Mirth is short and transient, cheerfulness fixed and permanent† Mirth is like a flash of lightning that breaks through a gloom of clouds, and glitters for a moment: cheerfulness keeps up a kind of day-light in the mind, and fills it with a steady and perpetual serenity.

-Addison,Joseph
  In The Spectator, no.381,17 May.

You are a philosopher, Dr Johnson. I have tried too in my time to be a philosopher; but, I don't know how, cheerfulness was always breaking in.

-Edwards, Oliver
  In conversation with Dr  Johnson,17  Apr. Quoted in  James Boswell Life of Samuel Johnson (1791).