Hygrometer
For horticultural purposes, the best instrument for ascertaining the degree of humidity in the air is the Dry and Wet Bulb Thermometer. This consists simply of a couple of fine tubes, carefully graduated, containing mercury. The bulb of the one should be covered with thin muslin; and round the neck, and over the muslin, should be twisted loosely, or tied in a loose knot, a conducting thread of lamp wick or some similar material: this must pass into a vessel of water, about 3in. from the bulb, and a little on one side, so that evaporation may not affect the reading of the dry bulb by its too near vicinity. The Hygrometer just described is less complicated and expensive than those in which the dew-point is ascertained by the use of ether, &c.; and, moreover, it allows of continuous observations. Hygrometrical tables, adapted to the use of the Dry and Wet Bulb Thermometer, have been compiled by James Glaisher, F.R.S., and published in pamphlet form.