154 
REPORT—1840. 
of atmospheric temperature, solar radiation, pressure, hu- 
midity, fall of rain, force of wind, or electric tension. 
- Daily observations of the barometer are valuable, especially 
in tropical regions, because there the calculation of heights 
may at once be completely made without corresponding 
observations. By these means a traveller’s route across a 
tract of country may be traced in section, and the value of 
many of his local remarks greatly increased. 
. Observations continued even for a few days in the equa- 
torial parts of the globe, suffice to determine approximately 
the diurnal barometric fluctuation. (152, &c.) The hours 
seem to be everywhere nearly the same. 
. Optical meteorological phenomena of all kinds admit of 
being peculiarly well studied, from the varying points of 
view in which the traveller is placed. The diameters of 
rainbows, halos and corone, observed with due accuracy, 
and the abnormal phenomena which occasionally accom- 
pany these appearances, are facts of which as yet we pos- 
sess but a slender stock. We may specify the following 
subjects of inquiry :— 
(a.) The colours of the sky, their optical composition, and 
connection with the hygrometric state of the air. (254.) 
(b.) The polarization of the clear sky, (observed with Sa- 
vart’s polariscope) the position of the neutral points, its 
variations, and the cause of the inversion of the plane of 
polarization. (260.) 
(c.) The diameter of the rainbow, and contemporaneous 
measures of the distance of the supernumerary bows from 
the primary. The distance between the Primary and 
Secondary rainbow, measured from the brightest part of 
the Red. [This last is an easy and important observa- 
tion, especially if accompanied with a measure of the di- 
stance of the red of the first supernumerary bow from 
the primary red.] (264, 271.) 
(d.) The diameters (in different directions) of the Great 
Halos, (280); the condition with respect to polariza- 
tion of the parhelic circle, and other rarer appearances. 
(e.) The phenomena of glorified shadows, (298, &c.) in all 
their particulars ; and the state of polarization of the suc- 
cessive rings. ‘To compare the diameters of the direct 
corone and those by reflection formed in the same cloud. 
338. The traveller on lofty mountains possesses peculiar faci- 
lities for the following kinds of observation :— 
8. The decrement of temperature in the atmosphere. (51.) 
9. The force of solar and nocturnal radiation at different 
