4. On the Existence of two dispersive Powers in all doubly refracting Crystals. When substances refract doubly, one image is always more coloured than the other. Hence it is obvious that such substances possess a doubly dispersive power. The following table exhibits the double dispersions of various bodies:

XIV. An Appendix to Mr. Ware's Paper on Vision. By Sir Charles Blagden, F.R.S.] Sir Charles states his concurrence in opinion with Mr. Ware, that near-sightedness comes on at an early age, and that it is almost confined to the higher ranks. He conceives it to be owing to the habit acquired by such young people of confining their attention to near objects. He describes his own case as an illustration. At the age of four, when he learned to read, he was not near-sighted; but being much addicted to reading, he became slightly so at eight. He was dissuaded from using a glass, and struggled on with a certain degree of near-sightedness till the age of thirty. He then used the glass called No. 2, which in a few years he laid aside for No. 3, then for No. 5, which he still employs.

XV. A Method of drawing extremely fine Wires. By William Hyde Wollaston, M.D. Sec. R.S.] The method employed by this ingenious philosopher was to take a small platinum wire, put it into the centre of a mould, and fill the mould with fine silver. The silver is then to be drawn into a wire as fine as thought requisite. The silver wire thus obtained is bent into the form of an U, making two hooks at its upper extremities; and

in this position it is dipped for a few minutes into hot nitric acid. The silver is melted off, except at the extremities, and the platinum wire remains. The extreme hooks, retaining their silver and size, serve to make the platinum wire visible and tangible, By this process he obtained wires onlyth of an inch in diameter.

XVI. Description of a single lens Micrometer. By William Hyde Wollaston, M.D. Sec. R.S.] This instrument possesses the characteristic simplicity and precision which mark all Dr. Wollaston's inventions. It resembles a common telescope with three sliding tubes. Instead of the object-glass is placed a scale of wires, each th of an inch in diameter, and easily counted by means of a regular variation in their length. Instead of the eye-glass is a small single lens, with a focus of about th of an inch. A small perforation is made by the side of this lens within

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th of an inch of its centre. The substance to be measured is inclosed between two flat glasses, which slide in before the lens, sufficiently near to enable the object to be seen distinctly. The wire, or other substance, to be measured, is seen through the lens, and the scale through the perforation, by the naked eye, and it is seen how many divisions of the scale it covers. In Dr. Wollaston's micrometer one division of the scale corresponds to Toth of an inch, when it is at the distance of 16.6 inches. Hence at the distance of 8.3 inches it will correspond to th of an inch. Upon the side of the micrometer are marked the value of one division, according to the distance.

XVII. Observations of the Winter Solstice of 1812, with the Mural Circle of Greenwich. By John Pond, Esq. Astronomer Royal, F.R.S.] From the observations on the summer solstice, corrected by subsequent observations, Mr. Pond deduced the obliquity of the ecliptic 23° 27′ 51.50": from the winter solstice he deduced it 23° 27′ 47.35′′. He thinks it not unlikely that this small discordance may be owing to some slight error in Bradley's refractions; and he is now employed in endeavouring to ascertain whether Bradley's mean refraction does or does not require alteration.

XVIII. On the Tusks of the Narwal. By Sir Everard Home, Bart. F.R.S.] Sir Everard had been informed by Mr. Scoresby, jun, that the female narwal had no tusks; and, in consequence of a scull which he received from that Gentleman, he wrote the paper on the subject, an account of which appeared in a former Number of the Annals of Philosophy. Subsequent information led him to doubt the accuracy of his information, that the female narwal has no tusks. Mr. Brown having examined all the books on the subject in Sir Joseph Banks's library, found an account of a female narwal with two tusks, brought to Hamburgh, in 1684, by Dick Peterson, and still to be seen there.

On examining the narwal sculls in the Hunterian Museum he found, besides the tusk which had protruded, another milk tusk, seven or eight inches long, and all enclosed in the scull. The scull of the female narwal exhibited two milk tusks in the same situation. Thus it appears that the narwal, both male and female, has two tusks; that one is protruded much earlier than the other, and that the tusks of the male narwal are protruded at a more early period than those of the female.

ARTICLE XII.

SCIENTIFIC INTELLIGENCE; AND NOTICES OF SUBJECTS
CONNECTED WITH SCIENCE.

1. Lectures.

Medical School of St. Thomas's and Guy's Hospitals.-The winter course of lectures at these adjoining hospitals will commence the beginning of October, viz. :

At St. Thomas's.-Anatomy and Operations of Surgery; by Mr. Astley Cooper and Mr. Henry Cline.-Principles and Practice of Surgery; by Mr. Astley Cooper.

At Guy's. Practice of Medicine; by Dr. Babington and Dr. Curry.-Chemistry; by Dr. Babington, Dr. Marcet, and Mr. Allen.-Experimental Philosophy; by Mr. Allen.-Theory of Medicine, and Materia Medica; by Dr. Curry and Dr. Cholmeley.-Midwifery, and Diseases of Women and Children; by Dr. Haighton.-Physiology, or Laws of the Animal Economy; by Dr. Haighton.-Structure and Diseases of the Teeth; by Mr. Fox.

