ballon d'essai

And in the early morning she had sent her note to Kitty—a ballon d'essai, despatched in a horror of great fear.

In the autumn of 1916 the newspapers put forth a ballon d'essai.

This is no doubt what is technically known as a ballon d'essai.

That was a ballon d'essai, as they call it, and they think it was a very happy one.