Reference for Gittin 106:1

ששקל בהן משקלות כשרה לא קשיא הא בגופן הא בכנגדן

against which weights have been balanced is not disqualified? — There is no contradiction; the one [Raba] speaks of weighing against the water, the other of weighing in it.<span class="x" onmousemove="('comment',' Like butchers, who place meat in water to see how far it will rise, and judge the weight accordingly. ');">¹</span>

Jastrow