64 



THE ALUMNI JOURNAL. 



mass, and is always called a weight. 

 What is the difference ? Mass is the 

 quantity of matter (brass), weight is the 

 force with which the mass is drawn to- 

 ward the centre of the earth The mass 

 is the same whether it is at the sea-level, 

 or on a high mountain, ou the moon, the 

 sun, or anywhere else in the universe ; 

 mass is constant, is absolute. The weight 

 of the mass is less on a mountain than at 

 sea-level, less on the moon than on the 

 earth, more on the sun, and in fact is 

 purely relative, depending entirely upon 

 the position of the mass with reference to 

 other masses. When we say ' ' a body 

 weighs ten pounds," we really mean 

 "the body is drawn toward the earth 

 with ten times the force with which the 

 standard pound is drawn toward the 

 earth." The force is proportional to the 

 mass, and hence, if the force is ten times 

 the mass, it is ten times, and the mass of 

 the body is ten pounds. 



The metric absolute standard of mass 

 is the mass of one cubic decimeter of pure 

 water at its temperature of maximum 

 density. The practical standard is a cer- 

 tain piece of metal preserved at Paris and 

 called "the kilogram of the archives." 

 Again, it is not equal to the absolute 

 standard, but we know its error and so it 

 answers our purpose. 



Another standard that interests us, and 

 is very intimately connected with the 

 kilogram, is the liter — equal, theoreti- 

 cally, to the volume of one cubic deci- 

 meter ; practically it is the volume, at its 

 temperature of maximum density, of a 

 mass of water equal to the " kilogram of 

 the archives." The difference is very 

 slight, and can be neglected in all but 

 the most accurate absolute scientific 

 work. Similarly the cubic centimeter is 

 the volume of one gram of water at its 

 temperature of maximum density. 



Our immutable standard of a period of 

 time is one revolution of the earth ou its 



axis — a day. It is also our practical 

 standard. 



The "unit" is usually some fraction 

 of the " practical standard," and varies 

 with the quantity measured ; for exam- 

 ple, the kilometer, meter, millimeter and 

 micron (i-iooo m. m.) are various units, 

 any one being chosen according to the 

 length we wish to measure. Similarly 

 the day, hour, minute, second are units 

 of time. 



Mark the simplicity of this system. 

 We measure the dimensions in convenient 

 decimal units ; the unit area is the square 

 on the unit of length ; the unit volume is 

 a cube on the unit of length ; the unit 

 mass is the unit volume filled with mat- 

 ter of unit density (water at 4° .o C). 

 Specific gravity is the number of units of 

 mass of the substance in the unit vol- 

 ume, etc., etc. 



In practice it is found desirable to use 

 only certain units, for example ; we often 

 use the kilometer, the meter, centimeter, 

 millimeter, whereas hectometer, deka- 

 meter and decimeter are seldom used. In 

 mass we use kilogram (called simply 

 " kilo "), gram, but decigram, centigram 

 and milligram are much less used. In 

 volume we use the cubic meter, hectoliter, 

 liter and cubic centimeter (never called 

 milliliter). Specific-gravity multiplied 

 by volume gives us mass (weight in com- 

 mon usage); conversely, mass (weight) 

 divided by specific-gravity gives volume. 

 Compare for a moment these operations 

 with an attempt to get from cubic inches 

 to scruples and drachms, or vice versa. 

 In a word the metric system is homo- 

 geneous throughout and that one fact 

 ought to be sufficient to speedily accom- 

 plish its adoption. 



Let us see what is the status of the 

 metric system to-day and what are its 

 claims to our favorable consideration. In 

 the first place it should never be called 

 the ' ' French system ; ' ' the French first 



