The determination, by means of the chemist's balance, of the specific gravity of rocks and minerals, now more than ever an important element in their classification, is an operation of some difficulty. Various instruments of different kinds have been devised to obtain the same results more easily and expeditiously, and as amongst these there is one I had constructed some time ago and of which I have lately had very favourable reports from practical workers, perhaps an account of it may interest some of the readers of the Geological Magazine.