face temperature, especially on northerly exposures, may register several degrees—on occasions, as many as 10 degrees—lower than the thermometers 6 feet or more above the surface.[1]
Thermometers on the business streets of cities, especially those in which the blocks are solidly built, register from 3 degrees to 6 degrees too high as a rule, owing to radiation and reflection from nearby buildings. They indicate the temperature of the street, but not of the free air.
The Six Maximum and Minimum Thermometer.—A maximum and minimum thermometer of the Six pattern consists of a glass tube bent in two or three sections as shown in the accompanying figure. The tube in the center is a cylindrical bulb about 0.1 inch internal diameter; the bulb at the top of the right-hand column is large enough to have a volume of about 1 cubic centimeter. The bore in the U part of the tube is about 0.02 inch in diameter; it is filled with mercury. The central bulb is completely filled with a solution of creosote, or with alcohol. The expansion of the liquid in the central bulb pushes the mercury down on the left side of the U and up on the right side; it also pushes liquid into the air bulb on the right side, slightly compressing the air and vapor in the bulb. Lowering temperature causes a contraction of the liquid in the central bulb, thereby drawing back the mercury in the U. This is made more positive by the compressed air and vapor in the right-hand bulb. The scale reads downward on the left and upward en the right side. These are marked respectively “cold” and “heat,” or “night” and “day.”
Maxima and minima are recorded by separate indices within the bore of the tube. The indices are pieces of steel wire coated with glass—in some thermometers they are plain wire—each armed with two appendages. On one end the appendage points upward; on the other, downward. Their object is to hold the index lightly to the place in the bore to which the mercury pushes it. Pushing the indices is the only work the mercury in the U tube performs. Rising temperature pushes the index in the right-hand tube upward; falling temperature pushes the index in the left-hand tube upward. The- ↑ In the latitude of middle England Sir Napier Shaw notes that thermometers on the grass register lower by 20 degrees than those in the shelter, a difference of 8 degrees being very common.
