The above photos show the Casella T8563 Humidity Slide Rule, to be used to calculate the relative humidity from the wet bulb and dry bulb temperatures.  The slide rule consists of two parts, a transparent plastic temperature scale with Celsius degrees on one side and Fahrenheit degrees on the other, and a printed white card with the relative humidity scale.  The white card has a humidity scale on one side for whirling (sling) or aspirated hygrometers and a scale on the other side for screen or masons hygrometers.  The white card may be reversed to be used with either Celsius or Fahrenheit temperatures.  The slide rule is 8.5 inches long.

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