474 



PHYSICAL SCIENCES. 



Impetus, vis viva, or living force, is equal to the mass of a body multi- 

 plied by the square of the velocity with which it is moving, and is the 

 true measure of work or labour. For if a weight be raised 10 feet, it will 

 require four times the labour to raise an equal weight 40 feet. If both 

 these weights be allowed to descend freely by their gravitation, at the end 

 of their fall their velocities will be as 1 to 2 ; that is, as the square roots 

 of their heights ; but the effect produced will be as their masses multiplied 

 by 1 and 4 ; but these are the squares of their velocities : hence the impetus 

 or vis viva is as the mass into the square of the velocity. 



Thus impetus is the true measure of the labour employed to raise the 

 weights, and of the effect of their descent, and is entirely independent of 

 time. Now heat is proportional to impetus, and impetus is the true 

 measure of labour. In percussion the heat evolved is in proportion to the 

 force of the impetus, and is thus measured by labour. 



Travail is a word used in mechanics, to express that work done is equal 

 to the labouring force employed. The work done may be resistance 

 overcome or any other effect produced, while the labouring force may be a 

 horse, a steam-engine, wind, falling water, &c. 



NOTE 223, p. 313. When a stream of positive electricity descends 

 from P to , fig. 72, in a vertical wire at right angles to the plane of the 

 horizontal circle A B, the negative electricity ascends from n to P, and the 

 force exerted by the current makes the north pole of a magnet revolve 

 about the wire in the direction of the arrow-heads in the circumference, 

 and it makes the south pole revolve in the opposite direction. When the 

 current of positive electricity flows upwards from n to P, these effects are 

 reversed. 



NOTE 224, p. 314. Fig. 73 represents a helix or coil of copper wire, 



Fig. 72. 



Fig. 73. 



terminated by two cups containing a little quicksilver. When the positive 

 wire of a Voltaic battery is immersed in the cup p, and the negative wire 

 in the cup n,' the circuit is completed. The quicksilver ensures the con- 

 nection between the battery and the helix, by conveying the electricity 

 from the one to the other. While the electricity flows through the helix, 

 the magnet S N remains suspended within it, but falls down the moment 



