climate in which he was born, and continued to live. He was thence removed to Tomos, which Dr. Wells, in his maps of an cient geography, places only in the 44th degree of northern lati. tude: the change was therefore only of 2 degrees, and yet Ovid immediately describes it as the winter of Hudson's Bay, with the Euxine sea frozen over, with people and cattle walking on it; as well as other instances of extreme cold.

Besides the quotations from Ovid, Mr.B. gives several others from the ancients, as Virgil, Strabo, Pliny, &c. descriptive of the excessive cold of that latitude. He then contrasts these with the accounts of modern travellers in that country, who have not noticed any such severities of climate there.

Mr. B. now leaving Tomos, compares the accounts of the weather in Italy, with those of the present times: it being first premised, that the country was better cultivated in the Augustan age than it is now, which should consequently have made the temperature of the air more warm than it is now experienced to be. He begins with some passages from Virgil's Georgics. This most ex. cellent husbandman is constantly advising precautions against snow and ice in the management of cattle; and he may be generally supposed to give these directions for the neighbourhood of Naples, or Mantua his native country, where he does not evidently from the context mean some other parts of Italy. Speaking afterwards of Calabria, the most southern part of Italy, he expresses himself, with regard to the rivers being frozen, as what was commonly to be expected. Pliny too in a chapter, De natura cæli ad arbores, and speaking of Italian trees, says, Alioqui arborum frugumque communia sunt, nives diutinas sedere. But perhaps the strongest proof of that very remarkable fact, the Italian rivers being constantly frozen over, is to be collected from a chapter in Ælian, which consists entirely of instructions how to catch eels while the water is covered with ice. Now, if we may believe the concurrent accounts of modern travellers, it would be almost as ridiculous to advise a method of catching fish in the rivers of Italy, which de pended entirely on their commonly being frozen over, as it would be to give such directions to the inhabitants of Jamaica. Mr. B. cannot find that the precautions, which Virgil gives in his Georgies, against the damages which sheep and goats might receive from the snow and frost, are now necessary; and both these animals are

known to stand the severest winters of the Highlands of Scotland, conceived to be in Virgil's time almost the ultima Thule. On the whole Mr. B. infers, that there appears to have been a general melioration of temperature in the air and the seasons, in many, perhaps most parts of the earth.

[Phil. Trans. 1768.

CHAP. XXXVI.

NATURE, PROPERTIES, AND VARIATIONS OF HEAT.

SECTION 1.

Sources and Effects of Heat.

THE sources ofheat are various, but its effects uniform whatever the cause that produces it; or rather, perhaps, the essence or ma terial of which it consists is the same in every instance. The most powerful and extensive source of heat with which we are ac. quainted is the sun, from which perhaps this invisible matter (if matter at all) is emitted, in consequence of the chemical processes that continually take place at its surface. Friction is another powerful source of heat; percussion is a third; and there are a few others which have not yet been sufficiently traced out and explained.

The recent opinions upon this curious subject are given with so much perspicuity, and at the same time such convenient succinctness by Sir Humphry Davy, that through the remainder of this chapter we shall take leave to borrow his words.

WHEN a body which occasions the sensation of heat on our or. gans, is brought into contact with another body which has no such effect, the result of their mutual action is, that the hot body con tracts, and loses to a certain extent its power of communicating heat, and the other body expands, and in a degree acquires this power.

This law may be exemplified with respect to every form of pon

derable matter. If a polished cylinder of tin, which accurately fits a ring, be heated so as to make water boil, it will no longer pass through the ring, and will be found enlared in all its dimen. sions. If spirits of wine be heated in a glass-vessel having a narrow tubulated neck, as it becomes capable of communicating the sensation of heat, it will be found to expand and to rise in the narrow neck; and if the body of the same vessel be filled with air, and it be inverted in water, its neck containing water, the air will ra. pidly expand, on the application of a heated body, and will cause the water to descend in the neck of the vessel.

2. Different solids and fluids expand very differently when heated by the same means.

Glass is less expansible than any of the metals; 100,000 parts raised from the degree of freezing to that of boiling water, expand so as to become 100,083 parts; 100,000 of platinum under similar circumstances expand so as to become 100,087; and equal parts of gold, antimony, cast-iron, steel, iron, bismuth, copper, castbrass, silver, tin, lead-zinc, and hammered zinc expand in the following order: 100094, 100108, 100111, 100112, 100126, 100139, 100170, 100189, 100238, 100287, 100296, 100308. The expansive power of liquids in general is greater than that of solids; alcohol appears to be more expansible than oils, and oils in general more expansible than water. 100,000 parts of mercury of the same degree of heat as ice, become at the degree of heat at which water boils 101,835. All the elastic fluids, or the different species of air that have been examined, as has been demonstrated by Messrs. Dalton and Gay Lussac, expand alike when heated to the same degree; 100 parts of each at the freezing point of water becoming about 137,5 at the boiling point.

