Page:Handbook of Meteorology.djvu/197

 To loosen the index, tap the edge of the metal scale with a small piece of wood—say, a clothes pin—until it becomes free.

With bulb end down, let the thermometer fall vertically an inch or more so that it strikes endways on the table or the shelter floor. Little by little the alcohol will flow along the tube; the broken parts of the column become shorter; and the bub- bles disappear.

If this fails, hold the thermometer at its upper end and bring it down forcibly as though striking hard blows with a hammer—being careful, however, not actually to strike anything. It may require vigorous exercise, but the centrifugal force will finally bring the broken parts to the rest of the column. It usually requires from a quarter to half an hour to put a minimum thermometer with a broken column in order. Great care must be used that no part of the alcohol remains in the chamber at the farther end of the tube.

Thermometer Shelter.—Maximum and minimum thermometers should be sheltered from the sun and from direct contact with precipitation of every sort. They also must be placed so that they are in contact with free air. They must be sheltered from heat radiated from buildings, metal roofs, and pavements. The board support should not be attached directly to the wall of a building; if on a porch it should be attached to an outrigger that leaves a space of a few inches from the building. A wide, covered porch with a northerly or an easterly exposure is the best position about a building.

The daily maxima on the south side of a house, within 3 feet of the wall will be from 2 degrees to 6 degrees higher in clear weather than those on the north side, close to the house. The minima will vary but little. If only a window exposure is available, a north-facing window should be selected, and the thermometers should be screened from the window if the room is heated. There should be several inches of space between the shelter front and the window.

In cities, the flat roof of a building frequently offers the best position for thermometers. A graveled roof reflects less heat than a metal roof, and should be preferred when possible. In any case the shelter should be placed where the thermometers are not affected by heat reflected from nearby walls. The best