48 ELEMENTARY GENERAL SCIENCE CHAP. 



demonstrate the difference in weight we must measure the weight 

 by a spring balance as in the last experiments. 



Similarly, because the earth is not a perfect sphere, but is 

 flattened at the poles, points at the surface of the earth in the 

 region of the tropics are at a greater distance from the centre 

 than points in the neighbourhood of the poles. Consequently 

 the weight of a mass situated on the earth in the tropics should be 

 less than the weight it wouljl have if it were moved into the polar 

 regions. This has been found to be the case ; a body weighing 

 191 ounces when near the equator will weigh 192 ounces if 

 moved near to the poles. These facts can be illustrated by a 

 simple experiment. 



EXPT. 49. Hang a piece of iron from a spring balance and 

 notice the weight indicated. The iron has a certain mass, or 

 consists of a certain quantity of matter. Bring a strong 

 magnet under the iron, and again notice the indication of the 

 pointer of the balance. Evidently the quantity of matter, or 

 mass, of the iron has not changed during the experiment, but 

 the attraction of the magnet causes the apparent weight to 

 increase. 



Masses are determined by means of the Balance. Though 



the reader may have seen the spring balance in use among 

 hawkers in the country, it is not a very common practice to use it 

 for estimating masses. The instrument generally in use for this 

 purpose is the balance or pair of scales. This simple apparatus 

 is best understood with the help of a few simple experiments : 



50. Balance a light stiff lath upon an edge of a 

 triangular block, or better, make a hole through a point above 

 its centre so that the lath will turn easily upon a stout 

 nail fixed in a wall or blackboard. Hang a mass by means of 

 a piece of thread upon the lath at any convenient distance on 

 one side of the pivot or fulcrum, and balance it with a mass of 

 the same amount on the other side. The distance of the wm.s.sv.s 

 from the fiilcrinn icill be found the same in each <-tixt>. 



EXPT. 51. Using the same lath supported on the nail at its 

 middle point, hang over it, at equal distances from the support, 

 two pans (which you can make out of pill boxes and cotton), 

 one on each side of the support. Put a mass of L'O grains in 

 one pan and ascertain how many must bo put into the other in 

 order that the lath may remain horizontal, or as we say, be 



