TRANSACTIONS OF THE SECTIONS. 29 
As stated in a paper communicated to the Section last year, the Meteorological 
Society of Mauritius has, since the Ist January 1853, been tabulating the obser- 
vations recorded in the log-books of vessels visiting the harbour of Port Louis, so 
as to form a journal containing information with respect to wind, cloud, rain, fog, 
lightning, &c., and the state of the sea, for each day and, as far as possible, for each 
hour; and it is from that journal that the materials for constructing the charts are 
mainly derived. Up to the present time, about 215,000 days’ observations, each 
comprising several observations taken in the course of the twenty-fcur hours, have 
been tabulated, and a separate collection has been made of numerous details rela- 
ting to the hurricanes which have occurred during the same period. Since 1859 the 
annual number of observations has considerably increased, and for several years 
there is a daily average of from 70 to 80 days’ observations, or, in other words, of 
70 to 80 vessels on board each of which various observations were taken daily. 
The importance of synoptic weather-charts, as a means of studying meteorolo- 
gical phenomena, had induced the author to attempt to bring out a series of such 
charts about twelve years ago. The observations collected for March 1853, toge- 
ther with several charts, were published in 1856, and daily charts for other months 
prepared ; but for various reasons, of which the fewness of the observations was 
one, the daily average being only 30, the work was abandoned. Subsequently, 
several hundreds of synoptic charts were constructed for periods for which the ob- 
servations were more numerous, and it was now proposed to issue those for 1861. 
The mean daily number of observations for that year, recorded in Mauritius, was 
70, and as the author was to be favoured with a copy of those in the Office of the 
Meteorological Committee of the Royal Society, the total daily average, including 
ela taken at the Indian and other Observatories, would fall little short 
of 100. 
In order to render the charts synchronous, the observations are, as far as practi- 
cable, referred to the meridian of 60° E., which divides the charts into two equal 
parts, and is taken to represent noon. Generally this is easily accomplished with 
respect to the winds and weather, which are frequently observed in the course of 
each day; the chief difficulty being in applying corrections for the positions of the 
vessels at local noon, when they happen to a at considerable distances from 60° E., 
and to be going at a rapid rate; but the correction seldom amounts to 40 miles, 
and is often so small that it may be omitted. There is much greater difficulty with 
regard to the pressure and temperature of the air. In ordinary weather these are 
observed on board ship only at noon. Hence the isobaric and isothermal curves, as 
given in the charts, represent the pressures and temperatures at local noon, and not 
at the same moment of absolute time; for it is impossible to apply corrections 
which would make the observations synchronous. Noon, however, happens to be 
nearly the hour when the barometer is at its mean daily height; and therefore 
in usual weather the isobars for local noon probably represent with tolerable accu- 
racy the mean pressure for the day at the localities over which they pass. When 
the weather is stormy, and the barometer is falling, observations are taken hourly 
or oftener ; so that corrections may be applied. 
After describing the manner in which the observations were reduced and the 
results delineated graphically, the author exhibited specimens of the charts, and 
called attention to the connexion which apparently subsisted between the several 
phenomena, particularly the isobaric curves and the directions of the wind, and 
ventured to think that a series of such charts would be of some value. The Mauritius 
observations, with those belonging to the Board of Trade, were alone sufficient for 
constructing ten years’ daily charts of about ninety observations each. 
As a method of investigating weather phenomena and discovering atmospheric 
laws, there could, he thought, be little doubt that synoptic or synchronous charts 
were more important than average charts, in which various disturbances, devia- 
tions, and even periodicities were entirely masked. He would go further, and 
say that a series of synoptic charts for a particular ocean would be of more service 
to the practical navigator than average charts, not only in giving him an insight 
into weather sequences, but in affording more reliable information as to the winds 
and weather likely to be experienced in a certain locality at a certain time ; that, 
for example, helf a dozen observations of the winds which actually occurred 
