Observations upon the Rainy Seasons, from the year 1711 to 1811—Notices on the Fall of heavy Rains, fine Showers, and drizzling Rain, from the 1st of January, 1806, to the 31st December, 1811—erroneous Notions upon the Effects of heavy Rains at St. Helena ; sometimes damage the lore Grounds, but always favourable to cultivated and pasture Lands. Abstract of the Fall of Rain front February 1811 to February 1812.
IT is generally believed by the oldest inhabitants of St. Helena , that rain of late years, has fallen in less quantity than in former times : and the cause is imputed to the diminution in the number of trees. This is no doubt a plausible conjecture ; since it is admitted that trees have a power of attracting clouds, as well as moisture from the atmosphere : but whether any change has actually taken place is a point that cannot now be ascertained ; because there is no record whatever of the fall of rain.
There are, however, upon the consultations, and in letters from the Governments of St. Helena to the Court of Directors, several passages that tend to throw some light on the periods at which the rainy seasons formerly set in, as well as some notices of unusual falls of heavy rain ; both during the seasons of rain, as well as in the dryest months of the year.
The following are the passages and notices I have selected.
1711. January 23d.—“This is deemed the proper season for planting.”
1714. November 12th.—“On the 3d February, 1713, the floods carried away part of the west curtain, and damaged other places.”
1719. May 5th.—“Great floods descended from the hills ; supposed to have been occasioned by a water spout.”
1734. April 3d.—“At this time the rains have ceased.”
1735. March 29th.—“We have had a good season : the rains set in on the 1st February.”
1736. January 29th.—“The weather still continues very dry.”
1737. January 27th.—“Our summer rains began on Christmas day."
1743. February 1st.—“The late rains have damaged the store house.”
1747. April 11th.—“Unusual drought for several months past.”
1753. August 27th.—“Great damage has been done by the late heavy rains.”
1754. February 11th.—“Rupert's and Banks's fortifications have received damage by the late heavy rains.”
1756. June 20th.—“A heavy fall of rain on the 19th instant did great damage.”
1763. June 6th.—“Great damage to the fortifications, by the late violent rains.”
1774. June 25th.—“The rains have done great damage to the fortifications at Sandy Bay.”
1781. March 5th.—“The fortifications at Sandy Bay sustained great damage by the late rains.”
1787. March 28th.—“The leeward defences suffered greatly by the floods.”
1789. March 24th.—“The late flood has damaged the fortifications at Sandy Bay.”
1797. April 27th.—“Great damage was done to the fortifications by the floods.”
1809. March 4th.—Heavy rains that fell in the short space of one hour, damaged the road upon Ladder hill, overflowed the water course in James's Town, and damaged several houses.
1811. February 22d.—“Heavy rains overflowed the water course in James's Town, and dammed some houses ; as well as some plantations in Sandy Bay .”
These passages will skew that the rainy seasons were expected, and usually set in much about the same periods as in later times ; that is in January, or February ; which are called “the summer rains,” and in June and July “the winter rains.”
So far, therefore, as relates to the times of the rains setting in, there seems to have been no difference : but the floods that happened on the 5th of May, 1719, and on the 6th of June, 1763, are rather remarkable ; for the first was entirely out of season, and the latter was much earlier than the rains usually set in.
In the preceding extracts I have given every record I can find of damages sustained by the heavy rains ; and by those it appears, that the fortifications of James's Town, Rupert's Valley, Banks's, and Sandy Bay, and some plantations in the low grounds, have all occasionally been subject to great damage.
It seemed to me, before I left England, that some vague accounts of these floods, and of the great damage done by them, had gone abroad, and had led to very inaccurate conclusions; for it was a generally received opinion that there would be much risk in loosening the soil of St. Helena, for the purposes of agriculture as it would be liable to be washed away by such violent torrents of rain as had frequently happened.
