You should read up on him. He was the dog of all dogs. He also wrote a long, satirical paper entirely about farting, called A Letter to a Royal Academy on Farting or Fart Proudly.

From the same letter:

And as in the dark all Cats are grey, the Pleasure of corporal Enjoyment with an old Woman is at least equal, and frequently superior, every Knack being by Practice capable of Improvement.

The Face first grows lank and wrinkled; then the Neck; then the Breast and Arms; the lower Parts continuing to the last as plump as ever: So that covering all above with a Basket, and regarding only what is below the Girdle, it is impossible of two Women to know an old from a young one. And as in the dark all Cats are grey, the Pleasure of corporal Enjoyment with an old Woman is at least equal, and frequently superior, every Knack being by Practice capable of Improvement.

I guess the legs are under the girdle too so maybe that is what he meant.

Ben "Horndog" Franklin