496 
Assuming this law, and supposing the relative humidity constant, 
we can calculate the temperature corresponding to any given pres- 
sure, and then find the corresponding vapour-tension by reference 
to the table. By trying this for each separate inch of pressure from 
30 inches down to 0, and calculating the vapour-tension in each 
inch, I find (on a rough approximation) that the average pro- 
portion of the vapour-tension to the dry-air pressure will not 
exceed one-half of that which we find at the earth’s surface. 
Thus, when the,vapour-tension‘at the earth’s surface is half an inch 
in 30 inches, or 7, the average vapour-tension throughout the 
aérial column does not exceed +4, of the whole ; and when we 
calculate the weight of the superincumbent vapour, we must 
further allow for its smaller specific gravity. Making this cor- 
rection, I believe that when the tension of vapour at sea-level 
is half an inch, the real weight of the superincumbent vapour- 
column seldom exceeds that of one-sixth of an inch of mercury. 
The proportions of course are not fixed. -Those which I take 
from Herschel answer best for a temperature at the earth’s 
surface of about 65° F., or 18° C. 
I intend to apply these observations chiefly to the explanation 
of the annual and diurnal variations of the barometer by the 
greater or less amount of aqueous vapour in the air. It is sup- 
posed, for example, that when the vapour-tension at the earth’s 
surface is an inch, about 3; of the whole air-column consists of 
vapour. This displaces an equal bulk of air, and thus the 
column is lighter than a dry-air column of the same height and 
temperature by the difference in weight of the air and vapour 
occupying this space, z.e. about $ of an inch, or 3‘; of the whole. 
But if my computation is correct, the diminution of weight owing 
to this cause would only be 4% of an inch, or y}5 of the whole. 
Taking the standard pressure at 30 inches, this would only 
account for a diminution of 0°187 of an inch when the air from 
being absolutely dry changed to one inch of vapour-tension. In 
this country we never experience absolute dryness, and we seldom 
if ever experience so much as an inch of vapour-tension at the 
surface. Yet the annual variations of the barometer exceed 0°187 
of an inch. Looking, for example, at the table for Greenwich 
Observatory at the end of Herschel’s work, I find the mean 
monthly pressure varying between 29°923 and 29°602 inches, 
a difference of 0°321 inches; while the monthly means of vapour- 
tension vary between 0°466 and 0°195, a difference of 0°271; 
which, as I have endeavoured to prove, would only account for a 
change of o’o51 in the mean pressure. The diurnal maxima 
and minima at the same place exhibit a difference of 0018, 
the vapour-tension giving 0042, which only accounts for 
about 0'008, 
The same thing is more evident in other places. At Madrid for 
example, the monthly barometric averages vary between 28°003 
and 27°701, a difference of 0°302, while the vapour-tension varies 
between 0°236 and 0'076, a difference of o'160. At Longwood, 
St. Helena, the diurnal variation of the barometer gives a mean 
of 0067, while that of the vapour-tension is only 0'030. This 
would only account for a change of 0°006, or less than +; of the 
actual change. At Bombay the diurnal variation of the barometer 
amounts to 0102 inches, while the corresponding variation of 
vapour-tension is only 0'004. This would not account for one 
hundredth part of the change. 
The same result is confirmed by taking another view. It 
is pretty evident that in a country of any considerable ex- 
tent the diurnal oscillation of the barometer (which is often 
nearly double the diurnal variation), if produced by changes of 
vapour-tension, must always be less than the mean diurnal rain- 
fall and dew-fall, since rain often falls at hours when the 
barometer is on its diurnal descent. Now in this country the 
mean diurnal rainfall does not exceed +; of an inch, correspond- 
ing in weight to 54, of an inch of mercury. Supposing all this to 
fall during the hours when the mean barometric pressure is 
increasing, the subtraction of that amount of aqueous vapour from 
the column, and the replacing of it by air, would not nearly 
account for the observed diurnal oscillations. 
I therefore conclude that the annual and diurnal variations of 
the barometer are not due to changes in the amount of vapour 
present in the aérial column. Indeed it does not seem certain 
that the vapour displaces air at all. It may simply permeate that 
column without displacing any of it, and thus add to the weight 
of it. Again, if it displaced air, condensation or the formation 
of dense clouds ought always to be attended with a rise in the 
barometer, since air would rush in te fill up the space which the 
vapour vacated on condensing. This does not seem to be the 
case. 

