All through the calculus we are dealing with quantities that are grow- ing, and with rates of growth. We classify all quantities into two classes: constants and variables. Those which we regard as of fixed value, and call constants, we generally denote algebraically by letters from the be- ginning of the alphabet, such as a, b, or c; while those which we consider as capable of growing, or (as mathematicians say) of “varying,” we de- note by letters from the end of the alphabet, such as x, y, z, u, v, w, or sometimes t.

Obviously 1 minute is a very small quantity of time compared with a whole week. Indeed, our forefathers considered it small as com- pared with an hour, and called it “one minu`te,” meaning a minute fraction—namely one sixtieth—of an hour. When they came to re- quire still smaller subdivisions of time, they divided each minute into 60 still smaller parts, which, in Queen Elizabeth’s days, they called “second minu`tes” (i.e. small quantities of the second order of minute- ness). Nowadays we call these small quantities of the second order of smallness “seconds.” But few people know why they are so called.

What one fool can do, another can.
(Ancient Simian Proverb.)