Dee. 2, 1869] 
NATURE 
12g 
observations as are at present made. 
with, as I have shown— 
We have to begin 
(1) The mass of vapour actually present at a station 
from hour to hour. 
(2) The mass that passes a station in one hour, going 
east and west. 
(3) The mass that passes a station in one hour, going 
north and south. 
There is wanting— 
(4) The vertical component of the motion of vapour. 
(5) Its production or consumption as it passes from 
place to place. 
These deficiencies may, however, be to some extent 
overcome by the following considerations :— 
first, the atmosphere moves as a whole when it moves, 
the dry and moist air moving together; secondly, dry air 
is neither capable of production nor of consumption, but 
always remains constant in amount. 
To illustrate this part of the subject, let it be supposed 
that we wish to investigate the vertical motion of the atmo- 
sphere at a certain station. Make this station the imagi- 
nary centre of a circle, the circumference of which may be 
supposed to be studded with other stations at sufficiently 
frequent intervals, so that we can tell, hour by hour, how 
much dry air passes in towards the centre of the circle 
through its circumference, and also how much passes out, 
Let us suppose that more is passing in than is passing 
out, or that the imports into the area of the circle are 
greater than the exports out of it. Now, the dry air that 
passes in is incapable of production or of consumption, 
and hence the stock of the material at the central station, 
and in the area generally, ought to be on the increase, 
since we have imagined the imports to be greater than 
the exports. If, however, we ascertain from actual ob- 
servation that the stock of dry air is diminishing instead 
of increasing, we may be sure that some is carried off by 
an upward current, which of course carries the moisture 
with the dry air. 
The establishment of accurate observations is so recent 
that I cannot at this moment produce any definite 
example in illustration of this mode of analysis. We 
may, however, take a cyclone. As I have said, there 
are two hypotheses with regard to the motion of air in 
this phenomenon: one set of philosophers advocating a 
strictly rotatory motion, and the other set an indraught 
of air from the circumference towards the centre ; and yet 
frequently we have a falling barometer in the centre. Now, 
what can carry off the air, if there be not an ascending 
current at the very heart of the cyclone? This is, how- 
ever, I may remark, merely put forward in illustration of 
the method. 
So much for the vertical component ; and now, in the 
next place, with regard to the production or consumption 
of aqueous vapour as it passes from place to place. Our 
consideration has hitherto been confined to guantity ; let 
me now define what is meant by the hygrometric quality 
of the air. It may be represented by the following 
quotient :— 
mass of vapour in a cubic foot 
mass of dry air in a cubic foot 
Now, this quotient can only alter by evaporation, by 
precipitation, or by mixture. This hygrometric quality of 
the air may perhaps be considered a quality sufficiently 
constant to aid us in tracing the actual motion of air, just 
as we may make use of the element of saltness to trace 
the actual path of an oceanic current. It gives us, in 
fact, a chemical analysis of the air, and one, moreover, 
which is independent of pressure, so that we can tell by 
its means the various qualities of air which we meet with 
in a balloon or mountain ascent. But besides this aid, 
we may make use of it to enable us to tell the preci- 
pitation_or evaporation. For instance, a very damp air, 
in passing over a very dry country, may be supposed to 
emerge less damp, having its hygrometric quality changed ; 
or a very dry air, in passing over a very damp country, 
may be supposed to emerge less dry, having its quality 
changed in the opposite direction. Thus, by actual obser- 
vation of the quality of the air at the time of its reaching 
some particular tract of land or ocean, and at the time of 
its leaving it, we may possibly get much better observa- 
tions of what goes on in the country, as far as this 
particular research is concerned, than if it were studded 
with gauges. 
I would therefore suggest that meteorological observa- 
tions should, by a system of reduction, be made to 
show— 
(1) The mass of dry air and of moisture in one cubic 
foot actually present at each station from hour 
to hour. 
(2) The mass of dry air and of moisture that passes 
each station, hour by hour, in two lines of di- 
rection at right angles to each other, namely, 
north and south and east and west. 
When these hourly elements are obtained, they might 
for seasonal changes be reduced after the method of five- 
day means; or for the investigation of changes of weather, 
they might be utilised in some other way, as, for instance, 
in that lately suggested by the Astronomer Royal. 
I ought to remark of this method of gauging, that all 
I claim to have done is to have put it in a somewhat new 
form ; for it has been acted on by Maury and others before 
now, and has, in fact, given us one proof of the anti- 
trades. For we know that there is a constant indraught 
of air from the tropics to the equator on both sides ; and 
as it does not accumulate there it must be carried off 
somehow, that is to say, it must return by the upper 
regions. 
Before concluding,—one word of recapitulation as to 
the present stage of development of meteorology. We have 
seen that, judging by astronomy, there ought to be three 
stages ; the object of the first being to ascertain the actual 
motions of the air, the second the causes of those motions, 
while we prophesy in the third. We have also seen how 
little progress we have made in the very first of these ; 
and we may naturally conjecture that the third or pro- 
phetical stage is so very far in advance of us that we may 
not reach it for a long time. Nevertheless there is one 
crumb of comfort for weather prognosticators ; for just as 
astronomers predicted certain phenomena in a rough way 
before the law of gravitation was established, so here also 
we may make certain rough and ready predictions of 
much practical utility before the advent of the Newton of 
meteorology. 
BALFOUR STEWART 
