Dilatabüity of Air by Heat.

37 5

Chap. 2.]

sels being tight. Now it is clcar that if a communication was opened bya tube between the interior of one of these boxes and a vessel of waterplaced a few feet below, that the liquid would be forced into it (by theatmosphere) until the contained air occupied no greater space than it didbefore any part was driven*'out by the heat. This mode of raising liquidsmay be illustrated at the tea-table : Let a saucer be half filled with coldwater, hold an inverted cup just over it and apply for a moment a smallslip of lighted paper to the interior of the cup, drop the paper on thewater and cover it with the cup, when the liquid contents‘of the saucerwill be instantly forced up into the inverted vessel.

If an inverted glass siphon be partly filled with water and the orificeof one leg be then closed and that leg be held to the fire, the air expand-ing will drive out the liquid and cause it to ascend in the other leg.Several philosophical instruments illustrate the same thing. Previous tothe discovery of atmospheric pressure and the invention of the barometer,the expansion of air by heat was the principle upon which the ancientweather glasses were constructed. They were made in great variety.The simplest consisted of a glass tube having a bulb blown on the closedend. It was held over a fire to dilate the air, and the open end wasthen plunged into a vessel of wateri Its construction was the same as themodern barometer. Variations in the temperature and density of theatmosphere caused the water to rise and fall in the tube, as the containedair was dilated and contracted, and thus changes in the weather wereindicated. From these instruments the barometer received its former nameof “ the weather glass.” a

The degree of elevation to which water can be thus raised dependsupon the temperature to which the contained air is subjected ; its dilatationor increase of bulk being, according to some authörs, in common with

a The following extract from a book published ten years before the diseovery of at-mospheric pressure, may interest some readers. Although the instruments to which itrefers are no longer in use, they ought not to be entirely forgotten.

“ A weather-glasse is a structure of at the least two glasses [a tube and the vesselcontaining the water] sometimesof three, foure, or more as occasion serveth, inclosinga quantity of water, and a porcion of ayer proporcionable ; by whose condensacionor rarifaction the included water is subject unto a continual mocion, either upward ordownward ; by which mocion of the water is commonly foreshown. the state, change,and alteracion of the weather;—for I speak no more than what my own experience hathmade me bold to affirm ; you may (the time of the year, and the following observacionsunderstandingly considered) bee able certainly to foretell the alteracion or uncertaintyof the weather a good many hours before it come to pass.

There are divers severall fashions of weather-glasses, but principally two. 1. Thecircular glasse. 2. The perpendicular glasse. The perpendiculars are either single,double or treble. The single perpendiculars are of two sorts, either fixt or moveable :The fixt are of contrary qualities ; either such whose included water doth move upwardwith cold, and downward with heat, or eise upward with heat and downward with cold.In the double and treble perpendiculars, as the water ascendeth in one, it descendeth asmuch or more in the other. In the moveable perpendiculars, the glasse being artificiailyhanged. it moveth up and down with the water.”

The author then describes the various kinds mentioned and teils his reader “ if you doewell observe the form of the figures you cannot go amisse.” He also gives directions formaking coloured water for the tubes, such as “ may be both an Ornament to the work anddelectable to the eye.” Treatise on Art and Nature, A. D. 1633 or 4. See account of thisbook page 321. A modification of an air-glass may be found in the Forcible Movementsof Decaus, (plate viii,) which he names an Engine thatshall move of itself. Lord Bacon ,in whose time these air glasses were common, presented what appears to have been animproved one and of his own invention, to the Earl of Essex, who it is said, was so capti-vatedwith it that he presented the donor with Twickenham Park and itsgarden, as a

E lace for his studies. The instrument was named * A secret curiosity of nature, whereby tonow the season ofevery hour of the year, by a Philosophical Glass,placed {with a smallpropor-tionof water ) in a ckamber. An account of Lord Bacon’s Works, London , 1679.