124 REPORT—1845.. 
cing wave-form becoming so perceptible, as in the first instance, namely the 
transit of the crest of the normal wave. 
ee 
Section III. 
Investigation of secondary Waves A.1, A 2, B 1 (reported last year). 
In my letter to Sir John Herschel published in the last Report, I stated that 
the coloured projections indicated three things as connected with the disposi- 
tion of the atmosphere :—“ Ist, the depth or extent of colour will show the 
depression of the lower station below the upper ; 2nd, the intersections of the 
curves will indicate that at the time of intersection the stations had an 
equality of pressure; and 3rd, the change of the position of the same colour 
will point out that the station which exhibited or experienced the higher or 
lower pressure, afterwards experienced the lower or higher, with its amount.” 
In addition to these three indications, the coloured projections and the barometric 
differences they exhibit may be very extensively and advantageously used 
in this investigation, as at the time when any one intersection of the curves 
shows an equality of pressure at the respective stations, the intersection also 
indicates that either a crest or trough was passing between them. Now if, 
from other considerations, it is found that at any intersectionatrough is passing, 
the next intersection will exhibit the passage of the crest; the differences 
therefore between the curves, or in other words, the differences of pressure 
between the stations, will augment and decrease as the anterior slope passes, 
the greatest differences occurring as the middle of the slope transits. The 
same result will obtain as the posterior slope passes, but the affections of 
pressure will be altered; the station which exhibited the greatest pressure 
under the anterior slope will manifest the Jeas¢t under the posterior. This ' 
principle will indicate the passage of a wave independently of the state (7. e. 
rising or falling) of the barometer at the time. The mereury may be falling 
from the transit of the posterior slope of a wave passing in a certain direc- 
tion, and this may occur at both stations; yet, although both curves may be 
descending from a posterior slope in one direction, the opening between them 
may indicate the transit of an anterior slope in another. 
Wave B°. 
The last report brought the investigation as far as the determination of 
the waves A 1 and B 1 (Report, 1844, p. 273). The dimensions and velocity 
of the latter were given; also the character of the trough between A 1 and 
A. Inthe note to (24.), page 274, it is shown that these waves, especially 
B 1, were small waves superposed on much larger ones. The principle just 
alluded to enables us to determine the phases of the larger wave on which 
B 1 rolled, not however uninfluenced by the transits of others, but sufficiently 
well-marked to contemplate it in its individuality as it passed over the area 
from the 8.S.W. This wave we shall call Be. 
Wave B°. Between Scilly and Longstone. 
Anterior Trough. Crest. Posterior Trough. 
1842. Between h h h m 
Nov. 6: 15. Nov. 9: 18 and 19. Nov. 11 : 0: 30. 
7 OS ZL. 
Amplitude in time ..... 102 hours (about). 
ss space..... 2600 miles fs 
Velocity, about 25 miles per hour. 
N.B. The above determinations subject to correction in examining this 
wave at other stations. 
