Page:Results of meteorological observations, 1894.djvu/17

 sonian Tables. The mean temperature of the dew-point is obtained by multiplying the difference between the dry and wet bulbs by Glaisher's factors and substracting the product from the dry bulb reading. The mean humidity is obtained from the readings of the dry and wet bulb thermometers by means of Mr. H. F. Blanford's Tables based on August's formula.

The daily maximum temperature of solar radiation is obtained from an exposed black bulb thermometer enclosed in a hermetically sealed glass tube.

The grass radiation thermometer No. 60687 is placed over grass, but above it and unscreened by it, and the amount of terrestrial radiation is found by subtracting the minimum temperature on the grass from the minimum temperature in shade.

The proportion of cloud is obtained by the inspection of the sky, a clear sky being 0, and an overcast sky 10.

The direction of the wind is given by the anemoscope invented by Mr. G. V. Jaggarow, the original founder of the observatory.

The amount of ozone is obtained by means of Negretti and Zambra's test papers according to the scale of ten tints distinguished by numbers from 1 to 10. The test paper is suspended by a hook attached to the roof of the inner cylinder of Sir James Clark's ozone cage hanging freely exposed to the air, protected from rain and the direct rays of the sun. The amount of ozone is registered once a day at 10

The velocity of the wind is registered by Robinson's anemometer, the readings of the dial being taken at four equidistant intervals during the 24 hours.

The amount of rain is measured by the pluviometer invented by the late Mr. G. V. Juggarow (vide page 188 of Buchan's "Handy Book of Meteorology" and pages 98 and 99, Chambers' "Encyclopædia," Vol. VIII ) consisting of a funnel having a diameter of 4·697 inches or a receiving area of 17·33 square inches. Since a fluid ounce contains 1·733 cubic inches of water, it follows that for every fluid ounce collected by this gauge, the tenth of an inch of rain has fallen.

No change has been made since 1891 in the position of the instruments or in the times or manner of observation.