=Relation of Sea to Climate.=—Water has the greatest specific heat of any known substance, being four times greater than that of the earth’s crust. On this account it takes longer to heat and to cool than the earth. Unlike the earth, likewise, it allows free penetration of the sun’s rays,—in clear water probably to a depth of at least 600 feet; consequently, the surface of the water becomes less rapidly heated. The freezing point of fresh water is 32°, while that of sea-water is 27·5°-28·4°. Thus, the sea remains open at a temperature at which inland lakes freeze, and has, therefore, a greater influence in moderating winter cold and summer heat. Another factor rendering it more competent to mitigate extremes of temperature than lakes, is the presence of currents, causing admixture of the water of different climates. Of these currents the most important for this country is the Gulf Stream, an immense stream of water which, when it leaves the Gulf of Mexico, is travelling at the rate of four to five miles an hour, and has a surface temperature of 88° F. It is important to distinguish between the _surface_ temperature and the _deep-sea_ temperature, the latter being fairly constant. The whole of the depths of the sea is filled with water at or near 32° Fahr., which in the tropics is 40°-50° below the temperature of the surface-water. The influence of seas on climate is so great as to lead to a classification of climates into oceanic, insular, and continental. An =oceanic climate= is least liable to violent changes of temperature. It can only be obtained by a sea-voyage. An =insular climate= presents smaller differences between the temperature of summer and winter than the interior of great continents, especially when the island is small and in the midst of the ocean. In the British Islands, the prevailing winds being westerly, places on the east coast are less truly insular than similarly situated ones on the west coast; and their climate approaches more nearly that of inland countries. A =continental climate= is drier and more subject to extreme alternations of temperature than insular and oceanic climates. =Isothermal lines= (lines of equal mean temperature) around the world bend up and down, the bendings being determined by the relative position of continents and oceans. New York has the same mean temperature as London, though New York is as far south of London as Madrid. This fact illustrates the fallacy in judging of the climate of a locality by the annual mean temperature. Means, it has been well said, are general truths but particular fallacies. One should know the extremes of temperature, and the extremes for each month of the year, as well as the amount and distribution of the rainfall, and the amount of sunshine, before judging of a local climate.