All of you are completely misreading, not only the intention, but the results of the experiment as well. The result of the experiment (that the plumb bobs were wider at the bottom of the shaft than at the top) does not prove that there IS a hollow earth. If the experiment was conducted correctly, it would prove that we are IN a hollow earth.
In regard to your criticisms, if you’d bothered to check, precautions were taken to keep the plumbs from swinging by suspending the bobs in oil at the bottom of the shaft. As to your concern that it would be difficult to measure the distance between the plumb bobs at the bottom, again, if you’d bothered to check, you’d have found that there was a clear shaft BETWEEN them that allowed accurate measurements to be taken.
I live about half an hour from the Tamarack Mine and have visited it several times.