"By dividing the neck A B into six or eight divisions, with the aid of a diamond or piece of flint, and then marking the lines so cut, with ink, an approximate graduation of degrees of pressure may easily be obtained.
"I show you a water barometer here, ([Fig. 74]) that is somewhat less hard to construct than the one I have already described, as the parts are easier to obtain.
Fig. 74. Barometer
"It consists of a bottle, containing water, inverted and suspended with its mouth in the jar of the same fluid. It is capable of roughly indicating atmospheric changes in a similar way to the mercurial barometer. When the atmosphere becomes denser, the greater pressure on the surface of the water in the jar causes it to rise in the bottle; while with a lesser density it falls. As with the mercurial barometer, temperature makes a slight difference, which, strictly speaking, should be allowed for; but, as the arrangement is of such a simple character, this may be ignored. Water, also, is more subject to evaporation than mercury, besides going stagnant, and will require occasional changing and replenishing.
"A barometer of a more scientific character, and more presentable, is, I think, within your range of skill, and it may be made as follows: Obtain a glass tube, closed at one end, about two feet ten inches long and three eighths of an inch thick, with a bore of about three sixteenth inch. A circular turned wood box, one and one half inches in diameter and one and one fourth inches deep, is required for the cistern. Cut out the bottom and glue on instead a piece of leather, sagging loosely. Then cut the lid in two, and make an opening in the centre to receive the tube.
Fig. 75. Thermometer