That's an excellent question, Susan! He is a translator in his own right, as well as the most recent editor of Rouse's translation.
Here is his translation available on Amazon;
https://www.amazon.com/Nature-Things-Lucretius/dp/0872205878
It was published in 1969, and Professor Smith is still alive. So the text is not in the public domain—unless he's made arrangements that I wouldn't know about.