58 NATURE 
LETTERS WO RES WED TOI. 
[The Editor does not hold himself responsible for 
opinions expressed by his correspondents. Neither 
can he undertake to return, or to correspond with 
the writers of, rejected manuscripts intended for 
this or any other part of Nature. No notice is 
taken of anonymous communications. | 
New Units in Aerology. 
In Nature of February 5, p. 629, is a reference to 
the new edition of the ‘‘ Observer’s Handbook”’ of the 
Meteorological Office, and complimentary mention of 
the proposed extension of c.g.s. units. On this side of 
the Atlantic, we have not yet seen the book, but feel 
that Dr. Shaw and his associates have with character- 
istic progressiveness done well in opening the cam- 
paign for the use of rational units. It will be hard for 
the present generation to depart from the old notation ; 
but for those who are to follow, the adoption of these 
units means clearer conceptions of atmospheric motion, 
fewer mistakes, and great ease of compilation. Briefly, 
the units are those proposed by Koppen at Monaco in 
1909, and advocated by V. Bjerknes at Vienna in 1912. 
Temperature is given in degrees Centigrade on the 
absolute scale, and pressure is recorded in bars and 
decimal parts thereof, as decibar, centibar, and milli- 
bar. 
We began using these units at Blue Hill Observa- 
tory, January 1, 1914, and within a fortnight had our 
attention directed by Prof. Kennelly to the fact that 
unknown to meteorologists at home (and presumably 
abroad), the bar was in use and had an established 
meaning among chemists and others. If we continue 
its use without definition we only add to the confusion 
already existing. 
So far back as 1888 the word barad was proposed 
by a committee of the British Association as a suitable 
term for the unit of pressure, one dyne per sq. cm. 
In 1903 Prof. T. W. Richards! independently sug- 
gested that the pressure of one dyne per sq. cm. be 
called a bar. He also suggested megabar for a c.g.s. 
atmosphere. So far as I can ascertain this is the first 
clear-cut definition of an absolute atmosphere. Ostwald 
in 1899 had the idea and advocated the use of one 
million of the units as a standard pressure, but gave 
no name to the large unit. Richards has used the 
bar consistently in his work, likewise Kennelly? and 
others. It has been definitely adopted by the Inter- 
national Congress of Physicists, independently of 
Richards’s proposal under the name barie (see Guil- 
laume’s “‘Récents Progrés du Systéme Métrique,” 
Paris, October, 1904). 
It seems almost unnecessary to argue that the 
smaller bar should be the basic unit and not some 
multiple. And again, it is doubtful if bar is the most 
appropriate designation for the pressure of an absolute 
atmosphere. Aer is a more significant word. Mega- 
bar is not altogether inappropriate, and, as we have 
seen, is established in the literature of chemistry, and 
cannot readily be displaced. The megabar in the nota- 
tion of the aerologist means the pressure of a million 
atmospheres, a magnitude not often dealt with; while 
on the other hand we sometimes need to express pres- 
sures smaller than the millibar of the aerologist. Now 
the bar of the chemist and physicist lends itself nicely 
to the measurement of these feeble pressures, since it 
is divisible down to its millibar, i.e. the thousandth 
of a dyne per sq. cm. 
To contrast the two systems, I have made the fol- 
1 Pub, 7 Carnegie Inst., 1903, p. 433 also Jour. Am. Chem. Soc., vol. 
XXVi., 1904 ; T. W. Richards, W. Kk. Stull. 
~ Am. Inst. Elec. Engineers, June, 1900; Kennelly, Wright, and Van 
Bylevelt. 
NO. 2316, VOL. 93) 
[MarcH 19, 1914 
lowing table, and at the suggestion of Prof. Richards 
have restricted it to the terms in common use. 
Chemist and 
é Aerologist 
(robe an by (To be Remarks 
¥ | abandoned) 
all hereafter) | 
| 
| 
_ | Imegabar | A million atmospheres; beyond 
direct measurement 
The absolute atmosphere ; equal 
to 750°1 mm. Hg, or 0°987 of 
usual sea-level atmosphere. One 
megadyne per sq. cm. acting 
through 1 cubic cm. does 1 
megerg of work. 
I megabar | 1 bar 
I kilobar 1 millibar 1 kilodyne per sq. cm. 
I bar | ? I dyne per sq. cm. acting through 
| I cubic cm. does I erg of work. 
i} 
There could be no objection to giving the term 
megabar or absolute atmosphere some convenient nick- 
name, such as ‘‘ Aer,’’ if megabar seems too ponderous. 
Prof. Richards has also suggested that for historical 
reasons the pressure of ten absolute atmospheres might 
be named after some pioneer in meteorology as 
Guericke or Torricelli or Pascal; but this need not be 
dwelt upon at present. 
Fortunately we can change from the aerologist’s 
system to that of the chemist by writing kilobar for 
millibar, and by substituting ‘‘aer’’ for ‘‘bar.’’ This. 
we are doing in the handy conversion tables now in 
course of preparation at this observatory. 
Now is the time to agree upon a logical and avail- 
able system. The megabar atmosphere seems to me 
to be the more appropriate; but perhaps some of the 
readers of NATURE can suggest something better. 
ALEXANDER McADIE. 
Weather Forecasting. 
Mr. Mattock quotes in Nature of February 26 
(p. 711) a sentence of the late Sir G. Airy concerning _ 
the amassing of millions of useless meteorological 
observations. Unfortunately, in scientific work a vast 
amount of work which is not immediately productive 
has to be done. Indeed, it is not possible to foresee 
with accuracy what the result of any particular in- 
vestigation will bring forth. But I do not think that 
this feeling will deter scientific minds from working, 
for each advance beyond the frontiers which limit our 
knowledge makes up for the disappointment resulting 
from many apparently unsuccessful expeditions. 
It is acknowledged that in this country, indeed in 
this latitude, the weather depends largely upon travel- 
ling cyclones which reach us from the Atlantic. Now 
our knowledge of the nature and origin of cyclones 
is very limited, and recent researches of the upper 
atmosphere have shown that a good deal of accepted 
theory concerning them is unsound. In spite of the 
millions of observations which have been, and are 
being, taken, we have no detailed information con- 
cerning the conditions obtaining in any one cyclone, 
and the changes which have occurred in it during its 
passage over the land or sea. In these circum. 
stances it is not surprising that weather forecasting 
should be difficult and uncertain. Now whilst such 
a lamentable want of knowledge concerning atmo- 
spheric disturbances exists, it surely cannot be main- 
tained that we already have too much information, 
and that further research is undesirable. 
The main question really is as to the direction such 
further research should take. Dines enters a plea for 
further research concerning the condition of the upper 
atmosphere. Considering that it is this kind of worl: 
