and by excessive evaporation; but such a movement must be widely and irregularly diffused and excessively slow, not in any sense comparable with the diaphragm produced in the atmosphere by the rushing upward of the northeast and southeast trade-winds in the zone of calms. Perhaps one of the most conclusive proofs of the extreme slowness of the movement of the deep indraught is the nature of the bottom. Over a great part of the floor of the Atlantic a deposit is being formed of microscopic shells. These with their living inhabitants differ little in specific weight from the water itself, and form a creamy flocculent layer, which must be at once removed wherever there is a perceptible movement. In water of moderate depth, in the course of any of the currents, this deposit is entirely absent, and is replaced by coarser or finer gravel. It is only on the surface of the sea that a line is drawn between the two hemispheres by the equatorial current, whose effect in shedding a vast intertropical drift of water on either side as it breaks against the eastern shores of equatorial land may be seen at a glance on the most elementary physical chart. The Gulf Stream loses an enormous amount of heat in its northern tour. At a point 200 miles west of Ushant, where observations at the greatest depths were made on board the _Porcupine_, a section of the water of the Atlantic shows three surfaces at which interchange of temperature is taking place. First, the surface of the sea--that is to say, the upper surface of the Gulf Stream layer--is losing heat rapidly by radiation, by contact with a layer of air which is in constant motion and being perpetually cooled by convection, and by the conversion of water into vapor. As this cooling of the Gulf Stream layer takes place principally at the surface, the temperature of the mass is kept pretty uniform by convection. Secondly, the band of contact of the lower surface of the Gulf Stream water with the upper surface of the cold indraught. Here the interchange of temperature must be very slow, though that it does take place is shown by the slight depression of the surface isotherms over the principal paths of the indraught. But there is a good deal of intermixture extending through a considerable layer. The cold water being beneath, convection in the ordinary sense can not occur, and interchange of temperature must depend mainly upon conduction and diffusion, causes which in the case of masses of water must be almost secular in their action, and probably to a much greater extent upon mixture produced by local currents and by the tides. The third surface is that of contact between the cold indraught and the bottom of the sea. The temperature of the crust of the earth has been variously calculated at from 4° to 11° C., but it must be completely cooled down by anything like a movement and constant renewal of cold water. All we can say, therefore, is that contact with the bottom can