In Practical Hints on Playing the French Horn David Bushouse brings up another very practical aspect of playing the horn in tune, that of temperature.
"The tuning of wind instruments is affected greatly by temperature extremes. A cold instrument contains a cold air column which has greater density than warm air. Sound waves pass from air molecule to air molecule, and there are more molecules in the cold air than in the same volume of warm air. Therefore it takes longer for sound to travel in a cold air column, resulting in a slower velocity. This means the cold air column has a lower pitch than the same air column when warm."
This post by Prof. Ericson is one of a series he's been doing comparing and contrasting the positions taken by various texts on the issues facing horn players. While there is some agreement on some issues, there are a lot of different opinions on others, which tends to confirm my notion that the horn is the least settled instrument in the orchestra, both in terms of what the instrument is (single, double or triple) and the best way to play it.