19 STORM SIGNALS
GREAT BRITAIN | ||
 A black cone point upwards indicates that a gale from NW., through North, to SE. may be approaching. |  A black cone point downwards indicates that a gale from SE., through South, to NW. may be approaching. | |
| Three lights of like color suspended from the corners of a triangle replace the cone at night. | ||
FRANCE | ||
 Gale from NW’d. |  Gale from SW’d. |  Hurricane. |
 Gale from NE’d. |  Gale from SE’d. | |
20 EXPLANATION OF INSET CHART.
The inset chart below shows the normal reduced barometric pressure and the normal temperature of the atmosphere for the month, and the annual change in the variation of the compass.
Isobars are shown by full and dashed blue lines; isotherms by dotted blue lines.
Any wide departure from the normal pressure shows some disturbance and may indicate a coming gale.
21 ISOBARS AND ISOTHERMS FOR THE MONTH.
Readings of mercurial barometers must be corrected to 32° F. and to standard gravity, by the tables given below, in order to compare them with the pressures shown on the inset chart. Aneroid barometers require no correction for gravity.
| To 32° Fahrenheit. | To standard gravity | ||
|---|---|---|---|
| Att. Ther. | Corr. | Lat. | Corr. |
| ° | In. | ° | In. |
40 | −.08 | 0 | −.06 |
50 | −.06 | 10 | −.06 |
60 | −.06 | 20 | −.06 |
70 | −.11 | 30 | −.04 |
80 | −.14 | 40 | −.01 |
90 | −.17 | 50 | +.01 |
The red lines show the annual change in the variation of the compass. The direction of movement of the north end of the magnetic needle is indicated by E. or W.