326 

acetic acid, but other products of oxidation are obtained 
in less quantity ; these are formic acid, acetal, acetic 
ether, saccharic acid, gloxal, glyoxilic acid, and glycolic 
acid, the final products being water and carbonic acid. 
When the elements of water are removed from absolute 
alcohol, ether is formed.” 
Now for a few words of criticism, or rather of sug- 
gestion. The book is not quite free from mistakes, 
probably in most cascs printers’ errors, which ought by 
all means to be avoided in a Dictionary, and will doubtless 
be corrected when a second edition is called for, which 
we hope may be very soon, But what we find most fault 
with is the cross-references, which are needlessly com- 
plicated, and often misleading. Being anxious to see what 
was said about the latest discoveries in Spectrum Analysis, 
we turned to the article under that heading, which we 
found to be very clear as far as it went. At the end we 
were referred “for further information” to five other 
articles ; of these, “Spectra of the Elements” and “ Metallic 
Spectra” are not to be found ; “ Spectra, Metallic,” does 
occur, but simply refers us again to “ Coloured Flames,” 
under which we found only two lines relating to spectro- 
scopy. Neither of the articles referred to gave us any 
further information of importance on the subject. 
One other criticism on another sentence in the preface. 
We observe with pleasure that this is to be the first of a 
series of similar dictionaries, which shall embrace the 
other departments of Science. One, it is stated, will have 
for its subject the “ classificatory sciences” of Botany and 
Zoology. Now we must protest against the use of this 
term as applied to the two sciences named. Zoology and 
Botany have their physiological as wellas their systematic 
side, and far the more important of the two. We trust 
that the forthcoming dictionaries will be framed on no 
such narrow basis as that implied in the denunciations by 
Mr. Emerson and Mr. Ruskin of the pursuit of botany as 
“a mere science of names.” When these volumes are 
published, some confusion may possibly arise from the 
title “ Dictionary of Science” having been given to this 
work, when it should have been more correctly “ Dic- 
tionary of Physical Science.” 
The book is however an indispensable addition to the 
library of every student, and we cordially recommend 
it to the notice of our readers. 
A. W. B. 



LETTERS TO THE EDITOR 
[The Editor does not hold himself responsible for opinions expressed 
by his Correspondents. No notice is taken of anonymous 
communications. | 
Ocean Currents 
I po not altogethe; agree with Mr. Johnston in concluding that 
his suggestion as to tne influence of barometric differences on 
Ocean Currents was stated with insufficient clearness. His words 
were :—‘‘ The waters which lie under the high pressure area have 
a tendency to escape from under the excessive weight, towards 
the space over which the pressure is less ;”—and about the meaning 
of this sentence there can be no dispute. It seems to me rather 
that Mr. Johnston, in his letter in NATURE for Feb, 2, is chang- 
ing his ground ; and whilst he formerly distinctly suggested the 
probability of the currents being due to mean differences of pres- 
sure stationary over certain oceanic areas, he now wishes to 
attribute them to the mofiox of centres of low pressure. This is 
quite adifferent thing, and is so far admissible, that what Piddington 
NATURE 



[ Feb. 23, 1871 
aptly called “ storm-currents,” attendant on tropical cyclones, 
are undoubtedly caused in such a way. Lut these storm-currents 
are very exceptional ; within the tropics, where their excessive 
development renders them most noticeable, they occur only in the 
hurricane months, and even then only at intervals; they have 
clearly no connection with the continuous current which flows 
west all through the year, and besides te great depressions of 
the barometer which give rise to these, there are no others. ‘The 
barometer in tropical latitudes is remarkably constant, and shows 
no centres of low pressure passing continually towards the west. 
in the direction of the Trade-winds or the Equatorial Currents. 
The Equatorial Currents are thus clearly not due in the slightest 
degree to differences of barometric pressure, cith-r to those that 
are stationary, as Mr. Johnston now seems to admit, or lo moving 
ones which exist only at rare intervals. 
It is however certain that centres of low pressure, round which 
the air circles, do very frequently pass along in temperate lati- 
tudes, driven (it seems to me) by the prevalent west wind. In 
our own latitudes, and more especially in the winter months, 
these succeed each other at intervals of a few days, aud though 
their action is intermittent, and for the most part peculiar to 
winter, I see no reason to doubt that they carry with them a 
species of storm-current, which does occasionally modily and 
even intensify the prevailing easterly drift. Butitis in the highest 
degree improbable that the formation of these centres of low 
pressure can give rise to any appreciable currents, Even towards 
the centre of a cyclone, the barometric gradient does not exceed 
one inch in fifty miles ; which, so far as its effect on the ocean is 
concerned, is equivalent to a difference of water-level of one- 
fourth of an inch ina mile; and even this can only exist if we 
suppose the barometric depression to be formed almost instan- 
taneously. There is no evidence that it is so formed ; and the 
longer it takes to arrive at its maximum, the more gradually does 
the water rise into the central space, and the more infinitesimal is 
the velocity with which it does so. 
Mr. Johnston rightly corrects the slip which appeared in my 
former letter, of twenty miles az hour, instead of a day, and 
thinks that even twenty miles a day is too large an estimate of 
the velocity of the equatorial current of the North Atlantic ; but 
the Admiralty chart to which he refers shows many instances of 
a velocity still greater ; and whatever force produces the current 
must clearly be adequate to the production of the greatest velo- 
city it attains. This, however, is of no consequence to my 
argument. The barometer decreases very steadily, regularly, 
and gradually, from the patch of permanent high pressure to the 
line ; and the effect of the formation of this patch would there- 
fore be equivalent—as I said before—to a difference of water- 
level of about four inches in 1,800 miles ; a difference which 
could no more generate a current of ten miles a day than it could 
one of twenty ; but as whatever effect it was capable of pro- 
ducing was produced, once for all, many ages ago, the consi- 
deration of it has no direct bearing on the currents of the present 
day. 
I would, therefore, repeat that neither as permanently existing 
nor as changing with the seasons, neither in their continuance, 
nor in their formation, nor in their fluctuations, can the areas of 
high and low pressure, which Mr. Buchan has delineated, and 
which formed the subject of Mr. Johnston’s first letter, produce 
an appreciable effect on the Ocean Currents ; and that, since the 
Equatorial Currents in the several oceans follow the course of 
the Trade-winds, and are not affected by transitory differences of 
pressure which do not exist in the Trade-wind region, the move- 
ment of these differences of pressure where they do exist, cannot 
be considered as causing the current, or as necessary to its flow, 
though it may occasionally give it an increased velocity. But 
any effect so produced is due not to the mere existence or forma- 
tion of a centre of low pressure, but to its onward movement ; 
and to urge that this onward movement exercises an influence on 
the currents, is merely to urge, with respect to one not very im- 
portant detail, the application of that principle which, in its 
widest sense, I have already maintained through a long chapter 
in my Physical Geography, and which Mr. Croll is now main- 
taining in the Philosophical Magazine—that the Ocean Currents 
are due entirely to the action of wind, 
J. K. Laucnion 
Royal Naval College, Portsmouth, Feb. 5 

In reference to the exchange current found to exist between 
the Mediterranean and Atlantic, at the Straits of Gibraltar, it 
seems a matter for inquiry how far the rise of the ocean bed, 
