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ON ATMOSPHERIC WAVES. 121 
Part I.—ReECURRENCE OF SYMMETRICAL WAVE. 
The following were the instructions drawn up for observing the Great 
Symmetrical Wave on its return in 1845. 
“The recurrence of the great November Wave observed in 1842 (an en- 
graving of which is inserted in the Report of the Thirteenth Meeting of the 
British Association for the Advancement of Science), during the autumns of 
1843 and 1844, renders the barometric movements of the months of October 
and November highly interesting. It is accordingly proposed that meteoro- 
logical observations, on a similar plan, should be made as extensively as pos- 
sible, with a view to observe this particular wave; and meteorologists are 
invited to direct their particular attention to the oscillations of the barometer 
during the months above-named. 
“ Times of Observation. 
“The following hours are the most suitable for the object now in view: 
3 A.M., 9 A.M., 3 P.M. and 9 p.M.; these hours divide the day into four equal 
parts; they have been recommended by the Royal Society as meteorological 
hours, and are the hours at which observations are made daily, by direction 
and under the superintendence of the Honourable the Corporation of the 
Trinity House, which have been most advantageously used in the examina- 
tion of atmospheric waves. 
In cases, however, in which the observation at 3 a.m. may be inconve- 
nient or impracticable, it will be important to substitute for it two observa- 
tions, one at midnight and the other at 6 in the morning, so that the hours of 
observation will in such cases be 6 A.M., 9 A.M., 3 P.M., 9 P.M. and midnight. 
“To individuals who cannot command these hours, it is recommended 
that observations should be made as near them as possible; these will still be 
valuable, although not to so‘great an extent as those made at the regular 
hours. In these cases, however, it will be absolutely necessary to substitute 
two readings for every one of the regular hours omitted—one previous to, 
the other succeeding the hour so omitted; and these should, if possible, in- 
clude an equal interval both before and after such hour. In all cases the 
exact hour and minute of mean time at the place of observation should be 
inserted in its appropriate column in the form sent herewith. 
“ At the regular hours of observation, or any others that the observer may 
fix upon, in accordance with the foregoing instructions, it will be necessary 
to observe, 
“Ist. The barometer, with its attached thermometer, and enter in the 
form the actual height observed with the temperature of the mercury. 
“2nd. The external and dry thermometer. 
“3rd. The wet bulb thermometer. 
“[ These observations are particularly essential in order to separate the 
pressure of the vapour from the aggregate pressure, as measured by the mer- . 
curial column. | 
“Ath. The direction and force of the wind. 
“These are important to determine the connexion between the undula- 
_ tory and molecular motion of the wave. ] 
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** 5th. The character of the weather at the times of observation; which 
may be recorded by Capt. Beaufort’s symbols. 
“Tt is proposed to commence the observations on the 1st of October next, 
_ and continue them daily until the end of November, unless it should be found 
that at that time the Wave is not completed, in which case it will be requisite 
to continue them a few days longer. 
“ft will be necessary, on returning the form when filled, to accompany it 
