Concavity of mercury surface

1429. Concavity of the surface of the mercury. Persons who have occasion to travel much in the winter, and who are doubtful whether it will rain or not, may easily ascertain this point by the following observation:-A few hours before he commences his journey, let the traveller notice the mercury in the upper part of the tube of the barometer; if rain is about to fall, it will be indented, or concave; if otherwise, convex or protuberant.