There are many, many books available in any large bookshop or art suppliers, that are intended to help those who are learning to paint (that's all of us). Some are better than others and all serve the very useful purpose of allowing the writer to earn a bit of money. When it comes to books on landscape painting, the question of composition soon comes to the fore. Now they nearly all follow the same pattern of suggesting that the canvas be split into 3rds, both vertically and horizontally and that the focal point of the picture is then placed at one of the intersections. It's good advice and helps to simplify things.

My bugbear is that somewhere along the line this 'advice' seems to have become a 'rule' and that to deviate from it is something close to a mortal sin. It was therefore with some pleasure that my tour around the exhibition revealed picture after picture which simply did not conform with this 'rule'. The division between land and sky can be at any point. It is perfectly possible to paint a landscape painting which splits the canvas into 2 equal parts of land and sky, with the focal point in the middle of the canvas. It's not the best idea to try it with your very first painting, but it's there to be done. Corot manages it very nicely with a picture of Avignon, almost to show how easy it can be.
So just to conclude by saying, there's good advice out there to be had, but there are no rules.
Enjoy!
PS As we're all fallible, there's a lovely painting by Corot in the last room which has the most wonderfully wonky water. Made me feel so much better about my own work.