COMPOSITION OF FORCES 



1522 



COMPRESSED AIR 



and are distributed all over the world. The 

 characteristic of the family, which gave it the 

 name, is the head of small flowers, which early 

 botanists thought to be a single compound 

 flower. In reality, however, the flower-heads 

 of this family of plants are closely-clustered 

 groups of small flowers, either ray or tubular, 

 or both. ID many species the flowers in the 

 outer margin of the head are different in form 

 from the others, being conspicuous ray flowers, 

 the inner or disk flowers being tubular and less 

 conspicuous. 



Having so many flowers bunched together 

 in one head, compositae have a perfect ar- 

 rangement to make sure of pollination and 

 fertilization, and so they are ranked as the 

 highest seed plants. Many plants of this fam- 

 ily have clever means of seed distribution; 

 among such are thistles, dandelions, etc., which 

 have seeds that float away on the winds, and 

 the burdock, whose seeds are carried away on 

 the fur of animals. A great many plants of 

 this large family are cultivated for ornament 

 only, and some few serve as food plants. 

 Others have considerable medicinal value. 

 Many compositae are described in this work 

 under their individual titles. 



Related Subjects. Reference is directed to 

 descriptions of the following members of this 

 flower family in these volumes, and to other 

 kindred topics : 



Arnica Lettuce 



Aater Seed Dispersal (subtitle 



Botany under Seeds) 



Chrysanthemum Sunflower 



Dahlia Tansy 



Goldenrod Thistle 



COMPOSITION, kom po zish' un, OF 

 FORCES . If two boys stationed at a in the ac- 

 companying diagram kick a football at the same 

 instant, one with a force that would drive it to 

 6 and the other with a force that would drive 

 it to" c, the ball 



will move to d. G 6> 



That is, the result 

 will be the same 

 as it would have 

 been had the first 

 boy driven the 

 ball to c, then 

 the second driven 

 it to d. The 

 forces ab and ac, 



~ 



d 



FIG. 1. 



therefore, combined to move the ball along 

 the path to d. Such a combination is known 

 as the composition of forces, and the path over 

 which the body acted upon moves is called 



FIG. 2. 



the resultant; in the illustration ad is the 

 resultant. 



Illustrations of this law of forces are com- 

 mon. The principle is frequently employed 

 for supporting objects in a given position. In 

 the second figure 

 is a street lamp; |-- ^^ L 

 ab and cb are 

 wires extending 

 from standards on 

 the opposite sides 

 of the street, at- 

 tached to lamp at 

 b. Three forces 

 are acting upon the lamp; these are gravity, 

 which pulls it down, and ab and cb, which pull 

 it towards the standards to which they are 

 respectively attached. These forces are brought 

 into equilibrium when the lamp is at I. The 

 resultant for any number of forces acting 

 upon a body may be found by finding it for 

 any two, then for this resultant and a third 

 force, and so on. 



COMPRESSED, komprest', AIR. The pop 

 gun and air rifle are the simplest illustrations 

 of devices for using compressed air. They 

 operate on the principle that air can be com- 

 pressed and that when pressure is removed it 

 will expand. When the plunger is forced into 

 the barrel of a pop gun the air is compressed 

 until the pressure is strong enough to force 

 out the pellet. The air expands with force as 

 soon as the pellet begins to move, and it 

 throws it from the barrel with a force that 

 sends it several feet. The air rifle is prac- 

 tically a pop gun on a more elaborate scale. 



How Air Is Compressed. The machinery 

 necessary for compressing air includes a closed 

 vessel called the receiver, in which the air can 

 be confined, and a pump for forcing' the air 

 in (see AIR COMPRESSOR, for illustration). The 

 receiver is usually made of steel plates riveted 

 together, the same as in a steel boiler. Great 

 strength is required, since the pressure for 

 ordinary purposes may be as high as 300 pounds 

 to the square inch. For special purposes it 

 may reach 3,000 pounds to the square inch, 

 but where such high pressures are required, 

 special receivers must be constructed. 



A rise in temperature always accompanies 

 compression, and the heat generated is often 

 a serious obstacle. Engineers have invented 

 various devices for cooling compressed air, 

 but none is entirely satisfactory. The most 

 common device is that of running cold water 

 through a jacket around the pump cylinder. 



