HORSE POWER 



2843 



HORSESHOE CRAB 



minute, the result divided by 33,000 will give 

 the horse power of the cylinder. An indicator 

 on the engine tells the pressure of the steam 

 upon a square inch of the piston, and this 

 multiplied by the area of the piston in square 

 inches produces the force exerted. For a gaso- 

 line engine exactly the same method may be 

 used, except that there is no indicator and. 

 the pressure must be estimated. The latter 

 varies from 45 pounds to 125 pounds to the 

 square inch. For an ordinary engine 60 pounds 

 is considered a conservative figure; for a 

 sleeve-valve engine, 85 pounds. In a four- 

 cycle engine the number of strokes is found 

 by dividing the number of revolutions of the 

 crank shaft per minute by two, since there are 

 two revolutions to every cylinder explosion. 



S. A. E. Rating. This is the method 

 approved by the Society of Automobile En- 

 gineers, formerly known as A. L. A. M. (Asso- 

 ciation of Licensed Automobile Manufactur- 

 ers). It is based on indicated horse power, 

 but assumes that in every engine a piston 

 travels 1,000 feet in a minute and that the 

 pressure on it is about 70 pounds per square 

 inch. It therefore takes into consideration 

 only the area of the piston, and determines 

 the horse power by multiplying the square of 

 the diameter (bore) of the cylinder by the 

 number of cylinders, and dividing the result 

 by 2.5. Most automobile engines are now built 

 with a longer stroke than when this method 

 was adopted, and they are very much under- 

 rated by it. As a result it is the method 

 generally employed when engines are taxed 

 according to their power, for the amount of 

 the tax is less. 



Brake Horse Power. This is sometimes 

 called the effective horse power. Instead of 

 being the force exerted at the cylinders it is 

 that which the engine is capable of applying 

 to its work. For steam engines it is from 

 eighty per cent to eighty-five per cent of the 

 indicated horse power. It is determined by 

 tests with a dynamometer. It should not be 

 confused with power exerted by the brakes 

 of an automobile. Obviously this method of 

 rating engines is the most satisfactory, and it 

 is now quite generally employed. 



Comparison oj the Three Methods. A cer- 

 tain automobile engine of standard type has 

 four cylinders, each with a bore of 3.5 inches 

 and a stroke of 5 inches. Its crank shaft makes 

 2,500 revolutions per minute. The process of 

 discovering indicated horse power may be ex- 

 pressed in an algebraic formula, as follows: 



in which the letters stand for mean effective 

 pressure per square inch, diameter of bore in 

 inches, length of stroke in feet, revolutions per 

 minute, and number of cylinders. Assuming 

 60 pounds to be the pressure, the horse power 

 of this engine will be 



1 



3.1416X3.5X3.5 5 2500 



4 X 12 X 2 X4X 33,000 



=36.4 



According to S. A. E. rating, the engine will 

 develop 



3.5X3.5X4 



B2C , ^ , D2 



- (often written -- 



2.5 



= 19.6 



The brake horse power actually developed by 

 this engine is 39.5, which shows that in this 

 instance more than 60 pounds per square inch 

 is the pressure on the pistons. See STEAM; 

 AUTOMOBILE; PHYSICS. C.H.H. 



HORSE-RADISH, a species of water cress, 

 originally a native of Central Europe and 

 Western Asia, but now cultivated everywhere 

 and naturalized in most parts of the world. 

 Its roots, which are long and cylindrical, are 

 extremely pungent in taste, and when grated 

 and mixed with vinegar are served as a relish 

 with cold meats, oysters, etc. They are also 

 used medicinally as a stimulant and to pro- 

 mote digestion, though physicians do not ad- 

 vise their use in this way. As almost any 

 small portion of the root will grow if left in 

 the ground, the 

 plant is very dif- 

 ficult to uproot 

 when once it has 

 become estab- 

 lished. 



HORSESHOE 

 CRAB, also 

 known as KING 

 CRAB and HORSE- 

 FOOT, is a large 

 member of the 

 crustacean fam- 

 ily, found along 

 the Atlantic 

 coast of North 

 America, from' 

 Maine to the 

 West Indies. It 

 often reaches a 

 length of near l y HORSESHOE ' CRAB 



two feet and is called horseshoe crab because 

 of the shape of its shell. It lives in deep 

 waters, feeding on worms and shellfish, coming 