It is evident that the density of bodies must be diminished by expansion; and in the case of fluids and gasses, the parts of which are mobile, many important phenomena depend upon this circumstance. If heat be applied to fluids or to gasses, the heated parts change their places and rise; and the colder parts descend and oc cupy their places. Currents are constantly produced in the ocean and in great bodies of water, in consequence of this effect. The heated water rises to the surface in the tropical climates, and flows towards colder ones, thus the warmth of the Gulf-stream is felt a thousand miles from its source; and deep currents pass from the

colder to the warmer parts of the sea and the general tendency of these changes is to equalize the temperature of the globe.

In the atmosphere, heated air is constantly rising, and colder air rushes in to supply its place; and this event is the principal cause of winds: the air that flows from the poles towards the equa. tor, in consequence of the rotation of the earth, has less motion than the atmosphere into which it passes, and occasions an easterly current; the air passing from the equator towards the poles having more motion, occasions a westerly current; and by these changes, the different parts of the atmosphere are mixed together: cold is subdued by heat, moist air from the sea is mixed with dry air from the land, and the great mass of elastic fluid surrounding the globe, preserved in a state fitted for the purposes of vegetable and animal life.

3 There are very few exceptions to the law of the expansion of bodies, at the time they become capable of communicating the sen. sation of heat; and these exceptions seem entirely to depend upon some chemical change in the constitution of bodies, or on their crystalline arrangements. Thus clay contracts considerably in dimensions by a very intense heat, and on the measure of its contractions the pyrometer of Wedgwood is founded; but in this case the clay first gives off water, which was united to its parts, and afterwards these parts cohere together with more force, and from being in a state of loose aggregation become strongly united. Water expands a little before it congeals, and expands considerably during its conversion into ice; but in this case it assumes the crystalline form; and its parts whilst they are arranging themselves to form regular solids, probably leave greater interstices than they occupied when at uniform distances in the fluid. Thus the same weight of matter will occupy much greater space when arranged in a certain number of octahedrons, than when arranged in a similar number of cubes, or hexagonal prisms. Certain saline solutions likewise that shoot into prismatic crystals, expand at the moment they become solid; and the case is the same with castiron, bismuth, and antimony.

The expansion of water during its conversion into ice, is shewn by the circumstance of ice swimming upon water; and if water in a deep vessel be examined at the time ice is forming, it will be `found a little warmer at the bottom than at the top; and these cir.

cumstances are of great importance in the economy of nature. Water congeals only at the surface, where it is liable to be acted upon by the sun, and by warm currents of air which tend to restore it to the fluid state; and when water approaches near the point of freezing it begins to descend, so that no ice can form till the whole of the water has been cooled to the point where it possesses the greatest density; and in the deep parts of the sea and lakes, even in some of the northern latitudes, the duration of the long winter is insufficient to cool the water to the degree at which ice forms.

4. When equal quantities of the same matter differently heated are mixed together, as much as the one contracts, so much the other seems to expand. It is easy to prove this by shaking together 100 parts of mercury so hot as not to be touched without pain, and 100 parts in its common state, having previously measured the space they occupy; if the mixture is made in the tube that contained the hot mercury, there will be no sensible change of volume.

It is on the idea, that when heat or the power of repulsion is communicated from body to body, as much is gained by one body as is lost by the other, that thermometers have been framed, and the doctrines of temperature, and capacity for heat founded.

5. The most common thermometer is a glass bulb, containing mercury, terminated by a glass tube, having a very narrow bore. The mercury is boiled to expel any air or moisture that might be attached to it; and at the moment it is in ebullition, the extremity of the tube being drawn to a fine point, is hermetically sealed by a spirit lamp. For the purpose of acquiring a scale, the bulb is first plunged into melting ice, and the place where the mercury stands is marked; the bulb is afterwards plunged into boiling water and the same operation repeated. On Fahrenheit's scale this space is divided into 180 equal parts, and similar parts are taken above and below for extending the scale, and the freezing point of water is placed at 32°, and the boiling point at 212°. In Fahrenheit's scale 1.8 degrees are equal to one degree of the centigrade thermometer, and 2.25 to one degree of Reaumur.

Other fluids besides mercury, such as alcohol, are sometimes used in thermometers, particularly for measuring low degrees when mercury freezes.