[The] mover of the blue flame does not move of itself, nor does the nourishment of the candle flame move of itself. Therefore such motion is generated by others, and the motion of that generator is the air, which moves with fury to fill of itself the vacuum that was left by the air formerly consumed by the flame. Fire cannot shine within its own sphere unless it is mixed with other elements proportioned to it. And the same happens outside its sphere.

The flame that shines less heats the air surrounding it less, and cannot penetrate the air that stands above it until it becomes lighter than the air, and for this reason it remains with a spherical, blue surface.
The elemented fire does not blend with the element of fire, but they stand in contact, as is seen with moist liquids and oily liquids. The light generates a vacuum, and the air rushes to the relief of that vacuum.

o – p – d c; b – droplet of flame
The lowest part of the flame is the first origin of that flame, through which passes all its oily nourishment; and this is of as much less heat than the rest of the flame as it is of less splendour; and this is of blue colour, and it is the part in which the nourishment is purged and disposed toward a more luminous flame; and this is the first that is born in the creation of the flames, and it is born of spherical shape, and after some life it generates above itself a tiny little flame of splendid colour and of the shape of a heart, with its point turned toward the sky; and this goes on continually multiplying toward infinity by means of the acquisition of the material that nourishes it.

... candle ...
The smoke is generated in the middle of the candle flame, and this... The air of which such a flame is composed cannot pass inside that flame, and this comes about because the air surrounding that flame strikes it with an impetuous motion, and such percussion condenses the surface of that flame and converts itself into it, and leaves empty the place where it became food for that flame, whence the air, following after, relieves the generated vacuum. And thus successively the swift motion continues, striking and pressing back the dilation of that flame, which dilation is born from the exhalation of the smoke that is continually generated within the flame; which smoke, multiplying within the said flame, wishes to dilate it, and the air that continually strikes it from without and drives it back comes to condense it, and such condensation makes the flame bright and shining; and the condensed smoke breathes out through the upper part of the flame and has no exit by any other place, because below it finds the material that generates it, and at the sides it finds the air that strikes it, and above it finds the dilation of the air that refracts it, and through that dilation the smoke has its exit.
There is generated a splendour that clothes the candle flame with itself, and it is clothed by the air that surrounds it; and this is the vacuum of the air that is filled by the element of fire, after which continually the elemented fire is generated, composed of the fiery vapour that is infused in the moist smoke, and the remainder of that smoke dims the edge of the contracted flame, which for this cause becomes a reddening flame and at last of dark redness.
Between the flame and the candle there is generated a revolution of air which, after it has made its percussion at the base of that flame, turns downward and, so heated, strikes the head of the candle and goes dissolving it and preparing it for the nourishment of the aforesaid flame. The rising of the dissolved moisture above the wick of the candle arises from the vacuum that would be generated if the relief of new nourishment did not fill the place where the preceding nourishment was consumed by the heat of the fire. In the contact of the two flames of the candles, for as much space as is the thickness of the potential flame that clothes the elemented flame of one candle, the other potential flame of the other candle does not enter. Then there will be born a yellowish glow of oval shape, which will be of yellowish colour and of doubled heat for as much space as is the contact of the glows of the two said flames. The blue flame that shows itself at the birth of the candle's light arises here from the thinness of that flame, being above the tincture that the wick of the candle has; and the excellent part of that blue is in the lateral edges of that flame, seen from any surrounding side about the dark wick. And this arises because that blackness, seen mirrored in the splendid part, is of great brightness which gives beauty to that blue, like that darkness which is mirrored in the lustrous concavity of the light that clothes such darkness, and so on. The blue flame is generated of spherical shape because it is not of such heat as to exceed the lightness of the air. And for this reason it does not generate of itself a pyramidal shape, but remains in spherical form until it heats the air surrounding it. And because the principal heating of the air is above the principal heat of that blue flame — which heat is generated from that part where the natural desire of the flame is to move, and this is toward the sphere of fire by the shortest way — therefore the fire is generated in the supreme part of the blue spherical flame in a minimal round shape, which roundness immediately takes on some extension and makes itself into the shape of a heart, turned with its point toward the sky; which shape immediately extends and, with swift dilation, masters its nourishment and penetrates the air that made itself a cover for it. But that blue colour remains at the base of that flame, as may be seen in the light of the candle, and this arises because in that place the flame is always less hot than elsewhere, that being the first encounter which the nourishment of the flame makes with the flame itself, and where it generates the first heat, which is the weakest and that which heats least, being the beginning of the heat, and so on.

