CHAPTER II.
MEASUREMENTS Sec.1.

Metrology: Lord Kelvin's view of its necessity-- Its definition Sec.2.The Measure of Length: Necessity for unit-- Absolute length--History of Standard--Description of Standard Metre--Unit of wave-lengths preferable--The International Metre Sec.3.The Measure of Mass: Distinction between mass and weight--Objections to legal kilogramme and its precision--Possible improvement Sec.4.The Measure of Time: Unit of time the second--Alternative units proposed--Improvements in chronometry and invar Sec.5.The Measure of Temperature: Fundamental and derived units--Ordinary unit of temperature purely arbitrary--Absolute unit mass of H at pressure of 1 m.

of Hg at 0 deg.

C .-- Divergence of thermometric and thermodynamic scales--Helium thermometer for low, thermo-electric couple for high, temperatures--Lummer and Pringsheim's improvements in thermometry.
Sec.6.Derived Units and Measure of Energy: Importance of erg as unit--Calorimeter usual means of determination--Photometric units.
Sec.7.Measure of Physical Constants: Constant of gravitation--Discoveries of Cavendish, Vernon Boys, Eoetvoes, Richarz and Krigar-Menzel--Michelson's improvements on Fizeau and Foucault's experiments-- Measure of speed of light..