« ПредыдущаяПродолжить »
ceptable in others, such as distance from the sea and from lofty hills, as well as their direction, &c.
The Committee will also be glad of any suggestions as to the conduct of rainfall work, and of information respecting any stations or old observations not included in the list published by them in 1866, and of which I shall be happy to send you a copy if you have not already received one. Yours very truly,
G. J. SYMONS, Secretary.
[Illustration of mode of filling up return.]
POSITION AND PARTICULARS OF THE RAIN-GAUGE
At [Camden Square, London,]
In the County of [Middlesex.]
Year in which observations were first made [1858.]
Hour of observation [9 a.m.] If entered against the day of observation, or
the one preceding [Preceding].
Position [In garden, 120 ft. by 24 ft.]
Surrounding objects, their distances and heights:
Inclination of ground [Quite level, but in N.E. rises 30 ft. in
Height of Ground above sea-level  ft. as determined by [Levelling from
Height of top of gauge above ground  ft.  in.
Pattern of gauge. (If similar to any on plate, quote the number; if not, give sketch.) [Similar to No. X., but the bent tube is made straight, and a jar inserted for the purpose of ensuring more accurate measurement.]
Have the same gauge and measuring-glass been used throughout? [No.] Has the gauge always been in the same position? [No.]
If not, state briefly
the previous position [300 yards further west.]
[Measuring-glass broken in 1861, and a new tested one obtained, the rainfall of each day until its arrival being bottled separately, and measured by the new glass.] Signed, [G. J. SYMONS.]
Another branch of investigation which has been arrested by the same cause is the relative amount of rain falling in different months, or, as we have usually termed it, the "monthly percentage of mean annual rainfall." Several articles upon the subject have appeared in our previous Reports; and last year we pointed out that the observations for the decade 1860-69 offered data of completeness unparalleled, either in this or any other country, the
result of which we had hoped to have submitted to the present Meeting. Excepting in our own Reports, we are not aware that the seasonal distribution of rain in this country has received any attention, while on the Continent it has at all times been looked upon as almost equally important with the gross amount.
Although several short and interrupted sets of observations have been made in Northern Derbyshire, the rainfall of that hilly district has not hitherto been examined with the thoroughness which its importance deserves. We have in previous Reports urged the desirability of several additional stations being established; and as no one else undertook the work our Secretary did so, and by the assistance of the observer at Buxton, and Mr. Hazlewood, of Castleton, was enabled to commence several sets of raingauge observations in the district. Some others are still required, which, if our funds permit, we intend to add.
Pit-gauges. In our last Report we drew attention to the fact that a gauge of which the orifice was horizontal, level with the ground, but in a small pit or excavation, had at Calne collected about 5 per cent. more than one of which the receiving surface was one foot above the ground; whence it followed that as a great many rain-gauges (the majority in fact) are placed with their apertures a foot above the surface, the records of all these gauges were below what they would have been if placed in. pits as just described. We gave some reasons which appeared to us to prevent the general use of pitgauges, and added the following concluding remark on page 176 :—
"This result appears so startling that further experiments will be conducted on the subject."
The funds at our disposal have not allowed us to do so; but fortunately the Rev. F. W. Stow, M.A., has tried one pair of gauges mounted in this manner at Hawsker, on the Yorkshire coast, a few miles south of Whitby. The following are the results during 1870
Of course it was not to be expected that the results of a single year should agree exactly with the mean of two other years, still less when the size of gauge used was different, and the locality so opposite as the inland district of Calne and the rock-bound Yorkshire coast. We therefore look upon it as satisfactory that in only four months out of eleven do the ratios at Calne and Hawsker differ more than 3 per cent. In April, June, and November they are identical. The Calne results are thus strongly confirmed; and it may be considered as certain that pit-gauges always exceed those at one foot, although the precise amount of excess remains to be determined.
In our last Report we expressed the hope that we should this year be able to state the result of the discussion of all the rainfall registers which were absolutely continuous from January 1, 1860, to December 31, 1869. We have the pleasure of doing so in two respects, viz. (1) with reference to their bearing on the question of the existence or otherwise of secular variation of rainfall in the British Isles, and (2) as data indicative of the distribution of rain over the country.
The secular variation of rainfall, or the relative dryness and wetness of different years and groups of years, is one of the most important and difficult branches of rainfall work. It has been treated in our Reports for 1865, and very fully in that for 1866. In the latter we gave the calculations in detail, from which the values shown on the accompanying diagram were obtained. Referring to that Report for full explanation, we have only now to mention that the subsequent years 1866 to 1869 have been computed in the same manner and added to the diagram (fig. 1). We may also remark that various observations collected since its publication have confirmed the general accuracy of the curve quite as much as could have been anticipated. On the present occasion we do not intend to discuss the relative rainfall of different years, but the relation of the fall during the ten years 1860-69 to previous decades. For this purpose we have grouped the yearly values in decennial periods, similar to those adopted in our 1867 Report, whence we obtain the following result:
TABLE II.-Ratio of Rainfall in each ten years since 1730 to the Mean of sixty Years, 1810-69.
Having previously pointed out the peculiarities of the earlier portion of the curve, it is only necessary on the present occasion to call attention to the last forty years, whence it will be seen that, according to this mode of investigation (which is principally based on English returns), three out of the four decades had a rainfall nearly identical, and the other (1850-59) considerably below them, the deficiency being nearly 7 per cent.
This result is based on a combination of records, as fully explained in our 1866 Report. We proceed to examine how far it is corroborated by individual stations, but are at once confronted by the paucity of stations of which perfectly continuous records for even half a century exist. We therefore confine ourselves to the forty years, from 1830 to 1869, for which period we
have twelve perfect records at widely separated stations. The mean fall in each decade and in the whole period, and the ratio of each decade to the whole period at each station, is given in Table III.
From careful examination of Table III., it appears that the amount of rain which fell in the ten years 1830-39 was very similar to that which fell in the ten following years, the difference being a decrease, but scarcely one per cent. The investigation in our 1866 Report shows an increase of 1.2 per cent.; and examination of returns ceasing in 1850, and therefore not quoted in either Report, show several cases of absolute identity.
With one investigation leading to a decrease of 1 per cent., another to an increase of the same amount, and a third to identity, we are led to the conclusion that the two decades may be considered to show similar results. This is a much more important fact than it at first appears; and for this
TABLE III.-Comparison of the Rainfall in each Decade since 1829 with the Mean Rainfall of forty years, ending with 1869.