18. One measure being thus made the standard of all the rest, they must be all equally invariable; but, in order to make this certainty perfectly¹ sure, the following precautions have been adopted. As the temperature was found to have an influence on bodies, the term zero, or melting ice, has been selected in making the models or standard of the metre. Distilled² water has been chosen to make the standard of the gramme, as being purer, and less encumbered³ with foreign matter than common water. The temperature having also an influence on a determinate volume of water, that with which the experiments were made, was of the temperature of zero, or melting ice. The air, more or less charged with humidity, causes the weight of bodies to vary, the models which represent the weight of the gramme, have, therefore, been taken in a vacuum.

19. It has already been stated, that the divisions of these measures are all uniform, namely by tens, or decimal fractions, they may therefore be written as such. Instead of writing,

1 metre and 1 tenth of a metre, we may write, 1 m. 1.
2 metre and 8 tenths, 2 m. 8.
10 metre and 4 hundredths, 10 m. 04.
7 litres, 1 tenth, and 2 hundredths, 7 lit. 12, &c.;

20. Names have been given to, each of these divisions of the principal unit but these names always indicate the value of the fraction, and the unit from which it is derived⁴. To the name of the unit have been prefixed the particles deci, for tenth, centi, for hundredth, and milli, for thousandth. They are thus expressed, a decimetre, a decilitre, a decigramme, a decistere, a deciare, a centimetre, a centilitre, a centigramme, &c. The facility with which the divisions of the unit are reduced to the same expression, is very apparent; this cannot be done with any other kind of measures.

21. As it may sometimes be necessary to express great quantities of units, collections have been made of them in tens, hundreds, thousands, tens of thousands, &c., to which names, derived from the Greek, have been given; namely, deca, for tens hecto, for hundreds; kilo, for thousands and myria, for tens of thousands; they are thus expressed; a decametre, a decalitre, &c.; a hectometre, a hectogramme, &c.; a kilometre, a kilogramme, &c.

22. The following table will facilitate the reduction of these weights and measures into our own.

 
The Metre, is 3.28 feet, or 39.871 in.
Are, is 1076.441 square feet.
Litre, is 61.028 cubic inch
Stere, is 35.317 cubic feet.
Gramme, is 15.4441 grains troy, or 5.6481 drams, averdupois.