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POPULAR SCIENCE REVIEW. 
dimensions of tlie atmosphere shown to be possible upon the Moon’s surface 
can be best shown by the fact that its total weight above one square mile 
is about four hundred thousand tons : and that it bears nearly one-eighth 
of the proportion of the Moon’s mass, as the Earth’s atmosphere does to the 
Earth’s mass. 
Variation of the Sun’s Diameter. — Signor Respighi says, in a recent note 
to the Academy ( u Comptes Rendus,” Oct. 6) that Father Secchi has stated 
that the duration of the passage of the solar diameter, measured by 
mono-chromatic images obtained in the spectroscope telescope with the 
direct vision prism before the slit, was less than the duration given in the 
“ Greenwich Nautical Almanack ” by about 0-6 s., whence he infers that the 
diameter of the mono-chromatic image of the sun is about 8 s. less than the 
diameter of the image with compound light, obtained by the simple 
telescope with coloured glass. Signor Respighi is unwilling to admit this 
difference. He suggests possible causes of error ; imperfect rectification and 
instability of instrument, influence of atmospheric refraction, but more 
especially the influence of variations of temperature of the prism. These 
sensibly displace the spectral lines. The results are constant, Father 
Secchi says ; but it is replied, that during each passage of the solar image 
the temperature of the prism could not continue constant, and its variations 
would be reproduced periodically in successive passages (independently of 
the absolute temperature). It is precisely these periodic variations which 
might displace the solar image by a quantity nearly constant in all the 
successive passages. (This influence is much less sensible in the author’s 
own instrument, in which the aperture is reduced by a diaphragm, and the 
prism has little absorption). Other possible causes of error are the undula- 
tion or agitation of the sun’s border, and the personal error in observation of 
the two contacts. Some of the causes mentioned are avoided by using the 
objective prism. The author’s observations by both methods gave results 
differing very little from the “ Nautical Almanack” (not more than 
+ or — 0T2). 
BOTANY AND VEGETABLE PHYSIOLOGY. 
The Gonidia of Lichens. — An important paper on this subject appears in 
' a Grevillea ” (September), and is worthy of being perused. M. Bornet, whose 
researches the paper deals with, is said to consider he has established the 
two following propositions : — 1. Every gonidium of a lichen may be re- 
ferred to a species of Alga. 2. The connection of the hypha with the 
gonidia is of such a nature as to exclude all possibility of one organism 
being produced from the other, and that the theory of parasitism can alone 
explain it satisfactorily. M. Bornet’s paper, of which the one in “ Gre- 
villea ” is an abstract, is illustrated with eleven coloured plates of micro- 
scopical dissections which assist very considerably the elucidation of his 
theory. 
Eucalyptus Globulus as a Means of Draining Moist Lands. — It is as- 
serted by those who seem to have practical acquaintance with the virtues 