4 W. H. S. Monex 
NATURE 


{ April 20, 1871 

THE ROYAL SOCIETY OF VICTORIA 
V E owe an apology to our scientific friends at the Antipodes 
for having allowed the president’s address, delivered last 
July, to have remained so long unnoticed. Mr. Ellery, after 
noticing the most important papers that had been read during 
the past two sessions (for no address was delivered in 1870 in 
consequence of alterations being made in the Society’s build- 
ings), including eight on physical science, seven on geology, 
mineralogy, and paleontology, one on natural history, three on 
medical science, one on social science, and four on arts and 
manufactures, expresses his regret that the state of their finances 
has for a time caused a stoppage in the printing of their Transac- 
tions which were commenced in 1868. He then proceeds to notice 
the present state of the chief scientific establishments in Victoria. 
‘* Botanical knowledge,” he observes, ‘‘is largely indebted to 
the labours of our member, Dr. Von Miiller, the head of the 
botanical department of Victoria. One of the prominent results 
of Dr. Miiller’s investigations is the publication of the Universal 
Flora of Australia (under the editorship of Mr. Bentham), to 
which Dr. Miiller is the principal contributor ; the fifth volume 
has, by this time, passed the press in London. This work, 
when completed, will be the only one extant that deals univer- 
sally with the flora of a large division of the earth’s surface. It 
will form a permanent basis of all future research with respect 
tothe adaptability of Australian plants to medicine, the arts, or 
other useful purposes, You will begladtolearn that Dr. Miiller 
is about to establish a permanent phytological collection in our 
new industrial museum, which will comprise objects illustrative 
of our natural resources in the vegetable kingdom, and of mate- 
rials used in the industries obtained from plants in this country as 
well as other parts of the globe. Such a collection properly 
arranged and accessible to the public will undoubtedly prove a 
valuable and instructive addition to the industrial museum, more 
especially if at the same time Dr, Miiller fulfils a project he has 
in contemplation of publishing in a popular forma volume on 
the culture of utilitarian plants in the colony not indigenous toit, 
as well as of plants likely to add to the resources of countries 
lying under similar latitudes to our own. The preservation and 
perpetuation of our more extensive forests has already become a 
question of serious import. A few years ago we thought our 
forests inexhaustible ; but already the bad effects of the indis- 
criminate stripping of our mountain ranges are becoming visible. 
The immense and increasing draft on our forests for fuel and 
other purposes has already denuded the land in the vicinity of 
towns and other centres of population of its former covering of 
timber. This, unless replaced by artificial planting, will even- 
tually leave our hills bare, and in all probability the climate will 
suffer in proportion. Dr. Miiller, in introducing and rearing 
very large numbers of forest trees that will be useful in themselves 
for the wood and bark, has exercised a wise forethought, of which 
the colony will reap the fruit in years to come, when the corks, 
oaks, hickories, red cedars, and firs, shall have in part replaced 
our eucalypti, mimosze, and other far less useful trees.” 
‘‘Our observatory,” he adds,+‘‘has been engaged with its 
usual work in astronomy, meteorology, terrestrial magnetism, 
and general physics. Considerable progress has been made in 
the Melbourne portion of the survey of the southern heavens ; 
the sky lying between the 60th and 52nd parallels of declination 
has been carefully surveyed, and the positions of 38,305 stars 
established and catalogued, of which 29,633 have been reduced 
to the epoch agreed upon, namely, 1875, and their positions at 
that date computed. Our staff of self-registering meteorological 
instruments may now be considered complete, and consist of 
three magnetographs (that is for declination, dip, and horizontal 
intensity), a thermograph, a barograph, electrograph, and anemo- 
graph. With these instruments a continuous and unceasing 
record is obtained by the aid of photography of all the variations 
in the force and direction of the earth’s magnetism, of the tem- 
perature of theair, and of evaporation, of the state and variations 
of the pressure of the air, atmospheric electricity, as well as of 
the direction, changes, and force of the wind.” 
The great Melbourne telescope, which, when the address was 
delivered, had been fairly at work for ten months, is then con- 
sidered, and Mr. Ellery observes that while the Society is disap- 
pointed in not getting, as it was hoped, the best telescope in the 
world, the members may feel satisfied that they have obtained an 
instrument that, ‘‘if it does not exceed, quite equals every 
other of its sort that has been yet made.” 
The progress of the survey is then noticed at considerable 
length, ‘*‘ The coast line from the boundary of South Australia 
