as much as the elements before they were burned. He thus laid the corner-stone of modern chemistry by demonstrating that matter is invariable in its total quantity, notwithstanding all chemical unions or partings. Phases of energy other than gravity are now measured with instruments as much improved of late years as the balance; they tell us the great truth that energy like matter is constant in quantity, however much it may vary from form to form, however many the subtle and elusive disguises it may wear. Foot and Cubit.