GEOGRAPHY
THE WATER CYCLE
IT BEGINs with evaporation, as the sun heats the sea. Condensation is the formation of clouds. Precipitation is rainfall. Percolation is rain running down into and through the ground, fattening the streams and rivers and flowing back to the sea, and so on and on.
OX-BOW LAKES
THE stretch of water left when a river has cut through itself at a bend.
CLOUD TYPES
sTraTUs are low, flat, often grey forms below 6,000ft. Cirrus or mares’ tails are fabulous long wisps found above 18,000ft. Cumulus are the puffy white stuff that toddlers paint. The great towering mega-clouds are cumulonimbus.
ROCK TYPES
IGNEOUs rock is cooled magma from Earth’s molten heart. sedimentary rocks are compressed sediments from an ancient sea or lake beds formed over aeons. subject either sedimentary or igneous rocks to ferocious pressure and they change, becoming the third type: metamorphic rock.
MATHS Tim Harford, economist and Financial Times columnist ALGEBRA
a ClEVEr way of solving a problem in a general way. It’s maths with lots of letters instead of numbers — x, y and z — but those letters are like a joker in a card game or a blank
in scrabble. They can stand in for any value.
PYTHAGORAS’S THEOREM
IN a right-angled triangle — the kind that’s handy for making sure that your bookshelves go up with nice square corners — the square of the longest side is the same as the sum of the squares of the other two sides.
WHAT’S PI?
IF yOU multiply the distance across the centre of the circle by Pi, you get the distance around the circle. simple! Except that Pi — which is slightly more than three — is a number that goes on for ever (and ever) and you can’t write it out in
full (3.14159 . . .). as you do more mathematics, you find Pi popping up everywhere.
TIMES TABLES
THE 9 times table is probably the hardest. But it turns out to be packed full of numbers that add up to nine. It goes: 9, 18, 27, 36, 45, 54, 63, 72, 81, 90, (let’s forget 99), and 108 — in each case if you add the digits together you get nine!
It’s neat, isn’t it?