Weighing the Earth

214 IMPROVEMENT ERA. upon plant growth. For this purpose hot houses were constructed of blue, green, red, and ordinary or white glass. The colorless glass allows the heat rays to pass through unob- structed, while the colored glass arrests and absorbs much of the heat. For this reason, on sunny days the temperature of the at- mosphere, in the house of ordinary glass, ranges about 5 degrees centigrade above that in the blue, 4 degrees centigrade above the green, and 2 degrees centigrade higher than the red. Plants grow tallest in the red light, and least in the blue. In the white light the plants are stocky and heaviest. Seeds also develop best in the ordinary sun light, but the blue light favors the development of more perfect flowers when the plants are placed in it just prior to blooming. Perfume is more abundant in flowers grown in red light. W. H. Homer, Jr., M. S.

Weighing the Earth. In the last issue of the Era mention was made of the strenu- ous efforts made by several great physicists to measure accurately in the laboratory the force of attraction L etween two masses of ordinary magnitude. It has been found that two small spherical masses of one gram each with their centers one centimeter apart attract each other with a force equal to one fifteen-millionth of a dyne.* The importance of this knowledge, which may at first seem insignificant, can hardly be overestimated. It enables us to calcu- late the attraction, in known units of force, due to a great variety of other distributions of attracting matter. Since we know the force between two unit masses one centimeter apart, and since we know the law by which this force varies with the distance, we have at once a means of calculating the attracting force between any two spherical masses at any distance ap ; rt. Moreover, if we know the magnitude of one mass and measure the force of attrac- tion between the two, we can calculate the magnitude of the other mass. This immediately suggests a universal method of measuring

• A dyne is defined as tnat force which, acting for one second on a mass of one gram originally at rest, will give it a velocity of one centimeter per second.