... Never the material from which its nourishment issues; but the smoke that breathes out from such material interposes itself between the flame and that nourishment, unless the material is charcoal wholly aflame. The flame of the fire, being lighter than the air, often divides itself, leaving a membranous tenacity which, with a lunar shape, joins together the bases of those flames with a membranous transparency and thickness, as the figure below here shows.

Flames are of two natures, of which one has its supreme edge of concave shape and the other has it convex. But the concave flame is always interposed between the two convex ones; and this convexity arises from the concavities of the cracks that are continually generated in the burning material, through which the heated moisture vaporizes; but the lunar flame is that which unites together the convex flames. But the upper part of the lunar concavity, composed of the sides of the other flames, is the densest and the most luminous and of greater heat than any of all the kinds of flames, and its motion is toward the sides of the convex flames; and this arises because beneath that convexity is the fissure, or cracks, whence the heated vapour breathes out, the natural nourishment of the flames, whose motion is by whatever way is given for its exhalation, which issues with swift blowing and with manifest sound, in the manner of a swift wind, and strikes back the flames, into which it is converted amid the upper air, opening it and penetrating it with fury until the vital nourishment lasts in it; which failing, the remainder is converted into smoke, which for some space follows the straight rigour of the generated impetus of the spent flame, after which rigour the smoke slows and obeys with its windings the motion of the winds that are more powerful than it. But by the experiment of the shadow that these fires cast before the sun, we have seen and found the true motion that the air penetrated by the flame makes, and likewise the smoke penetrating that air.
Three quadrilaterals invested by flows of air: Gravity of the air.

Of heat and of cold.
That thing cools more which removes itself more from the heat that is in the power to heat. That heat is of less power which is more distant from the incident and reflected power of that heat. That part of the body heats more which comes closer to the heat; and conversely it heats less which removes itself more from that heat. The cloud freezes above and not below, and more at the sides than below, because the heat extends more upward than to the sides, and more to the sides than downward. That thing becomes lighter which is more heated, and acquires more heaviness which is more cooled; but that heats more which condenses more. The converse follows, which says: that thing heats itself less which becomes more rarefied. But that heats more which comes closer to the heat, and conversely cools less which removes itself more from that heat. That flame extends more which finds itself in a space of greater heat, and shortens more which is surrounded by more powerful cold. That wind will be of briefer movement which is of more impetuous beginning. And this the fire that bursts from bombards has taught us, which shows us the shape and the speed of the motion by the smoke that penetrates the air standing opposite it, with brief and scattered removal. But the wind is of discontinuous impetuosity, as the dust raised by it amid the air shows us, with various dilations and windings. Again one feels on the ridges of the Alps the percussions of these winds made with the impulses of various powers. One sees also the banners of ships to be of various ventilations. One sees amid the sea one part of the water struck and not another; and the like is seen done through the squares and sandbanks of the rivers, removing with fury the dust in one part and not in another; and because these such effects give us experience of the natures of their causes, we may say with certainty that the wind which has the more impetuous origin has the briefer motion, by the experience stated above, which is the brief motion that the smoke has from the mouth of the bombard. And this arises from the resistance that the condensed air makes to the percussion of such smoke, which also (as is seen) shows us manifest condensation in the resistance made to it by the air; but if the wind is of slow movement, then it will have a very long and straight extension, because the air penetrated by it does not condense over against it, preventing its motion, but rather with ease dilates, giving it passage for a very long space.
Of the eddies of the winds.
When a wind is divided by mountains or by other buildings, if its rejoining is at right angles, then the motion made after such conjunction is whirling, with a rolling-up of columnar shape; and if the rejoined winds are equal, then such column will not change its place; but if the winds are unequal, then such column will move toward the weaker wind.