Here is, in a few words, what the notices relative to the observation of the barometer contain: 

1.  When, after a rather long continuance of fine weather, the barometer begins to fall in a sudden and continuous manner, rain will certainly fall; but, if the fine weather has had a long duration, the mercury may fall two or three days in the tube of the barometer before any change in the state of the atmosphere may be perceived.  Then, the longer the time between the falling of the mercury and the arrival of the rain, the longer will be the duration of rainy weather.

2.  If, on the contrary, during a rainy period which has already had a long duration, the barometer commences to rise slowly and regularly, very certainly fine weather will come, and it will last much longer if a long interval elapses between its arrival and the rising of the barometer.

3.  In the two cases given, if the change of weather follows immediately the movement of the barometrical column, that change will last only a very short time.

4.  If the barometer rises with slowness and in a continuous manner for two or three days, or even more, it announces fine weather, even when the rain will not cease during those three days, and vice versa; but if the barometer rises two days or more during the rain, then, the fine weather having come, if it commences to fall again, the fine weather will last a very short time, and vice versa.

5.  In the spring and in the autumn, a sudden fall of the barometer presages wind.  In the summer, if the weather is very warm, it announces a storm.  In winter, after a frost of some duration, a rapid falling of the barometrical column announces a change of wind, accompanied by a thaw and rain; but a rising which happens during a frost which has already lasted a certain time, prognosticates snow.

6.  Rapid oscillations of the barometer should never be interpreted as presaging dry or rainy weather of any duration.  Those indications are given exclusively by the rising or the falling which takes place in a slow and continuous manner.

7.  Toward the end of autumn, if after prolonged rainy and windy weather, the barometer begins to rise, that rising announces the passage of the wind to the north and the approach of the frost.

Such are the general consequences to draw from the indications of this precious instrument.

Dick Sand knew all that perfectly well, as he had ascertained for himself in different circumstances of his sailor’s life, which made him very skilful in putting himself on his guard against all contingencies.

Now, just toward the 20th of February, the oscillations of the barometrical column began to preoccupy the young novice, who noted them several times a day with much care.  In fact, the barometer began to fall in a slow and continuous manner, which presages rain; but, this rain being delayed, Dick Sand concluded from that, that the bad weather would last.  That is what must happen.