Skip Zone: In order to provide reliable service by reflec¬tion from the ionosphere, several frequencies spanning both bands must be used, depending on the time of day. Ordinarily, a frequency higher than 10 MHz is required during the hours around midnight.
Radio service in the MF and HF bands is characterized by a skip zone, which is the region between where the wave can be received as a direct ground wave and where it can be received as a reflected sky wave. This skip zone is shown in Fig. 5.
There are eleven hardiness zones, which are defined according to lowest winter temperatures. Zone 1 represents the coldest conditions, with winter temperatures below -50°F (-46°C); zone 11 is the warmest, wit] a minimum winter temperature of above 40°F (above 4°C). The classification of a plant into a particular hardiness zone means that the plant will normally thrive in that zone and also in any zone which is marked with a higher number.See Also Danger Zone:Your boat's danger zone is the arc from dead ahead to 22.5° abaft the star¬board beam. A vessel in your danger zone has the right-of-way if coming toward you — it is the stand-on vessel. Your Boat is the give-way vessel and must alter course, slow down, or stop (if neces¬sary) to avoid collision. The stand-on vessel must maintain course and speed, and may sound one
short blast of its horn; if it does so, you must answer with one short blast.
Quicksands are considered especially danger¬ous because they cannot be distinguished at sight, but the belief that they actually suck in men or animals is erroneous. They have figured considerably in literature, notably in Wilkie Col-lins' Moonstone and in The Bride of Lammer-tnoor, by Sir Walter Scott, and with probably some exaggeration of the danger. The danger of sinking in quicksand is increased by struggling, but decreased by lying flat, face up, as if floating; this is because the sustaining power of the water is increased by the particles of matter suspended in it. To make it possible to work in quicksands, engineers may congeal them by forcing into them brines and liquids at low temperatures.
QUICKSILVER. See MERCURY (element).
On The Other Hand See From Zone:from zone this data he can obtain in his negative any one toni and will know exactly the tones that other subject lumi nances will produce. The infinite gradation of light an< shade found in nature Adams divides into ten zones Zone O is black, Zone IX is white. Between these ex tremes are eight tones of gray, Zone V being the "middle tone—not by objective measurement, but by subjectivi judgment—and next to it, marked VI, the value tha conveys to the photographer the feeling of the tone o average, well-lighted skin.
Gutenberg found that abnormally low velocities of seismic waves occur in a zone centered at about 62-124 mi (100-200 km) be¬low the surface. In this low-velocity zone the mantle transmits earthquake waves more slowly because it is close to its melting point and therefore less rigid than usual. A few years after this discovery Sir Harold Jeffreys of Cambridge University and K. E. Sullen of Sydney, Austr., discovered that between the depths of 210 and about 380 mi (338 and about 612 km) there is a rapid change in density—a transition zone in which the density suddenly increases by 20%.
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