The adjustment for the difference of capacity of tube and cistern is effected previous to every observation, and the correction for capillarity and reduction to the temperature of 32° is made at the close of each month.
The difference between its readings and those of the Greenwich standard is scarcely appreciable, being only 0·002 inch.
The Dry and Wet Bulb Thermometers, also made by Barrow, are considered to have identical readings under similar circumstances, and both, too, agree with the Greenwich standard thermometer. The Dew-point apparatus, now discontinued, approximates very closely in its readings to the dry and wet bulb thermometers.
The Self-registering Thermometer is a large Six made by Dollond in 1840, and its average difference from the standard is within 2/10ths of a degree. A duplicate and precisely similar thermometer (which has also been repeatedly compared with a standard at every part of the scale) is fixed by its side, so that in case of No. 1 getting out of order, No. 2 can be resorted to without detriment to the results.
These instruments all have a northern aspect, and are placed about 4 feet above the ground. The naked thermometers employed for indicating the relative amount of solar and terrestrial radiation, are precisely similar to those in use at the Government Observatories.
The Rain and Evaporation Gauges are 8 inches in diameter, and the metres are graduated to the 1/1000th part of an inch. Both are read off daily. The aperture of the rain-gauge is about 7 feet above the ground. The evaporation dish is mounted on a moveable stand, 4 feet 4 inches in height, and the circular shelf on which the vessel rests, is just large enough to hold it. The gauge receives a fair proportion of wind and sunshine, and is always exposed in the open air during the day, except when rain is falling. At night and in wet weather, it is placed under a capacious shed, 9 feet in height, and open in front. Thus, it is conceived that the evaporating surface is freely acted upon by all the circumstances concerned in promoting this important natural process.
The direction of the wind is taken twice daily, and its force is registered on an arbitrary scale from 0 to 6; the highest number is reserved for storms approaching the hurricane in violence, and is very rarely recorded.
Remarks on the Weather in 1849.
January.—A damp wet month, except the first week, when sharp frost prevailed. The mean temperature is 0°·68 above the average. On the night between the 2d and 3d, a naked thermometer on the grass fell to 4°, and one on raw wool to 2°·8 below zero, being the lowest temperature I have recorded. The radiation indicated by raw wool was 21°·5. Between one and two o'clock on the morning of the 10th, a terrific thunder-storm burst suddenly over the town, and spread great alarm amongst the slumbering inhabitants. Seven or eight dazzling discharges of the electric fluid, followed by deafening crashes, succeeded each other in about as many minutes. The storm was almost vertical; and between several of the flashes and the accompanying thunder, there was scarcely an appreciable interval, certainly not more than a single second of time. The war of the elements ceased as suddenly as it commenced, and altogether, the storm did not last more than ten minutes. The wind, which previously blew a heavy gale, lulled almost to a calm as the last peal died away. The storm was followed by a heavy fall of rain and hail. It appears to have been pretty much confined to this town and neighbourhood. Thunder was also heard on the evening of the 14th, and lightning was seen on the nights of the 21st, 26th, and 29th. Saturn's ring was perceived at this Observatory on the night of the 31st, after a long continuance of damp, wet weather. As this singular appendage was readily seen, and was well and sharply defined, I have no doubt the instrument would have shewn it ten or fourteen days earlier, had the nights been at all favourable. The ring was also seen on the night of the 11th of September 1848, during its temporary reappearance.
February.—A fine, dry, and mild month. The temperature 3°·49 above the average of twelve years. On the 11th, the barometer attained the remarkably high point of 30·82 at this Observatory, which is about 90 feet above the sea level. At the Royal Observatory, Greenwich (40 feet above sea), the maximum was 30·85, being greater than any reading since January 1825, when the barometer at the Royal Society's apartments attained to 30·841, at 81 feet above the sea level; and there is no other instance recorded in the Philosophical Transactions of a reading so high as 30·8, from the commencement of the series in 1774. The maxima of pressure recorded on the 11th in various parts of the country, were all found to give a reading of 30·90 at the mean sea level.