Various forms of hygrometers are in use, in which a pointer is intended to indicate on a dial the percentage of atmospheric humidity. That shown in Fig. 160 is one of the common forms. Instruments of this kind depend for their action on the absorptive property of catgut or other materials that are sensitive to the moisture changes of the air.

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.

Fig. 160. - Dial hygrometer.

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.

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.