The instrument described in the paper is made of German silver, and is about a foot in length, and of an inch in breadth; along the middle of it there is a groove for a slider. On the right edge of the groove is engraved a scale of inches, and on the left the degrees of temperature from 0° to 85° Fahrenheit, each being placed exactly opposite that point of the scale of inches which measures the corresponding tension of vapour. On the left edge of the slider is engraved a scale of equal parts, each of an inch; on the right edge, and having the same zero, is a vernier, applicable to the scale of inches. The lines on these scales are ten times larger than those which the symbols in the dew-point formula represent, but their numerical designations are not changed.