M I C 



MECHANICAL Powers. 



The simple mechanical powers into which more complex machines arpr 

 resolved, are these- 1. The Lever; 2. The Wheel and Axle; 3. The 

 Pulley ; 4. The Inclined Plane ; 5. The Wedge ; 6. The Screw ; lor which 

 see the respective heads. 



The following property is common to all the mechanical power?, and 

 indeed to all machines whatsoever ; it is known by the name of the prin- 

 ciple of Virtual velocities. 



If a power and weight sustain each other on any machine, and the 

 Whole be put in motion, the velocity of the power : the velocity of the 

 weight : : the weight I the power. In other words, if the equilibrium 

 of a machine be disturbed by a quantity indefinitely small, and if the ve- 

 locity of each force be multiplied into its quantity, the sum of these pro- 

 ducts, reckoning the forces which are in opposite directions positive and 

 negative with respect to one another, will be equal to nothing. 



MERCURY. See Planets, elements of. 

 MERCURY, Phases of. See J'enus. 

 .: MERCURY, Transit of. See Transit. 



MERIDIAN, Transit of a star or planet over. See Time, 

 MERIDIAN, to place a Telescope in. See Telescope. 

 MERIDIONAL parts. See Projection. 

 METEOROLOGY. See Atmosphere; Rain. 

 MICROSCOPE. ( Wood, Coddington.) 



1. The visual angle of an object, when seen through a single micro* 

 scope, is to its visual angle, when seen with the naked eye at the least 

 distance of distinct vision, as that least distance to the focal length of 

 the glass. 



Let c least distance of distinct vision, which in common eyes is about 

 7 or 8 inches, F the focal length of the lens ; then 



Ex. Let F .02 inch, and c 7 inches, then the number of times that 



rj 



the length of the object is magnified = -^ =: 350 j and the number of 

 times the surface is magnified 122500. 



2. Required the same hi the double microscope. 

 176 



