After tea, the family went down to the river's bank and chatted awhile on home matters; then shortly after the sun went down, they adjourned to "the lion's den."
"Now," said George, "father will tell us about barometers and thermometers, as he promised."
"Well," said Mr. Gregg, "I'm pleased to know you are so ready to listen to my talks, and I hope you'll remember some of the facts I've been telling you.
"There are many kinds of barometers, but all are constructed about on the same principle, and on the old theory that 'nature abhors a vacuum'. There may have been some kind of an instrument that did service as a barometer in the early ages, but we have no knowledge of it. The instrument as we now know it had its beginning with Galileo, Torricelli, and Pascal, but was not perfected until about 1650. Good barometers require the greatest possible care in their construction, and there ought to be two or more standing together as checks on one another in order to obtain correct results. The mercury used must be pure and good, free from all other substances and from air bubbles or films of air on the sides of the bulb. Simple barometers, suitable for ordinary purposes, can be easily made. I will describe one, and make a sketch of it on the blackboard.
Fig. 73. Simple barometer
"This simply consists of a wide-mouthed glass bottle filled with ordinary drinking water up to the point indicated by the letter A ([Fig. 73]); in this is dipped an inverted glass flask, or an incandescent light bulb, the extremity of the neck being allowed to dip just below the surface of the water.
"The flask should be inverted quite empty during wet weather, and as long as the atmosphere remains in a stormy condition, no change in the water takes place; but immediately the weather becomes finer, the water will rise in the neck of the inverted flask, and, if a continuance of fine weather be probable, will rise to the point indicated by letter B.
"I have found this simple contrivance to give sure and early warning of the approach of rain, and I need hardly remark that the principle upon which this little weather glass acts is exactly similar to that of the ordinary mercury barometer, for the rise and fall of the water is due to the respective increase or decrease of atmospheric pressure.