=61. The sling psychrometer.= The standard form of this is shown in the illustration, and is the one used by the Weather Bureau. This instrument can be obtained from H. J. Green, 1191 Bedford Ave., Brooklyn, or Julien P. Friez, 107 E. German St., Baltimore, at a cost of $5. It consists of a metal frame to which are firmly attached two accurately standardized thermometers, reading usually from –30° to 130°. The frame is attached at the uppermost end to a handle in such fashion that it swings freely. The wet bulb thermometer is placed lower, chiefly to aid in wetting the cloth more readily. The cloth for the wet bulb should be always of the same texture and quality; the standard used by the Weather Bureau can be obtained from the instrument makers. A slight difference in texture makes no appreciable error, but the results obtained with different instruments and by different observers will be more trustworthy and comparable if the same cloth be used in all cases. The jacket for the wet bulb may be sewed in the form of a close-fitting bag, which soon shrinks and clings tightly. It may be made in the field by wrapping the cloth so that the edges just overlap, and tying it tightly above and below the bulb. In either case, a single layer of cloth alone must be used. The cloth becomes soiled or thin after a few months’ constant use and should be replaced. It is a wise precaution to carry a small piece of psychrometer cloth in the field outfit. [Illustration: Fig. 6. Cog psychrometer.]