In message , The Natural
Philosopher writes
Well if you know the specific gravity of the cat, or assume its nearly
one, submerge it in a full bucket of water and measure what overflows.
PMSL

This also eliminates the unwanted cat.
You're a braver man than I, last time I suggested harming a kitty in
here I got death threats via email..

Another useful trick is to use potential energy.

Take a known weight on one end of a see saw and place the cat on the other.

The height to which the cat is cat-a-pulted is in the ratio of the cats
weight to the other weight, and the distance that weight falls.

Another good trick is to swing that cat on a bit of string tied to its
tail with a spring balance. By measuring the RPM and looking at the
scales, and knowing the string length, the cats weight may be calculated.




--
Clint Sharp