Upon Putney Heath, by the road-side, stands an obelisk, to record the success of a discovery made in the last century, of the means of building a house which no ordinary application of ignited combustibles could be made to consume. The inventor was Mr. David Hartley, to whom the House of Commons voted 2,500l., to defray the expenses of the experimental building, which stood about one hundred yards from the obelisk. In 1774, King George the Third and Queen Charlotte took their breakfast in one of the rooms; while in the apartment beneath, fires were lighted on the floor, and various inflammable materials were ignited, to attest that the rooms above were fire-proof. Hartley's secret lay in the floors being double, and there being interposed between the two boards sheets of laminated iron and copper, not thicker than tinfoil or stout paper, which rendered the floor air-tight, and thereby intercepted the ascent of the heated air; so that, although the inferior boards were actually charred, the metal prevented the combustion taking place in the upper flooring.

Another experiment took place on the 110th anniversary of the great fire of London, when a patriotic lord mayor and the corporation of London witnessed the indestructible property of the structure. Yet, the invention was never carried into further practice. The house was, many years after, converted into a tasteful villa, although the obelisk records the success of the experiment.