adie a 
7 
ON MAGNETICAL AND METEOROLOGICAL OBSERVATIONS. 29 
midity are indispensable for this purpose. As the range of the diurnal 
variations is much less in clouded than in clear weather, and the diurnal 
march of the barometer is even different in the two cases, more years than 
one appear to be indispensable for the establishment of their Jaws ; more years 
than one are also required to give the necessary elements of correction for 
calculating the monthly means of temperature, pressure, and humidity, from 
the observations at the several hours. I believe that in the present state of 
science, even two years of hourly observation will give us very valuable ma- 
terials, and that five years will completely satisfy what is required in this 
respect. In regard to variations within the annual period, the three monthly 
sections ordinarily employed as meteorological seasons, are only truly such 
for certain latitudes; in other latitudes they combine heterogeneous data and 
dissever corresponding ones; I therefore hold it better to return everywhere 
to monthly means ; but as the shortening of the intervals increases their va- 
riability, a longer continued series becomes necessary to give assured ele- 
ments. As however the system of hourly observations cannot be continued 
so long, certain hours must be selected ; which are they to be? 
If it is desired to keep the laws of the diurnal variations still in view, hours 
at equal intervals will be desirable for the convenience of empirical formule ; 
but this would necessitate the inconvenience of night observation ; and if this 
is avoided, the advantage of equal intervals must be given up. 
The hours of 6, 9, 12, 3, 6, 9, or 9, 12, 3, 9, have been chosen with a view 
to the barometric oscillations ; they are also convenient for the calculation of 
the mean temperature. But as respects the barometer, I.regard it as proved 
that we have here to do with the difference of two variations, and that it is 
therefore of greater interest to learn separately the daily extremes of the va- 
riations of the pressure of the dry air and of the elasticity of thé aqueous 
vapour mingled therein. As however the hours 3, 9, 3, 9 have been recom- 
mended in the ‘ Report of the Committee of Physics,’ I hold it good to keep 
to them, thinking it better to continue consistently a system of observation 
once begun than to alter it, even though it be afterwards shown that other 
hours are preferable for certain objects: the command of as long a series as 
possible of precisely similar observations is always that which is chiefly to be 
desired for the solution of a meteorological question. 
To question 3, “ Would. you be disposed to recommend any modification,” 
&c. &c., my answer would therefore be negative. 
To question 1, “ What important objects are to be accomplished by the 
continuance of the existing establishments for a longer period,” I permit my- 
self the following remarks: — 
We possess from no point of the southern hemisphere, and from no point 
in North America, a barometric, thermic, or atmic windrose, no calculation 
of the variations of the barometer, thermometer, and hygrometer, dependent 
on the law of rotation of the wind, founded on a sufficient number of obser- 
vations. I should regard it as an essential service to science, if from only one 
extra-tropical station in the southern hemisphere, and from one in North 
America, we had a five or ten years series of observations three times a day 
of the barometer, thermometer, and hygrometer, direction of the wind, and 
quantity of rain; to enable us to determine the question of the opposite law 
of rotation in the southern hemisphere, and the influence of the relative po- 
sition of continent and sea. The annual march of the barometer and annual 
distribution of the quantity of rain are also important questions which would 
thus be answered. It would however require the intercomparison of different 
stations. 
I will now permit myself to allude to some questions, which if not solyed 
