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TRANSACTIONS OF THE SECTIONS. 5 
leading features being first laid down on the smooth surface from Messrs. Beer and 
Maedler’s micrometrical measures and charts. The diameter of the model is 12 inches 
8% lines (Rheinland measure), or one 10,000,000th part of the moon’s actual dia- 
meter. The scale of heights is, however, necessarily enlarged to double this amount, 
as otherwise the relief would be too low for distinctness. The material is a compo- 
sition of mastic and wax, and the whole is worked out in such perfection of detail as 
to represent every visible crater and mountain peak, nay, even the minuter lines of 
elevation which streak the so-called seas, &c. in their true forms and conventional 
proportions. In consequence, when properly illuminated, and placed at thirty or 
forty feet distance, and viewed through a good telescope, the artificial is scarcely 
distinguishable from the real moon. ‘The delicacy and precision cf the work can 
‘only be appreciated by a microscopic examination. In fact, the whole model is 
stated by Madame Witte to have been executed with the aid of magnifying glasses. 
Sir J. Herschel accompanied his explanation of this model with several remarks on 
the physical constitution of the moon in respect of climate, atmosphere, moisture, 
&c., and compared its surface with the chart of part of Mount Etna, lent him for 
that purpose by Baron von Waltershausen, and with a drawing of his own of one of 
the principal craters as seen in his 20-feet reflector, placing the volcanic character 
of the ring mountains beyond all doubt. By the aid of a large chart by Messrs. 
Beer and Maedler, several of these, such as Aristarchus, Tycho, Kepler, Coperni- 
- cus, &c., were pointed out and their peculiarities described, their places on the model 
being fixed by the aid of brass circles, representing the moon’s equator and meri- 
dians. This work, it is understood, will be submitted to the inspection of the 
Astronomical Society, on the resumption of their meetings in November. Speaking 
of the climate of the moon, Sir J. Herschei considered as probable the attainment 
of a very high temperature (far above that of boiling water) by its surface, after ex- 
posure to unmitigated and continual sunshine during nearly a whole fortnight. The 
moon therefore, when at the full, and for a few days after, must be, in some small de- 
gree, asource of heat to the earth ; but this heat, being of the nature rather of culinary 
than of solar heat (as emanating from a body below the temperature of ignition), 
will never reach the earth’s surface, being arrested and absorbed in the upper strata 
of an atmosphere where its whole effect will necessarily be expended in the conver- 
sion of visible cloud into transparent vapour. The phenomenon of the rapid dissi- 
pation of cloud (in moderate weather) soon after the appearance of the full moon 
(or of a moon so nearly full as to appear round to the unassisted eye), which he 
stated himself to have observed on so many occasions as to be fully convinced of 
the reality of a strong tendency in that direction, seemed to him explicable only on 
this principle*. 
On the Projection of a Star on the Dark Limb of the Moon just before its 
Occultation. By Professor Strvetuy. 
This the Professor considered to be a result of diffraction. Sir Isaac Newton 
having observed the shadow of a hair placed in a strong beam of sunlight to be 
broader than the hair itself, was led to investigate the course of a ray as it passed 
by the edge of a body, like the edge of a knife placed across a hole in the window- 
shutter, through which a sunbeam is admitted. Beyond a certain distance the rays 
proceeded in their usual straight courses; at that distance they were bent towards 
the edge; but the courses of the nearest rays were bent away from the edge, so as 
to form curves convex towards it. The undulatory theory enables us to trace these 
curves, and they are known to be of the nature of the hyperbola, with asymptotic 
branches extending onwards from the diffracting edge. Prof. Stevelly conceived the 
dark limb of the moon to be such a diffracting edge to the slender beam of light which 
reached us from a fixed star; and that as the curve was, at the last moment the light 
was allowed to pass, convex towards the moon, the portion of the ray which last 
enters our eye before the star disappears, being the direction in which we should 
then see the star, if produced backwards, would meet the moon on her dark surface. 
* On the conclusion of Sir J. Herschel’s explanation, Baron von Waltershausen entered 
into further particulars of the nature of the volcanic phenomena on the surface of Etna, as 
represented in the elaborate chart above alluded to, of the environs of Nicolosi, and pointed 
out many particulars of resemblance to the lunar volcanoes. 
