These instruments give fairly accurate readings in a small range for a limited time, but they are apt to go out of adjustment from causes that cannot be controlled. Unless they are occasionally compared with a standard humidity determination, their readings cannot be relied upon for definite amounts of atmospheric moisture.
The Swiss Cottage “Barometer.”
—Fig. 161 is one of the instruments of absorptive class that are sometimes used as weather indicators. The images which occupy the openings in the cottage are so arranged that with the approach of damp weather the man comes outside and at the same time the woman moves back into the house. In fair weather the reverse movement takes place. The figures are mounted on the opposite ends of a light stick which is fastened to an upright pillar. The movement of the images is caused by the change in length of a piece of catgut which is secured to the pillar and also to the frame of the house. Any change in atmospheric humidity causes a contraction or elongation of the catgut which moves the pillar and with it the images.
Fig. 161.—Swiss cottage “Barometer.” This device is arranged to show the condition of atmospheric humidity by the movement of the images. It is not really a barometer.
Since stormy weather is accompanied by a high degree of humidity and fair weather is attended with dry atmosphere, the movement of the images indicates in some degree the weather changes; but the device is not in any way influenced by atmospheric pressure and hence is not a barometer.
Dew-point.
—Dew is formed whenever falling temperature of the air passes the point where saturation occurs. The reduction of the temperature of air raises the relative humidity because of the diminished capacity to contain moisture. As the temperature declines there will come a point at which the air is saturated and any further decrease of temperature will cause supersaturation. At this point the moisture will be deposited on the cooler surfaces in the form of drops. The temperature at which dew begins to form is known as the dew-point. The sweating of cold water pipes, the dew that forms on a water glass and other relatively cold surfaces is caused by a temperature below the dew-point of the air.
Dew-point Table
Dew-point in degrees Fahrenheit, barometer pressure 29 inches