“Hush!” cried somebody behind them; and the ladies were silent: but it was plain they thought Voltaic Electricity had something to do with conjuring, and that the lecturer might be a professor of Magic. The lecture over, we returned to the Gallery, where we found the Diving Bell just about to be put in operation. It is made of cast iron, and weighs three tons; the interior being provided with seats, and lighted by openings in the crown, upon which a plate of thick glass is secured. The weighty instrument suspended by a massive chain to a large swing crane, was soon in motion, when we observed our skeptical lady-friends join a party and enter, in order, we presume, to make themselves more sure of the truth of the diving-bell than they could do of the identity between light and heat. The bell was soon swung round and lowered into a tank, which holds nearly ten thousand gallons of water; but we confess our fears for the safety of its inmates were greatly appeased, when we learned that the whole of this reservoir of water could be emptied in less than one minute. Slowly and steadily was the bell drawn up again, and we had the satisfaction of seeing the enterprising ladies and their companions alight on _terra firma_, nothing injured excepting that they were greatly flushed in the face. A man, clad in a water-tight dress and surmounted with a diving-helmet, next performed a variety of sub-aqueous feats, much to the amusement and astonishment of the younger part of the audience, one of whom shouted as he came up above the surface of the water, “Oh! ma’a! Don’t he look like an Ogre!” and certainly the shining brass helmet and staring large plate-glass eyes fairly warranted such a suggestion. The principles of the diving-bell and of the diving-helmet are too well known to require explanation: but the practical utility of these machines is daily proved. Even while we now write, it has been ascertained that the foundations of Blackfriars Bridge are giving way. The bed of the river, owing to the constant ebb and flow of its waters, has sunk some six or seven feet below its level since the bridge was built, thus undermining its foundation; and this effect, it is presumed, has been greatly augmented by the removal of the old London Bridge, the works surrounding which operated as a dam in checking the force of the current. These machines, also, are constantly used in repairing the bottom of docks, landing-piers, and in the construction of breakwater works, such as those which are at present being raised at Dover Harbor.