It has long been recognized that the basic units in science – such as the and the – should be defined in terms of fundamental physical phenomena. Ind… - Terence Quinn

In my house I have four , I sometimes adjust them so that at least two, but rarely more than three, strike the hour within a few seconds of each other. Fortunately, the pleasure I get from my clocks does not depend on them all telling the same time. When I switch on the in my car, however, things are different. It very quickly latches on to at least four atomic clocks high in the sky, all of which tell the same time to about a hundred millionth of a second. If they did not, my GPS would guide me to somewhere other than my desired destination.

On 19 March 1791 five of the great luminaries of French science, Laplace, Lagrange, Condorcet, and , met at the in Paris and drew up a document that laid down the definition of the new basic unit of length, the , for the proposed new system of measurement that would become the .

... of may allow us to say that "this object is hotter or colder than that one." But even this apparently simple statement is fraught with pitfalls for the unwary. For example, take hold in turn of a block of , a piece of expanded and a rod of , all near room temperature but differing slightly slightly in temperature from one another. It is not easy to make any useful statements about which is hotter or colder. This means, of course, that the is a poor thermometer, but the reasons for this being so are by no means straight-forward: they are related to the way in which sensations of hotness and coldness are generated in the human body ...