is such as to determine the rays of light in directions giving rise to halos (coloured circles or rings surrounding the sun or moon), and, where they cross each other, of parhelia (mock suns), and paraselenæ (mock moons). In looking for these phenomena, particularly those connected with the sun, it is imperatively necessary to know where to look for them, and this will be known by the following particulars :

18.-SOLAR HALOS are circles round the sun, of different diameters, some coloured and some not.

1st. A coloured circle may appear at the distance of 7° to 10° from the sun.

2nd. A coloured circle may appear at the distance of 22° 30' from the sun.

3rd. A coloured circle may appear at the distance of 46° from the sun.

And these may all appear at the same time. The red colour is at the innermost part of the ring.

4th. A colourless circle may appear at the distance of 90° from the sun. [This is of rare occurrence. I have never seen one.]

5th. A horizontal white circle passing through the sun may appear.

6th. Inverted arcs of circles, touching the halos of 2230, and 46° at their highest and lowest points may appear.

19. MOCK SUNS (Parhelia) are images of the true sun appearing simultaneously with him.

7th. The parhelia, or mock suns, are coloured, the red part being nearest the sun. They appear

as follows:

8th. With halos-They usually occur a little beyond the points where the halos intersect the horizontal circle passing through the sun (5); if, therefore, the horizontal circle be visible, the place is immediately determined; but if not, by considering where such a circle would cut the visible halos, the place is indicated.

9th. Parhelia sometimes appear in the halos vertically above the true sun, and also in the inverted arcs, vertically above the true sun. 10th. A mock sun sometimes occurs in the part of the sky exactly opposite to the true sun, and

in this case it is called anthelion.

11th. Mock suns frequently occur at the distance of 22° from the sun, in a horizontal plane passing through the sun, at the time when no halos are visible.

12th. Vertical lines passing through the sun, and circles containing the sun in their circumference, are to be looked for. Their several phenomena may continue visible for several hours. 20. LUNAR HALOS and Paraselenæ, or images of the moon,

13th. Are much more easily seen than solar halos. The same remarks apply to them as to solar halos, and also to mock moons.

21. GLORIES, CORONE, &c.-The coloured rings seen round the sun and moon, when thin, white clouds, generally of a cirro-cumuli kind, pass over their discs, are carefully to be distinguihed from the true halos. 14th. They are much smaller, being immediately round the sun or moon, and their whole diameters vary from 10 to 12°. When large, their colours are beautiful, and several series of colours appear at once.

15th. The diameter of any ring is not constant for any length of time.

16th. The red occupies the outermost ring, instead of the innermost, as in the true halo.

22. Of minor phenomena not dependent upon instrumental aid for observation, are those relating to the congelation of water. It is well known that water freezes at a temperature of 32°, but the first process of its change from a fluid to a solid state is not so well known. As seen on the surface of ponds, or wayside water during periods of frost in winter, the method of

its change may be described as follows:-The first commencement of congelation is attended with the almost simultaneous appearance of long needles, radiating for the most part from the sides of the bank, within the margin of the water: these increase in length, sometimes appearing divergent and sometimes parallel. Those at the sides are generally the first to make their appearance, but, by degrees, others similar in form, and the thinnest possible, gradually form at intervals on the surface, transversely, and in all directions, until the very smallest interstices are filled. The needles are laminated, as may be distinctly seen on the surface of thin and newly-formed ice. But the freezing of water is not always so accomplished: it frequently happens that the needles on the surface, generally

those towards the centre of the pond, group themselves into stars of three or six radii, feathered on either side with fine spiculæ, which quickly form a crystalline encrustation of serrated outline, giving to each radial arm or pinna the appearance of a frond of fern. Fig. 18 is a representa

Fig 18.

tion of this the most elementary form of water crystal, as commonly seen on water just below the freezing point, and may be seen extending to a length of upwards of 18 inches radius, and again small and beautifully defined, not exceeding an inch in diameter. The interstices between these crystals are frequently filled in here and there with a single pinna of the same, and every where besides with needles crossing and recrossing in all directions. If the frost continue, in the course of a few hours as the ice thickens these beautiful markings become obliterated. They are best witnessed on the cold morning of a fine clear winter day, when the process of congelation is proceeding slowly: at such times the water by the road side, or the surface of a pond, affords matter of the highest interest and infinite study.

23. These processes in part may be repeated within doors and during severe weather, by exposing in shallow vessels of wood, porcelain, &c., about an inch depth of water, under an open window, in a room of about 32° temperature. By this means the process may be seen in miniature, but the observer should be prepared to watch repeatedly before he can hope to see it displayed in full perfection. The annexed engraving (fig. 19) is copied from the surface of a bath of water so exposed: the entire surface was covered with groups equally interesting and graceful.

24. The forms of hoar frost, as deposited on various out-door substances, are infinitely varied: hoar frost or white frost is simply frozen dew, and may be seen to great perfection before sunrise on mornings following cold and cloudless nights, during which the radiation of heat from the earth is large, the cooling effect of which on the passing air is to cause an abundant deposition of dew, which on becoming frozen gives rise to innumerable combinations of spiculæ, varying in arrangements and grouping according to the nature

Fig. 19.

of the substances on which the dew is deposited. It varies very greatly on the leaves of different plants, which of itself may be said to constitute a study. During the years 1841 to 1845, whilst carrying on a course of experiments on the radiation of heat, I paid considerable attention to the subject; the results of these experiments are published in the Philosophical Transactions for the year 1847.

25. Every one is familiar with the beautiful en

crustations on the surface of window-panes in frosty weather, another of this class of phenomena, and produced by the same process of congelation; and which the observer may perceive for himself by effacing with his hand a portion of that already crystallized, and breathing upon it anew. After a short interval there will simultaneously appear along the inner margin of the part an almost imperceptible fringe of spikes or needles, which speedily lengthen-a few generally exceeding the remainder very greatly in length. They are most frequently curved, and soon assume the arborescent form: sometimes the lines of crystallization will run parallel to each other, in undulations, so as to give rise to beautiful semblances of the most varied objects.

The graceful and flowing curves occasionally exhibited about the freezing point on various substances may be illustrated in the following engraving (fig. 20), the

Fig. 20.

original of which was observed upon a wooden handrail in the open air, and was continued on for a length of seven feet.

26. The crystallization of water or vapour in the upper regions of the air is a still more interesting field of inquiry, and leads us to the consideration of snow. Very little is as yet known respecting the formation of snow, excepting that it is water congealed in the higher regions, and can therefore only be formed at or below a