Fig. 51.
Stevenson’s Thermometer Screen.
The earth thermometer chiefly used is shown in Fig. 52. It consists of a sluggish thermometer mounted in a short weighted stick attached to a strong chain, and of a stout iron pipe which is drawn out at the bottom to a point and driven into the earth, usually to a depth of 4 feet.
Solar radiation is measured by black-bulb and light-bulb thermometers in vacuo, which are mounted on a post 4 feet above the ground and record the maximum temperature.
Humidity in the air is measured by direct or indirect hygrometers. Of the former Dines’, Daniell’s, and Regnault’s are the best known, but as they are not used in observations acknowledged by the Royal Meteorological Society, the reader may be referred to their description in books on physics. The indirect hygrometer which is universally employed in this country is that furnished by the dry and wet bulb thermometers. In frosty weather they require much attention, and then a Saussure’s hair hygrometer may be used as supplementary. The general arrangement of the dry and wet bulb thermometers is shown in Fig. 53.
The wet bulb is covered with a single layer of soft muslin, while a noose of six to eight strands of darning cotton connects the neck of the wet bulb with a covered water receptacle 2 to 3 inches distant, below and at its side. This receptacle is kept filled with rain-water.
Fig. 52.
Symons’ Earth Thermometer.
Fig. 53.
Dry and Wet Bulb Thermometers.
From the readings of the dry and wet bulb thermometers three deductions can be made: