the angles of a number of triangles observed near the north end of the arc; the calculations of the sides of a series of triangles extending from Dunnose to Clifton; and, from these data, a calculation of the meridional distance between Dunnose and Clifton. Then come the observations with the zenith sector at Dunnose, Clifton, and Arbury Hill, near Daventry, a point almost in the middle of the arc, which was chosen with a view to ascertain how far the observations at the terminations would agree with others made for finding the value of its parts. A few additional observations made at the Royal Observatory are also given, serving to demonstrate the precision of the former ones, and the accuracy of the instrument.

Next follow the extensive and laborious calculations by which, in order to assimilate the numerous observations made at different times, they are all reduced, from the respective days on which they were made to the 1st of January 1802: the equations here introduced are those for aberration, nutation, semi-annual solar equation, precession, and refraction.

The general conclusions deduced from this ample stock of observations and calculations are, that the whole arc, subtending an angle of 2° 50′ 23′′, measures 1,036,337 feet; so that the length of a degree on the meridian, in latitude 52° 2' 20", is 60 820 fathoms. This degree, at the latitude of Arbury Hill, is found to be 42 fathoms longer; whereas, admitting the earth to be an ellipsoid, with the ratio of its axis as 229 to 230, it should be 10 fathoms less. On maturely weighing all the causes that may have occasioned this deviation, it is thought most likely that, owing to different attractive forces, which increase as we proceed northward, the plumb-line of the sector has been drawn somewhat towards the south at each of the stations,—a circumstance that must be carefully attended to in the prosecution of this survey, whenever the zenith sector is to be used. It is observed in general, that meridional observations carried on in insular countries are not so likely to afford just conclusions, with regard to the different lengths of the degrees, as the same operations if conducted in places very remote from deep seas.

Adverting, lastly, to the operations of the French astronomers who have measured the arc of the meridian between Paris and Barcelona, which distance was found = 3,527,921 English feet, this, combined with the arc lately measured, gives the whole meridional distance between Clifton and Barcelona, being 12° 5' 42"-79, something more than the thirtieth part of the whole circumference of the globe, = 4,411,968 feet. According to this determination, the mean length of a degree of the meridian, in latitude 47° 24', will be = 60,795 fathoms; and in the latitude 51° 9', the degree will measure 60,825 fathoms.

In an Appendix are subjoined the latitudes and longitudes of those places intersected in the surveys of Essex, Suffolk, &c. whose distances from their respective places of observation are given in the Philosophical Transactions for 1800; which, it is asserted, cannot

but be highly useful, as they may be depended on; the interior surveys of those parts having since proved that no erroneous intersections had been made in those operations.

The Bakerian Lecture. Experiments and Calculations relative to physical Optics. By Thomas Young, M.D. F.R.S. Read November 24, 1803. [Phil. Trans. 1804, p. 1.]

It consists of six sections, the first of which is intended to convey an experimental demonstration of the general law of the interference of light. This demonstration rests on two experiments, the results of which are brought in proof, that fringes of colours, and even the crested fringes described by Grimaldi, are produced by the interference of two portions of light. These results are, that if one of the two edges of a shadow produced by a narrow opake body be intercepted by a screen at a small distance from that body, the opposite edge will no longer exhibit the fringed appearance which it had in common with the former edge, when the latter was not intercepted.

Under the second head we have a comparison of measures of the intervals of disappearance of light when refracted between two edges of knives, or intercepted by a hair or a thin wire. The experiments, which were partly suggested by some observations of Sir Isaac Newton, are here collected in tables: and the author states, as a general inference, that if we thus examine the dimensions of the fringes under different circumstances, we may calculate the differences of the lengths of the paths of the portions of light which have been proved to be concerned in producing those fringes; and we shall find, that where the lengths are equal, the light always remains white; but that where either the brightest light, or the light of any given colour, disappears and reappears a first, a second, or a third time, the differences of the paths of the two portions are nearly in an arithmetical progression.

