In the last number of this Journal I described some apparatus for demonstration and class experiments. I Certain pieces were not completed at the time when that paper was written; so that I was not able to give illustrations. My present purpose is to make good this deficiency. The accompanying half-tone plate shows four large apparatus, to which reference was made in my former paper. I. The new model Whipple gasometer. The double gasometer for acoustic and general laboratory work was described and figured by Dr. Whipple in this Journal, XIV, I903, 107 ff. A new model of the instrument is shown to the left of the accompanying plate. It has been rendered more compact, by the substitution of a metal for the original wooden frame. It has been mounted on a wheeled platform, so that it can be moved from room to room of the laboratory. Outlet cocks have been soldered into each tank, near the floor, so that the water can be drawn off and the apparatus correspondingly lightened for moving. The butterfly valves are encased, not in wooden boxes, but in felt-lined brass tubes. These can readily be taken apart if anything goes wrong with the flaps. Minor improvements have been introduced as regards the lateral guidance of the moving tanks, the overhead travelling of the chain, and the raising and shifting of the counterweight. There is thus no change in principle, though the apparatus as a whole is a good deal more serviceable than it was in its first form. I may add that our inner stationary tanks have shown a tendency to rust into holes. We have learned wisdom by experience, and now keep some borax in solution in the contained water. The mending of a tank is, at best, an awkward business, and means the dismantling of at least half the apparatus. 2. The frame for adaptation and after-images. This is seen standing on the platform, to the right of the gasometer. The plate shows the frame in readiness for an experiment. The black-white sheet drops down, as was explained in my former paper, behind the black front, and exposes a gray sheet upon which the complementary after-images are seen.