But those who entertained such erroneous notions could never have been informed of the real causes of the damages they had heard of: nor could they have known that these damages had been partial, confined merely to the bottoms of valleys or ravines, and particularly to the mouths of the vallies, where the torrents descending from naked and steep sides of the mountains, had accumulated, and were forcing a passage into the sea. This was evidently the case at all the four places above-mentioned.
At each of those places during those severe floods, the rains that fell upon the upper surface of the ravines, which extend half a mile and upward across, and penetrate from two to four miles inland (and to which many small branches communicate) must have been immense ; and the force of the accumulated waters when confined in narrow channels in the low grounds, must have been irresistible. It is to these circumstances that may be justly ascribed the devastations that have taken place: but such evils can never occur upon lands laying upon a gentle declivity, and so situated, as to receive only those rains that fall perpendicularly upon them. Even three or four inches of rain, falling in one day upon fields of this description, (and particularly if they are ploughed) so far from doing injury, would undoubtedly be of the greatest advantage—because the loose soil by readily absorbing every drop of rain, would long retain the moisture—and consequently promote vegetation.
There are betwixt two and three thousand acres of the above description on this island ; which I have no hesitation in declaring might be broken up with the greatest safety—and made to yield excellent crops of potatoes, mangel wurzel and corn-from which the supplies of vegetable and animal food, would become abundant—and the inhabitants might very soon be relieved from their present dependence on foreign imports.
Upon an average, the number of days, throughout the year, on which rain falls, is 135. The wettest months are usually January, February, and March ; and July and August.
As wet or dry seasons depend on heavy rains and fine showers, the following is a comparison of the six years :
|1806,||heavy rains and fine showers,||–||92 Days.|
Now, as my measurement of the fall of rain, from the 22d February 1811, to the 21st of February 1812, inclusive, gave 22,4 inches, and this in one of the driest seasons in the above comparison, it may be presumed that in wetter years the fall of rain at St. Helena exceeds that in London, and in several other places in England.
|“Rain falls—At London , being the average of the following years, 1774, 5, 6, 7, 8, 9, 80 ;|
|1789, 90, 91, 92||Inches||21.25|
|Lincolnshire, in medium season||–||–||18|
|Ditto, extreme wet||–||–||24|
|Townley, in Lancashire||–||–||42.5|
|Kendal, in Westmoreland||–||–||61.25|
|Dumfires, in Scotland||–||–||36.25|
In India , there is a remarkably striking difference between the seasons of rain, and at St Helena . According to an exact measurement, taken at Madras , by the late Benjamin Roebuck, Esq. from the year 1791 to 1803, it appears that the fall of rain from the beginning of January to the end of May, is so trifling, as scarcely to affect the rain-gage. Indeed, during the month of March, not a drop of rain fell in the period of thirteen years. There were moderate showers during June, July, August, and September ; and heavy rains fell in October, November, and December. But at St. Helena, excepting in very dry seasons, there are usually some heavy rain, fine showers, or drizzling rain in every month of the year.
The natives of India provide for their dry months, by retaining the monsoon rains in tanks or reservoirs, several miles in length which serve for their crops until the rainy season returns.
|Abstract of Mr. Roebuck's account of Rain which, fell at Madras,
from 1791 to 1803, inclusive.
|Average fall of Rain in each Month during Thirteen years.|
The average fall of rain was 49.3 inches per annum. It sometimes happened in the month of November, after unceasing rains, that the quantity which fell during 24 hours, measured 7 inches.
|Abstract of the fall of Rain at Plantation-house, from 22d Feb.
1811, to 21st Feb. 1812.
|12||:||8||Feb. to 21||2||:||6|
|Inches||22||:||4||Total fall of rain during 12 months.|
This is less than it actually was—because evaporation is very considerable here—and no allowance is made for what unavoidably took place upon an open surface, 9 inches in diameter. Being now provided with proper apparatus, which receives the rain through a small tube into a bottle, the present year's measurement will be far more accurate. I consider, that if evaporation had been prevented, the last year's measurement (even in what was reckoned a dry year) would have been about 24 inches instead of 22 : 4.
24th Feb, 1812.