In the third section, these principles are applied to explain the repetition of colours sometimes observed within the common rainbow, particularly those described in the Philosophical Transactions by Dr. Langwith and Mr. Daval. The train of reasoning here adduced would lose too much of its evidence by being abridged.

The fourth section is entitled, "Argumentative inference respecting the Nature of Light." Here we meet with something of a controversial nature, in which those who have adopted theories different from that which our author is desirous to establish, are called upon to explain his experiments according to their principles. What appears to him to operate chiefly against the advocates for the projectile hypothesis of light, is, that light moves more slowly in a denser than in a rarer medium, and that hence refraction is not the effect of an attractive force directed to a denser medium.

The fifth section treats of the colours of natural bodies. The nature of the light transmitted by various bodies is here described, but

more particularly that which passes through blue glass. This, we are told, may be separated by the prism into seven distinct portions, nearly equal in magnitude: the two first are red, the third yellowish green, the fourth green, the fifth blue, the sixth bluish violet, and the seventh violet. This division, it seems, agrees perfectly with that of the light reflected by a plate of air 16,4th part of an inch in thickness and hence we may infer the extreme minuteness of the particles of light.

The sixth and last section describes an experiment on certain dark rays, which were first noticed by Ritter, and relates to the existence of solar rays accompanying light, but cognizable only by their chemical effects. This fact our author has confirmed by observing the effect of the reflection of these invisible solar rays from a thin plate of air capable of producing the well-known rings of colours. This image he threw on paper dipped in a solution of nitrate of silver, and in less than an hour he distinctly perceived portions of three dark rings, nearly of the same dimensions, but manifestly different from the coloured rings. This seems to coincide with Dr. Herschel's late discovery of rays of invisible heat; but our author doubts whether we are yet possessed of thermometers of sufficient delicacy to place implicit confidence in the experiments hitherto made on these rays by means of that instrument.

Continuation of an Account of a peculiar Arrangement in the Arteries distributed on the Muscles of slow-moving Animals, &c. In a Letter from Mr. Anthony Carlisle to John Symmons, Esq. F.R.S. Read December 8, 1803. [Phil. Trans. 1804, p. 17.]

Since the communication of his former paper on that subject, the author has collected further illustrations respecting the connexion between the disposition of the blood-vessels and the actions of the muscles. His first observations relate to the spermatic and intercostal arteries, and those of the diaphragm in men; which, he finds, are distributed in a different manner from those of the ordinary muscles. Compared with the distribution of the coronary arteries, it is found that the latter are much more subdivided or arborescent than any other set, and that accordingly these supply the heart,—a muscle whose actions we know are more rapid than those of any other part of the muscular system.

It is hence inferred, that any impediment to the accustomed course of the blood, flowing through muscles, induces a corresponding diminution in their power of action; and that wherever we find cylindrical arteries emitting few lateral branches, we may conclude that they appertain to muscles of slow but in general of long-continued motion. Of this, instances are given in the human eye, the swimming-bladder of fishes, the intestinum ileum of the Cavia Aguti, and various animals of the amphibious class. The better to illustrate his observations, the author has added figures of the swimming-bladder of the tench, and of the ileum of the Aguti,

An Account of a curious Phænomenon observed on the Glaciers of Chamouny; together with some occasional Observations concerning the Propagation of Heat in Fluids. By Benjamin Count of Rumford, V.P.R.S. Foreign Associate of the National Institute of France, &c. &c. Read December 15, 1803. [Phil. Trans. 1804, p. 23.] The fact here stated is as follows:-At the surface of a solid mas s of ice, of vast thickness and extent, viz. the Glaciers of Montanverd, certain pits are frequently met with, about seven inches in diameter, and more than four feet deep, perfectly cylindrical, and always quite full of water: their sides are smooth, or rather polished, and their bottoms hemispherical and well defined. They are always found on the level parts of the ice, and only in the summer season, increasing gradually in depth as long as the hot weather continues, and disappearing at the return of winter, when they are completely frozen up.

