A sponge hygrometer

1432. A sponge makes a good hygrometer on this account, as being less liable to be changed by use than cord. To prepare the sponge, first wash it in water, and when dry, wash it again in water wherein sal ammoniac or salt of tartar has been dissolved; and let it dry again. Now, if the air becomes moist, the sponge will grow heavier; and if dry, it will become lighter.