There is a greater difference in the daily temperature of the sea within the temperate zone : though when compared with the changes which occur in any place situated in the same zone, but not immediately on the shores of the sea, it will be found considerably less than in the latter. The following table from Berghaus, of the changes on the sea, may be compared with the changes in the temperature of London in the middle of March :— The difference between the maximum and minimum of the daily temperature in this table amounts to IN', and the mean temperature of the day is Tho maximum is above the mean tempe rature, and the minimum is below it.
IV° come to the same conclusion that tho temperature of tho air over the sea is subject to less considerable changes than that which surrounds the land, when we compare the changes that occur during the seasons. Looking at the difference of temperature between the hottest and coldest season in Berghaus's tables, we find that this differ ence is much greater, and at the same time more irregular, in the northern than in the southern hemisphere. But this seems to be clue, in a great measure, to the influence of the Arctic current and to the difference between the winter and summer temperature, owing to the presence of vast masses of ice, which during winter extend as far south as 40' N. lat. ; between and 25' the increase of the mean temperature of the spring is tolerably regular, rising from to 5' for every 5 degrees of latitude. The other irregularities are evidently produced by a greater increase of the heat in summer. The mean temperature of the summer increases pretty regularly between 55' and 45' by from 2 to 4 degrees for every 5 degrees of Latitude. But between 45' and 40" it ruses suddenly to more than 6 degrees. This sudden rise is, no doubt, produced by the warm vapours arising from the Gulf-stream [ATLANTIC OCEAN, In GEO°. Div.), which iu these parts runs krona the Atlantic. Another rise of more than 6 degrees occurs between 40' and N. lat. The air of the Sahara, when raised to the highest degree of heat by the continuance of the sun near the northern tropic, seems to affect the mean temperature of the summer between and 15', and to raise it nearly to the moan temperature of that season under the equator. The effect of this heated air has been distinctly traced for an Immense distance, we may, however, suppose that it ceases near Cape Verde; but the effects of another phenomenon begin to operate. The region of calms [Csurs] frequently extends in
summer to 12' and 13' N. lat. Of these three agents, the heated air of the Sahara seems to have the greatest effect in raising the summer temperature of the Atlantic. Again, the fact that most of the Isother mal lines run In a north-eastward direction from the western side of the Atlantic towards the eastern, and then bend downwards towards the south, indicates, as Maury remarks, the presence along tho African shores in the North Atlantic of a large volume of cooler waters, tho return current of the Gulf stream.
When two elements, such as air and water, approach each other, there can be no great difference in their temperature. Still there must be some difference. as the air is tho better conductor of heat, and the water, as a more dense body, is capable of retaining it for a greater length of time. ,Many observations have been made for the purpose of establishing this difference more precisely. It has been found that the temperature of both is subject to regular changes during the day, but that the air attains its highest temperature about two o'clock, whilst the sea attains its highest temperature in some parts at three o'clock, in others not before four o'clock. Further, it appears that the temperature of the air is greater shortly before and after noon, and that of the sea about midnight ; but in the morning and evening the two elements have nearly the same temperature. On the land, again, the coldest and hottest months are February and August, but on the sea tho vast amount of observations collected by Maury (' Physical Geog. and Meteorology of the Sea,' ch. xvii., ed. 1860), show distinctly that the extremes occur in March and September. It has further been found, that "at sea tho climatic conditions of the land are reversed, for the coldest side of the ocean is next the warmest aide of the con tinent, and rice rend" (Maury), conditions, however, which a little reflection will render readily comprehensible. When these slight changes are excluded, it is found that the difference of temperature in both elements is inconsiderable, but differs in different latitudes.