E 4
The rainfall recorded at the Public Gardens in 1911 was as follows: - January 0.67, February 0.04, March 2.79, April 5.52, May 23.07, June 5.88, July 9.50, August 22.63, gauge choked on 6th, September 8.19, October 6.31, November 2.93, December 0.41, the total for the year being 87.94 inches. These figures are furnished by the Superintendent of the Botanical and Forestry Department. The total fall at the Gardens was 2.61 inches less than that registered at the Observatory, but this is in part due probably to the choking of the gauge on August 6th. On an average of the five preceding years the fall at the Gardens was 10 per cent. in excess of that recorded at the Observatory.
15. The comparison of the records of the Robinson Anemograph (Beckley pattern) and the Dines pressure-tube anemograph has been continued in 1911.
Particulars of the positions the instruments occupy on the roof were given in last year's report. No change has been made, but in August the recording apparatus of the pressure-tube instrument was taken asunder, cleaned, and the water changed, distilled water being used as before.
The records of the pressure-tube instrument have been tabulated independently by two observers and the result of the comparison of the two instruments is now given under different velocities and wind directions, for the period May to December 1910 in Table XII, and for that of January to December 1911 in Table XIII.
To reduce the actual run of the cups of the Robinson to velocities recorded by the pressure-tube instrument, using all the observations, the factor for the year 1911 was found to be 2.26, as against 2.22 found for the period May to December 1910; or omitting velocities below 6 miles per hour (Robinson) 2.29 and 2.25 respectively.
On reference to the tables it will be seen that the adoption of a constant factor to reduce the Robinson to the pressure-tube velocities would, in the case of our instrument, be impracticable - the factor varying considerably with wind direction.
On plotting the results on a chart and smoothing the curves, maxima are found with the wind direction near the East and West points, and minima near the North and South points, the difference in the factor varying approximately from 2.3 in the former to 2.0 in the latter.
The higher factor obtained with winds from the East and West may be due, in part, to the sheltering influences of the chimney stacks which are situated at a distance of 22 feet due East and West of the Robinson, the cups, however, being placed at a height of 42 feet above the highest point of the stacks. For these winds the pressure-tube instrument would be little affected, as it is placed on the Northern edge of the roof. There are probably other contributory causes, such as the rectangular shape of the building (83 feet by 45 feet, the greater length lying in an East and West direction), the configuration of the surrounding ground, also the character of the wind, which is very much steadier from Easterly than from other directions.