Page:Handbook of Meteorology.djvu/231

 forms are now made by several manufacturers. Those of the best quality retain the name of the manufacturer; others are stamped with the name of the retailer. When new and clean, those of the best quality are about equal in accuracy to the psychrometer. Hair hygrometers usually deteriorate with continued use. The chief trouble comes from dust, and from gumming or fouling of bearings. Like any other delicate mechanism, careful cleaning and oiling will prolong the life of a hair hygrometer and preserve the accuracy of its registration. In the case of a commercial hygrometer, when once it has gone wrong, it is usually less expensive to purchase a new instrument than to repair an old one.

In spite of its shortcomings, the convenience of the hair hygrometer outweighs its disadvantages. For use in dwellings, school-rooms, textile establishments, candy factories and tobacco factories it is far better than the ordinary Mason type of instrument. The humidity is read instantly; computation from reference tables is not required.

The hygrograph is a hygrometer with recording mechanism like that of the thermograph. Drum, clock and record sheets are much the same in both, except that the record sheets of the hygrograph show per cent values in their horizontal rulings. The instrument is delicate and very sensitive to changes. Its records are not always trustworthy, but its errors are readily checked and adjustments are easily made. It should be sheltered so that by no possibility can rain or snow be driven upon it.

A hygrograph is usually a part of the equipment of each Weather Bureau station. In spite of the difficulties of transportation it is a very useful instrument in field stations. It is useful not only in noting the changes in relative humidity; it also may be an indication of change in absolute humidity.

The normal movement of the thermograph pen is upward from sunrise until 3 o’clock, then downward to the minimum of the next morning; the normal curve of the hygrograph is opposite in movement—downward from sunrise to 3 o’clock and then upward. These movements usually are so regular that experience enables one to read temperature approximately from the hygrogram sheet, and humidity from the thermogram. But when the temperature line is normal and the humidity line is abnormal, a change in the absolute humidity has occurred.