ME A 



MEC 



2. The geometrical mean of two quan- 

 tities is the square root of the product of 

 those quantities. In Geometry, the term 

 is synonymous with mean proportional, 

 and it is a quantity, such that, if placed 

 between two given quantities, a series of 

 three continued proportionals would be 

 formed ; in other words, the first of the 

 two given quantities is to the mean pro- 

 portional as this is to the other given 

 quantity. 



3. The harmonical mean is a number, 

 such that, the first and third terms being 

 given, the first is to the third as the dif- 

 ference of the first and second is to the 

 difference of the second and third, the 

 second being the harmonical mean. 



MEAN SUN. Solar days are not 

 equal in duration ; a clock regulated by 

 the sun would, consequently, need fre- 

 quent adjustment. To avoid this, an 

 imaginary or mean sun is supposed to 

 move regularly round the equator in the 

 same time as that in which the true sun 

 moves irregularly round the ecliptic. 

 Such a time represents a mean solar day, 

 and it is the average of all the apparent 

 solar days in a year. See Equation of Time. 



1. Mean Noon. This is an expression 

 connected with the above fiction of a 

 mean sun; and the term " mean" is here, 

 as above, opposed to "apparent" or 

 ** real." Thus, apparent noon is the real 

 or true noon, when the true sun, the sun 

 which appears, is on the meridian ; 

 whereas mean noon takes place when the 

 mean sun, the average imaginary sun, 

 which does not appear at all, is on the 

 meridian. 



2. Mean Moon. The still greater ab- 

 sence of uniformity in the actual motion 

 of the moon induced astronomers to em- 

 ploy an imaginary or mean moon, in the 

 same manner as the " mean sun" above 

 mentioned. This mean moon is made 

 to move uniformly in the equator, or in 

 the ecliptic, as required. The Kalendar 

 Moon is another fictitious moon, em- 

 ployed for regulating the finding of 

 Easter ; this moon is generally a day or 

 more distant from tlie mean moon. 



3. Mea7i Time. Equal or mean time 

 is that which is reckoned by a clock, 

 supposed to indicate exactly twenty-four 

 hours, from twelve o'clock on one day to 

 twelve o'clock on the next day. Appa- 

 rent or real time is that which is mea- 

 sured by the apparent motion of the sun 

 in the heavens, as indicated by a meri- 

 dian line, or sun-dial. 



4. Mean Anomaly of a Planet. Its 



212 



angular distance from the aphelion or 

 perihelion, supposing the planet to re- 

 volve in a circle with its mean velocity. 



5. Mean Conjunction or Opposition. 

 The mean place of the sun when in con- 

 junction with, or opposition to, the mean 

 place of the moon in the ecliptic. 



6. Mean Distance of a Planet from the 

 Sun. An arithmetical mean between 

 the planet's greatest and least distances ; 

 or, the semi-transverse diameter of its 

 orbit. 



MEANS and EXTREMES, In any 

 proportion the first and fourth terms are 

 called the extremes, the second and third 

 the means, and the product of the former 

 is equal to the product of the latter. Thus, 

 in the proportion 15 .•20 : ; 21 ; 28, 

 since the two ratios are equal, we have 



15 _ 21 



20 ~" 28' 

 and, if we multiply each of these equals 

 by 20 X 28, we have 15 x 28 = 20 x 21, 

 or the 1st x 4th = 2nd x 3rd. See 

 Proportion. 



MEASURE OF A NUMBER. One 

 number is said to be a measure or a factor 

 of another, when it divides it exactly, 

 without a remainder. Thus, 1, 2, 3, 4, 

 6, 12, are all measures or factors of 12. 

 Unity, however, is not generally named 

 among the divisors of a number. 



1. Greatest Common Measure. Any 

 number which divides without remainder 

 each of two or more numbers, is said to 

 be a common measure or common factor 

 of those numbers ; and, of course, the 

 greatest number which so divides them 

 is their greatest common measure. Thus, 

 3, 5, 15, are each of them common mea- 

 sures of 30 and of 45, and 15 is their 

 greatest common measure. 



2. Measure, in Geometry. A magnitude 

 or quantity assumed as a unit, and em- 

 ployed to express the relations of other 

 magnitudes or quantities of the same " 

 kind. Euclid defines the measure of a 

 quantity to be that which, being repeated 

 a certain number of times, becomes equal 

 to the quantity measured. 



MEASURES (in Geology). A term 

 sometimes employed as synonymous with 

 beds or strata. 



MECHANICAL CURVE. This is now 

 called the transcendental curve. It is a 

 curve in which the relation between the 

 abscissa and the ordinate cannot be ex- 

 pressed by an algebraic equation. 



MECHANICAL ORIGIN, ROCKS 

 OF. A term applied to rocks composed 

 of sand, pebbles, or fragments, to distin- . 