After calling upon those who maintain that water is a conductor of heat, to solve this phænomenon according to their principles, and pointing out to them, that as the water in these pits, being surrounded by ice, must continually be at the freezing point of temperature, it is not the general heat of the fluid that can melt the ice at the bottom of the pits, our author proceeds to give the following explanation of this singular effect.

The warm winds, he says, which in summer blow over the surface of this column of ice-cold water, must evidently communicate some small degree of heat to those particles of the fluid with which this warm air comes into immediate contact; and the particles of the water at the surface so heated, being rendered specifically heavier than they were before by this small increase of temperature, sink slowly to the bottom of the pit; and here they come in contact with the ice, and communicate to it the heat by which the depth of the pit is continually increased.

Count Rumford mentions next the singular but well-authenticated fact, of the equal temperature, at all seasons, of the water at the bottom of lakes; and shows how difficult, if not impossible, it must be to explain this phænomenon on a supposition of water being a conductor of heat. With a view to illustrate this subject, he gives us hopes that he will soon favour us with some observations, showing why all changes of temperature in transparent liquids must necessarily take place at their surfaces. Some further strictures are

next given, and certain difficulties are pointed out, on the cause of the descent of heat in liquids. And, lastly, notice is taken of the observations of Mr. Thompson of Edinburgh, on the experiments our author had contrived to render visible the currents into which liquids are thrown on a sudden application of heat or cold. The whole of this discussion rests on the accuracy of his observations, which Mr. Thompson had called in question, but in which he confidently asserts there was no fallacy whatever.

Description of a triple Sulphuret, of Lead, Antimony, and Copper, from Cornwall; with some Observations upon the various Modes of Attraction which influence the Formation of mineral Substances, and upon the different Kinds of Sulphuret of Copper. By the Count de Bournon, F.R.S. and L.S. Read December 22, 1803. [Phil. Trans. 1804, p. 30.]

The copious contents of this paper are arranged under the three following heads:-1. A description of the sulphuret of lead mentioned in the title; 2. Observations on the various modes of attraction which influence the formation of mineral substances; and 3. Observations upon the different kinds of sulphuret of copper.

1. The cupro-antimonial sulphuret of lead, described in the first part, has hitherto been found only in Cornwall; and though many specimens of it are to be met with in various collections in the kingdom, yet no writer has hitherto taken any particular notice of it, nor has it been classed by any of the late compilers of mineralogical systems. Mr. Hatchett is the first who, on a careful analysis, has ascertained it to be a triple sulphuret, in which the sulphur is combined with lead, antimony, and copper; each of these ingredients exhibiting their characters in so striking a manner as to afford, in some measure, a new example of a natural compound in the mineral kingdom.

The following are its principal characters. It is of a dark gray colour; it has a brilliant lustre, and is very brittle; its hardness is such, that it very easily cuts calcareous spar, but is not sufficient to scratch fluor spar; it slightly marks white paper; when rubbed, it does not emit any smell; when powdered and thrown upon a hot iron, it emits a phosphorescent light; and its specific gravity is 5765. The form of its primitive crystal is a rectangular tetrahedral prism, with terminal faces perpendicular to its axis; but as no specimen has yet been discovered in which the above-mentioned form is totally destitute of secondary facets, the author enters into a minute investigation of the various modifications of this form, hoping by this means to promote essentially the knowledge of the crystalline character, so important in the study of mineralogy. These modifications are four in number, and can only be understood by inspecting the figures which are subjoined to the paper.

To these characters is added the more essential one, which is supplied by the proportions of the constituent parts of the substance. These, according to Mr. Hatchett's analysis, consist of 42 62 of lead, 24-23 of antimony, 12.80 of copper, and 17 of sulphur: 1.20 of iron was likewise yielded in the process; but this is thought to have been a mere accidental mixture. It is next observed, that all the characters in this substance indicate very plainly the mutual combination of the three sulphurets of which it is found to be composed; the whole of the external characters above described differing materially from those of either of the three sulphurets, and also from those of any metallic substance hitherto known; and the pro