We don’t know what pilings are safe or unsafe. We know that solidity depends on pilings, but we have no basis to say that the depth at which the Rialto pilings were driven was safe or unsafe.

We don’t know what depth of pilings is safe or unsafe. We know that solidity depends on pilings, but we have no basis to say that the contemporary standard of refusal was safe or unsafe.

We don’t know the standard that Da Ponte used. We know that his pilings met the standard for refusal as of 1588, but we don’t know whether Da Ponte used this standard or whether his standard was more or less strict than anyone else’s.

We don’t know whether there were any bridges built to the point of refusal after 1588. Maybe there were some driven to the point of refusal in 1589; we have no idea.

This is supported by the last sentence. The contemporary standard of refusal still allowed the pilings to be driven deeper — just not more than 2 inches deeper per 24 hammer blows. But, for example, the pilings might have been driven 1 inch deeper after 24 hammer blows, or even just 1 millimeter deeper.