It is singular how any one, even though he regarded this conclusion as but a rough approximation to the truth, could entertain the idea that ocean-currents can be the result of difference in specific gravity. There are one or two reasons, however, which may be given for the above not having been generally received as conclusive. Herschel’s calculations refer to the difference of gravity resulting from difference of temperature; but this is only one of the causes to which Maury appeals, and even not the one to which he most frequently refers. He insists so strongly on the effects of difference of saltness, that many might think that, although Herschel may have shown that difference in specific gravity arising from difference of temperature could not account for the motion of ocean-currents, yet nevertheless that this, combined with the effects resulting from difference in saltness, might be a sufficient explanation of the phenomena. Such, of course, would not be the case with those who perceived the contradictory nature of Maury’s two causes; but probably many read the “Physical Geography of the Sea” without being aware that the one cause is destructive of the other. Again, a few plausible objections, which have never received due consideration, have been strongly urged by Maury and others against the theory that ocean-currents can be caused by the impulses of the winds; and probably these objections appear to militate as strongly against this theory as Herschel’s arguments against Maury’s. There is one trifling objection to Herschel’s result: he takes 39° as the temperature of maximum density. This, however, as we shall see, does not materially affect his conclusions. Observations on the temperature of the maximum density of sea-water have been made by Erman, Despretz, Rossetti, Neumann, Marcet, Hubbard, Horner, and others. No two of them have arrived at exactly the same conclusion. This probably arises from the fact that the temperature of maximum density depends upon the amount of salt held in solution. No two seas, unless they are equal as to saltness, have the same temperature of maximum density. The following Table of Despretz will show how rapidly the temperature of both the freezing-point and of maximum density is lowered by additional amounts of salt:— +-----------+-----------------+------------------+ | Amount | Temperature of | Temperature of | | of salt. | freezing-point. | Maximum density. | +-----------+-----------------+------------------+ | | ° | ° | | 0·000123 | −1·21 C. | + 1·19 C. | | 0·0246 | −2·24 | − 1·69 | | 0·0371 | −2·77 | − 4·75 | | 0·0741 | −5·28 | −16·00 | +-----------+-----------------+------------------+