690 KANSAS CITY REVIEW OF SCIENCE. 



all the millions who lived before Newton, not one knew this fact to be valuable. 

 But it is one of the most important elements of human knowledge, since none can 

 learn much of nature, nor understand what kind of a world they are living in, 

 nor comprehend the solar system, without first knowing all the results derived 

 from this great fact. Let us see what it enables man to know. First, the veloc- 

 ity, 386 inches per second, acquired at the close of the first second of fall, is a 

 measure of the gravity of the earth. It shows that a mass of matter of the size, 

 shape and density of the earth, attracts with a force which, acting one second on 

 bodies near its surface, is able to impart to them that velocity. For ordinary pur- 

 poses of life we express gravity in terms of weight, and use scales graduated to 

 arbitrary standards to compare the force of attraction of the earth on different 

 bulks of matter. We take a piece of iron and say, by agreement, that it is one 

 pound. That is, the mass of the earth attracts it with a force the measure of 

 which we call a pound. Now if we let the iron fall from a high building, we 

 shall find that at the end of a second it will be in motion with a velocity of 

 thirty-two and one-sixth feet per second, therefore it is plain that if the nations 

 of the world agree to call this velocity generated in one second a measure of the 

 earth's attraction, scientists can write letters from one country to another, and 

 speak of gravity in terms of motion as well as they can in terms of weight. And 

 physicists all over the world have made such agreement, and have adopted one 

 second as the unit of time, and velocity of 386 inches as a measure of gravity 

 exerted by the whole mass of the earth. But this method of measuring gravity 

 is used only in astronomical and physical research into the mysteries of the uni- 

 verse, the plan of expressing attraction in terms of velocity being preferable. 



When clocks were perfected, it was found that one keeping exact sidereal 

 time in London would lose if taken to the equator. Or, carry a clock that keep 

 precise time at the equator, either north or south, and it will gain ; the further we 

 recede from the equator, the faster the clock will run. These facts seemed inex- 

 plicable until a clock that kept accurate time on the surface of the earth was 

 taken into a deep mine and also found to gain. It is evident that the reason 

 why a clock gains as it is carried toward the poles, is because in so doing we 

 draw a little nearer the centre of the earth. Then the idea came into men's 

 minds that gravity is variable ; and that its variation is connected in some way 

 with distance from the earth's centre. And by pendulum experiments it is found 

 that if we could reach the poles of the earth, we should be thirteen miles nearer 

 its centre than if at the equator. Hitherto philosophers had thought gravity con- 

 stant, but now they made a discovery of vast importance ; they found that it is not 

 the same everywhere on the earth. But then, if the pendulum varies its number of 

 vibrations with latitude, will not the velocity of a falling body vary in the same 

 ratio ? Delicate tests were made in many latitudes and at the equator, when 

 behold ! the further we go away from the equator, the greater velocity will a 

 falling mass acquire at the end of a second. But the whole increase in velocity 

 from the' equator to the poles, is only 1.97 inches; while the difference in dis- 

 tance from the centre of the earth is thirteen miles. The great question arose 



