xxx vi INTRODUCTORY CHAPTER. 



Besides these units, there are others on which a few words 

 may be said, as the units before referred to are implicated. The 

 Unit of Time or Duration is the same for all civilised coun- 

 tries. The twenty-fourth part of a mean solar day is called 

 an hour, and this contains sixty minutes, each of which is divided 

 into sixty seconds. The second is universally used as the unit 

 of duration. 



Having now units of space and time, we are in a position to fix 

 upon a Unit of Velocity. The units of velocity adopted by different 

 scientific writers vary somewhat ; the most usual, perhaps, in regard 

 to sound, falling bodies, projectiles, &c., is the velocity of feet or 

 metres per second. In the case of light and electricity, miles or kilo- 

 metres per second are employed. 



We have next the Unit of Mechanical Work. In this country the 

 unit of mechanical work is usually the foot-pound, viz. the force 

 necessary to raise one pound weight one foot above the earth in 

 opposition to the force of gravity. A horse-power is equal to 33,000 Ib. 

 raised to a height of one foot in one minute of time. In France the 

 kilogrammetre is the unit of work, and is the force necessary to 

 raise one kilogramme to a height of one metre against the force of 

 gravity. One kilograramelre== 7*233 foot-pounds. The cheval vapeur 

 is nearly equal to the English horse-power, and is equivalent to 

 32,500 Ib. raised to a height of one foot in one minute of time. 

 The force compatent to produce a velocity of one metre in one 

 second, in a mass of one gramme, is sometimes adopted as a unit 

 of force. 



Unit of Heat. These units vary : the French unit of heat, called 

 a calorie, is the amount of heat necessary to raise one kilogramme 

 (2'2046215 Ib.) of water one degree Centigrade in temperature ; 

 strictly from C. to 1 C. In this country we sometimes take one 

 pound of water and 1 Fahrenheit as the units ; sometimes one pound 

 of water and 1 C. 



Thermometric degrees. The value of different thermornetric 



