In the choice of a boiler of this description it should be remembered that rapid heating, economical combustion of fuel, and facilities for cleaning, are requisites, the absence of any of which considerably lowers the efficiency of the apparatus.
It is followed by a cold south wind which often lowers the temperature 25°.
The steamer on reaching the given position lowers one, or perhaps two, mark buoys, mooring them by mushroom anchor, chain and rope.
In the polar areas the melting of sea-ice and of ice formed by precipitation lowers the density of the seawater and causes a difference of level which sets up streaming movements towards the equator.
Evaporation of the water and anything that lowers the hydrogen-ion concentration have the same effect.