N.B. These several Lectures, with those on Anatomy, and on the Principles and Practice of Surgery, given at the Theatre of St. Thomas's Hospital adjoining, are so arranged, that no two of them interfere in the hours of attendance; and the whole is calculated to form a Complete Course of Medical and Chirurgical Instruction. Terms and other particulars may be learnt from Mr. Stocker, Apothecary to Guy's Hospital.

Dr. Clarke and Mr. Clarke will begin their Winter Course of Lectures on Midwifery, and the Diseases of Women and Children, on Monday, October 4th. The Lectures are read at the house of Mr. Clarke, No. 10, Upper John-street, Goldensquare, from a quarter past ten o'clock till a quarter past eleven, for the convenience of Students attending the Hospitals.

Dr. Roget will commence his Autumnal Course of Lectures on the Practice of Physic at the Theatre of Anatomy, Great Windmill-street, on the first Monday in October,

Mr. Thomas Shute will commence his Winter Course of Lectures on Anatomy, Physiology, and the Principles of, and Operations in, Surgery, at the Anatomical Theatre, Bristol, on the morning of Friday, October 1st, at eight o'clock.

Trinity College, Dublin.-Lectures upon Anatomy, Physiology, and Surgery; by James Macartney, M.D. F.R.S. M.R.I.A. Professor of Anatomy and Surgery in the University, and late Lecturer upon Comparative Anatomy and Physiology at St. Bartholomew's Hospital, London.

These Lectures will be divided into three courses:-1st. A preliminary Lecture of General Physiology, which will be open to the public. 2d. A strict course of Anatomy and Physiology, as applicable to Medical Science. 3d. A course of Surgical Anatomy and Operative Surgery.

The Lectures will commence on the first Monday in November, and terminate at the end of the following April.

Demonstrations will be given daily in the Dissecting Room by Mr. Cross, Member of the Royal College of Surgeons in London. It is intended to establish regular examinations in the Anatomical Class, and to grant Certificates and Premiums according to merit.

II. Vinegar from Wood.

It is well known that when wood is distilled, an acid liquor is obtained, having a strong empyreumatic taste and smell. To this liquor the French chemists gave the name of pyrolignous acid; and Fourcroy and Vauquelin, having examined it about twelve years ago, determined that it was not a peculiar acid, but merely vinegar impregnated with an empyreumatic oil. This discovery was not so new as the French chemists conceived. Glauber, in his Miraculum Mundi, describes a method of making charcoal and preserving this acid liquid, quite similar to that now practised by several of our gunpowder manufacturers, who were probably obliged to that old chemical writer for the first hint of that improvement. Glauber expressly calls the liquor obtained by distilling wood vinegar. (See Glauber's Works, p. 188, English Translation.) This opinion prevailed universally among chemists till it was rejected by the French chemists, without any reason. The experiments of Fourcroy and Vauquelin, instead of being a new discovery, merely corrected the mistake of their countrymen, and restored the old and true opinion respecting the acid from wood, which had formerly been universally prevalent.

III. Matrix of the Diamond.

Diamonds have hitherto been found only in India and Brazil. They usually occur in an alluvial soil, from which they are separated by washing and picking. Hence mineralogists have been

hitherto ignorant of the true repository of diamonds. Werner, indeed, conjectured that they occurred in the rocks of the newest floetz trap formation; but this conjecture was not supported by any direct proof. Dr. Benjamin Heyné, botanist and naturalist to the East India Company at Madras, has lately brought over to London a piece of the diamond bed at Banaganpally, in the Dekan, with a diamond actually imbedded in it, from which we are enabled to determine the nature of the rock with tolerable accuracy. As it is perhaps the first specimen of the kind brought to Europe, mineralogists will doubtless consider it as of some importance to present them with a description of it. To the eye it has the appearance of a conglomerate; but as the grains are commonly roundish, and the cement a clay, approaching to wacke in its appearance and nature, it seems rather entitled to the name of amygdaloid. The round grains of which it is composed are chiefly of chalcedony, of a bluish grey colour, and approaching a little to hyalite in their appearance. They vary in size, from a pin head to a hazel nut. These nodules are mixed with angular fragments of jasper, hornstone, and quartz. No fragments of corundum were visible; though, it is said, they now and then occur in some of the beds containing diamonds.

From the above description there can be little doubt that the rock in question is an amygdaloid belonging to the newest floetz trap formation. Though the nodules are not absolutely identical with those which occur in the amygdaloid of this country, yet they very nearly agree with it.

From Dr. Heyné's description, it would appear that though this amygdaloidal rock is of some considerable thickness, yet the diamonds are confined to a bed in the centre of it, not more than a foot thick. This bed is distinguished from the rest of the rock by being harder.

IV. Carbonate of Iron.

It has been known for some time to mineralogists that the substance called sparry iron ore, and flos ferri, is a compound of black oxide of iron and carbonic acid, or a carbonate of iron; and Dr. Wollaston has shown that it crystallizes in a form differing a little from that of the primitive crystal of calcareous spar. It has lately been determined, by the chemical analysis of Descotils, and several other chemists, that the species known by the name of clay iron-stone, and so much employed in this country as an iron ore, is in fact a carbonate of iron likewise, and differs in nothing from sparry iron ore, except in not being crystallized, and in being mechanically mixed with some sili cious matter and some manganese. Alumina is usually present only in a very small proportion, and seems to be in combination with the silica; though in some specimens the proportion of