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Here, as in the preceding table, we see that a temperature of between 60° and 68°, that is, within the range of 8° of temperature, sometimes no evaporation took place; while, on other occasions, quantities equal to one hundredth and six hundredths of an inch were registered.
In casting, however, the averages of the different rates of temperature, the connection of the one phenomenon with the other is obvious. Thus, the average temperature of 53° corresponds to zero of evaporation, above which that temperature is seen increasing in proportion to the increase of evaporation.
Finally, the examination of the registers which include evaporation and winds, shows a striking recurrence of the same series of facts under .similar circumstances, viz., that no evaporation takes place in calm, however favourable the temperature and the hygrometrical state of the atmosphere may be to its process, and that all the maxima, diurnal and monthly, occur invariably during the (N.W., N., and N. E.) equatorial wind, and all the minima of that evaporation during the (S. E., S., and S. W.) polar wind.
From what has been said, it follows —
That the state of the atmosphere in relation to evaporation is very nearly the same in New South Wales and Van Diemen's Land.
That a calm state of the weather, whatever may be its hygrometrical and thermometrical condition, is, by virtue of its vis inertia?, an obstacle to evaporation, as much in the one as in the other colony.
That in neither has the tension of the aqueous vapour any connection with evaporation.
That high temperature, although very favourable to evaporation, does not appear to be, in either colony, sufficient to produce evaporation, or to regulate its amount.
That evaporation, as observed in New South Wales and Van Dieinen's Land, is mainly dependent upon the agency of those currents of air which possess either a capacity of absorbing and diffusing vapours, or an electrical condition capable of promoting their dispersion; and, lastly,
That, compared with other localities,—for instance, to London, — the evaporation of Port Stephen and Launceston is by no means excessive; the British metropolis having 25*92, and the mean of Port Stephen and Launceston being 32*67 of an inch.
SOLAR HEAT, AND DIAPHANEITY OF THE ATMOSPHERE.
The observations referring to this branch of meteorology, were made with the view of ascertaining the difference which exists between New South Wales and Van Diemen's Land, as regards —
1. The intensity of solar radiation.
2. The calorific effects which solar rays produce
3. The diaphaneity of the atmosphere.
In the absence of Sir John Herschell's Actinometer, which of all instruments is the best calculated to ensure precision and accuracy, the intensity of solar radiation was obtained by means of a thermometer with a blackened bulb, which was exposed to the action of the sun's vertical rays.
The calorific effects of solar rays were estimated by the difference shown between two thermometers, equally exposed to the action of the rays falling vertically; one theremometer being covered with white, the other with black wool, in corresponding quantities.
The diaphaneity of the atmosphere was ascertained through the means suggested by Arago, viz., the difference between a thermometer screened from radiation, and another covered with swansdown, and exposed, under a perfectly clear sky, at night, to radiate freely towards space.
Of the five thermometers used in the experiments, that with a blackened bulb, and the two covered with wool, were placed permanently in a shallow box screwed to a tripod, as shown in the adjoining wood-cut. The box had both a vertical and horizontal movement. At the top of it, and perpendicularly to its surface, was fixed a common pin, which, by its shadow, guided the adjusting of the box to such a a position as exposed the thermometers it contained to the vertical rays of the sun, and thus dispensed with the necessity of regulating the apparatus by a clynometer.
Independently of the illustration of the colonial meteorology, which demanded this kind of inquiry, I was not the less induced to engage in it from the peculiar interest I felt in the general question relative to the calorific effects which solar rays may produce by absorption in different parts of the world; a question which, about the year 1830, was warmly debated by meteorologists; and has since remained without solution, for want of proper data, or rather, proper method of securing the same.
In the "Compte rendu de I'Academie des Sciences, seance du 23 Novembre, 1835," this question, and the elements upon which its solution depends, are put in their true light.
In that part of the report which referred to the instructions given to the officers of "La Bonite" a
French frigate sent out ou a scientific expedition,
"De vives discussions se sont élevées entre les météorologistes, au sujet des effets calorifiques, que les rayons solaires peuvent produire par voie d'absorption dans différens pays. Les uns citent des observations recueillies vers le cercle arctique, et dont semblerait résulter cette étrange conséquence: le soleil échauffe plus fortement dans les hautes que dans les basses latitudes. D'autres rejettent ce résultat ou prétendent, du moins, qu'il n'est pas prouvé: les observations équatoriales prises pour terme de comparaison, ne leur semblent pas assez nombreuses; d'ailleurs, ils trouvent qu'elles n'ont point été faites dans des circonstances favorables. Cette recherche pourra donc être recommendée à MM. les officiers de La Bonite. Ils auront besoin, pour cela, de deux thermomètres, dont les récipiens, d'une part, absorbent inégalement les rayons solaires, et de l'autre, n'éprouvent pas trop fortement les influences refroidissantes des courans d'air. On satisfera assez bien à cette double condition, si, après s'être muni de deux thermomètres ordinaires et tout pareils, on recouvre la boule du premier d'une certaine épaisseur de laine blanche, et celle du second d'une épaisseur égale de laine noire. Ces deux instrumens exposés au soleil, l'un à côté de l'autre, ne marquerons jamais le même degré: le thermomètre noir montera d'avantage. La question consistera donc à déterminer si la différence des deux indications est plus petites à l'équateur qu'au Cap Horn. Il est bien entendu que des observations comparatives de cette nature, doivent être faites à des liauteurs égales du soleil, et par le temps le plus serein possible."
The above instructions have greatly served me in the inquiry undertaken, and have been mainly instru