52 REPORT—-1848, 
terize it, the central maximum forming the apex of the symmetrical curve, 
the remainder being subordinate thereto (Report, 1846, p. 125). Upona 
close inspection of the curves of the “ great wave” as laid down from the 
Greenwich observations, six subordinate maxima can be traced, three on 
each side the central apex, which in all the years is by far the most pro- 
minent. The mean curve leads to the conclusion that Greenwich is not the 
point of greatest symmetry, its closing portion being depressed more than 
0°2 inch below the commencement. The most striking feature is the decided 
rise of the mercurial column during a period of 68 hours preceding the 
transit of the crest. The value of this rise is 0°7 inch, or about 0°010 inch 
per hour. The fall is not so precipitous; the barometer appears to be hept 
up in this locality by the first subordinate maximum succeeding the crest ; 
so that at the epoch of 68 hours after transit, the value of the reading is 
more than 0°2 inch higher than at 68 hours before transit. At 80 hours 
after transit a precipitous fall commences which continues during the next 
24 hours, the mercury sinking 0°36 inch, or about 0°015 per hour; the fall 
afterwards continues, with two slight interruptions, answering to the sub- 
ordinate maxima until the close of the wave 148 hours after transit, the 
entire depression being nearly 1 inch. 
It is worthy of remark, that in the determination of the symmetrical 
curve from the Greenwich observations, extending in each year over double 
the period embraced by the curve itself, and including five instead of 
three years, the period of the wave should so closely agree with the period 
assigned to it in 1845 when the ordinates of the mean normal curve 
were deduced from the recurring curves of 1842, 1843, and 1844 (Report, 
1845, p. 122), the observations in 1842 having been made at Leicester 
Square, and those in the last two years at Cambridge Heath. The two — 
periods are identical, viz. 296 hours. It is necessary to mention here 
that so close an accordance in the values of the ordinates is not to be ex- 
pected. The Greenwich observations have been corrected for temperature, 
and the mean ordinates are reduced to the level of the sea. The quantities 
employed in the deduction of the ordinates recorded on p. 122, Report, 
1845, were obtained from observations as read off from the scale without 
any correction whatever. 
The barometric rise, to which allusion has been made, was very distinct 
on the occasion of the return of the great symmetrical curve in 1846, I 
have recorded in my last report (Report, 1847, pp. 359 to 363) the pheeno- — 
mena of this rise as exhibited in Great Britain and Ireland, especially during 
the first 50 hours, and it is not difficult to recognise them as repetitions of — 
the phzenomena from which the mean rise has been deduced. 
As regards the departure from symmetry manifested by the mean curve 
at Greenwich, the first portion of the curve, viz., from 148 hours. to 68 — 
hours before transit, being thrown higher than the latter portion, that from 
92 hours to 148 hours after transit, and the bulging character of the fall 
occasioning the readings at similar epochs to be higher after transit than 
before, during a period of 90 hours preceding and succeeding the transit of 
the crest, it appears pretty evident that this station is removed a definite — 
distance from the point of greatest symmetry. If we take the commence- 
ment and termination of the wave at equal altitudes as indicating the greatest 
symmetry, the departure from symmetry at any station will be measured 
by the difference between the altitudes of the beginning and end; thus the 
mean departure from symmetry at Greenwich, from the five years’ observa- 
