a variety of interesting observations were made during the passage of the ship round Cape Horn, and numbers of valuable results obtained for the benefit of navigators in those high latitudes. Thus, for example, the fallacy was established of the assertion of certain navigators that "the fluctuations of the barometer off Cape Horn did not depend on the state of wind and weather." In like manner by ascertaining the mean of a variety of collated data, it was found that the temperature of the surface of the ocean demands the most careful attention, inasmuch as the alterations in it from hour to hour may be relied on to indicate corresponding changes in the wind and weather.
The low reading of the barometer off the Horn seems to be a sort of compensation for the great pressure of the air in what are known to seamen as "the Horse latitudes," and, in point of fact, the barometrical readings at 56° S. betray a drooping tendency, which corresponds with the movements of the sun, as the latter also does with that of the zone of greatest atmospheric pressure. Hence it is obvious that from this parallel the atmospheric pressure will increase as we advance to the Pole, and this law is farther confirmed by the prevailing winds further south. Hence, while we find north-west or strong west winds blowing off Cape Horn, at the South Shetland Islands, still further south, the prevailing winds are N.E. or E., thus producing contrary atmospheric currents, almost resembling chronic whirlwinds, and consequently that both north and south of the central zone, the
barometer will be found to indicate a greater atmospheric pressure.
For this reason vessels intending to round the Horn from E. to W. usually keep further to the south than those sailing in the opposite direction. On the other hand, during the winter season of the southern hemisphere, the east wind must blow more frequently at the Cape itself, in consequence of the influence exercised by the zone of least atmospheric pressure, and the weather be less likely to prove stormy. And such is found in fact to be the case.
Hitherto, with the exception of Cape Horn, so few observations have been made in high southern latitudes, that it is impossible to arrive at any definite conclusion, important as the subject is to science as well as in the interests of commerce, and which must exercise so much influence upon the whole system of atmospheric changes over the entire surface of the earth. To attain this object, an expedition consisting of but one ship cannot suffice. It would be necessary to employ several, each provided with instruments carefully compared, and which should sail simultaneously to the southern waters at definite distances from each other, and at given times make precisely similar observations and devote their entire attention to investigating the laws which regulate this puzzle to the scientific student.
Under more favourable political auspices, a joint expedition by the various naval powers would be the best means of solving the problem, and a fleet of some ten or twelve ships
commencing upon a definite plan, might obtain results such as might hand down the scientific renown of our age and century to all future generations.
While sailing in these southern latitudes, the Commodore hit upon the idea of ascertaining the increase of gravity as the poles were approached, by the comparison of simultaneous observations taken with the mercurial and Aneroid barometers. Both instruments, in fact, gave a regular rule for calculating the weight of the atmosphere at the points of observation, with this single difference, that the ordinary barometer gives the weight by the pressure of the air upon a column of mercury, representing the weight of a similar column of air; while in the Aneroid barometer, the weight of the atmosphere is measured by an exhausted receiver, which, in resisting this pressure, indicates the amount by the tension of a spring.
The indications of the Aneroid are moreover independent of the influence of universal gravity and the disturbing conditions it introduces into the instrument, to which the column of mercury is of course subject. Assuming, for example, that the ordinary barometer and the Aneroid gave the same readings, the similarity will no longer exist at a given distance from the Equator—the Aneroid, owing to the elimination of the disturbing element of gravity, indicating an increased pressure of the column of air, whereas the ordinary barometer will continue to indicate the same pressure as at the Equator. The difference between the two readings will, however, be
directly proportionate to the amount of gravity thus got rid of, and is consequently susceptible of calculation. Although the data collected during the voyage for widely different purposes, and those now collected by means of the Aneroid, do not realize the anticipations that had been formed, to the length of utmost precision, the result has shown that much may be achieved in this direction by observations easily made in the course of a voyage even by ordinary navigators, such as would greatly benefit science; and captains of all grades, who in the course of their voyages have occasion to traverse these special latitudes, and are able to use good, reliable, thoroughly-tested instruments, might by a series of such observations add materially to our acquaintance with physical phenomena.[118]