New Holland, 139. From Port Jackson to Valparaiso, 145 (two instances), 147, 149. From Valparaiso to, 15, 153, 155. Ship L'Astrolabe, 1828, 131 (two instances), 137. Cook, 1773, iv. 231, 490. 6 Duperrey, Observat. Météorol.' 25, 31, 49. Ship La Bonite, 1836, 101, 102, 106, 111, 120, 147, 148, 149. 6 In all the descriptions which have been hitherto given of the fluctuations of the instruments, reference has been made to the direction of the wind as indicated by the vane. I have described (p. 83) the manner in which the currents displace each other, in the same terms as I used in my Meteorological Investigations,'* and have explained there how the equatorial current makes its appearance in the upper strata of the atmosphere and displaces the polar current from above, while the polar current, on the contrary, first appears in the lower strata and gradually rises. In this respect the statements of Lartigue agree perfectly with mine, with the sole exception that he uses the term (p. 47) vents primitifs (primitive winds) for the current which I have called the polar current, and vents secondaires (secondary winds) for the equatorial current. I do not see any reason to change the nomenclature which I have introduced, inasmuch as it is employed in Germany, England, Russia, and also in France, as appears from a complete review of my works by Laugel in the Revue des deux Mondes.' 6 An immediate result of this mutual displacement of the winds is that, very frequently, winds which are blowing in opposite directions are flowing one above the other. Should the difference of temperature between the two currents produce a mist at the bounding surface, * Meteorologische Untersuchungen. 8vo. Reimer, Berlin, 1837. and should this take place within the upper current, we are able to ascertain the direction of this upper current by observing the drift of the clouds from below. There are two distinct causes to which the precipitation of moisture may be referred (neglecting those due to the courant ascendant [ascending current]), one of which is that the equatorial current, in its passage into higher latitudes, has its temperature lowered; the other that the contact of the two currents causes a condensation of moisture. The first-named of these I have termed the 'precipitations of the current,' the second the 'precipitations of the change of currents.' It is evident that in the first of these the drift of the clouds corresponds to the under current, and that, inasmuch as the equatorial current is a south wind which has received a westerly deflection proportionate to the distance which it has travelled, the directions of the wind for the west side, obtained from the drift of the clouds, must exceed in number those observed on the vane at the surface of the earth. The opposite must be the case with the east winds for, as they bring clear weather, it will be impossible to demonstrate the existence of the current at a high level when it is prevalent in the lower and upper strata, inasmuch as the materials by which the current might be rendered visible below are absent. This shows us that, although we may admit that the direction of the wind, which is given by the drift of the clouds, is not affected by so many of the disturbing actions exerted by the surface of the earth on the air which is flowing over it, yet we must not forget that a material complication is introduced by the fact that the clouds give the direction at times of the lower, at times of the upper current, while the vane of the weathercock only indicates the point from which the under current is blowing. In this way the results obtained by Bertrand le Doue (De la Fréquence comparée des Vents Supérieurs et Inférieurs' - On the Comparative Frequency of the Upper and Under Currents) are to be explained. He finds from observations carried on by himself at Le Puy for five years, by Quetelet at Brussels for eight years, and by Müller at Görsdorf for four years, the following proportions: With SW. below and SE. winds above, the amount of rain which was collected at Le Puy was 296 millimetres; with SW. above and SE. below, it was 131: so that in the former case it was greater than in the latter, while the proportion with other winds was the opposite. This is due to the fact that with the winds mentioned (SW. and SE.) the precipitations of the current unite with those of the change to produce the total result. There is another circumstance affecting this result to which Broun (General Results of the Observations in Magnetism and Meteorology made at Makerstown in Scotland,' p. 104) has drawn attention. Inasmuch as the earth seeks to impart its own velocity of rotation to the air which is in contact with it, it has a tendency to exert an action in diminishing the deflection which has been imparted to the current in its passage over parallels of latitude whose velocity of rotation is continually decreasing. Broun found the direction of the wind at the surface of the earth W. 21° S., of the 'under scud current' W. 7° S., of the Cirrostratus current W. 2° N., and lastly that of the Cirrus W. 9° N. The same law seems to hold good in the torrid zone, in the region where the Monsoons prevail, viz., that when the alternation of the currents takes place, the displacement of one current by its successor from the opposite point of the compass does not take place simultaneously in all the strata of the atmosphere. Le Gentil says ('Voyage,' i. 485) that, for three or four weeks before the change of the Monsoon, the drift of the highest stratum of clouds was in the opposite direction to that of the existing Monsoon at the surface of the earth, and in the direction of the Monsoon which would ensue after the change. |