Of the preceding observations, those to July 16 inclusive were made by myself, and the remainder were made by Mr. Quirling. A wire micrometer, furnished with a single fixed transit-wire and with a declination wire moveable by the screw, was used till the faintness of the comet made it necessary to observe in a dark field over thicker wires, when a negative eye-piece with metallic cross was substituted. This change occurred on Aug. 16. But, in all cases, the declination wire was used in a fixed position, and the reading of one microscope of the hour-circle and of one microscope of the declinationcircle was taken for each observation of the comet and the stars of comparison. These stars were generally taken from the Radcliffe Catalogue, and were in general so near to the comet on the night of observation that the differential effect of instrumental errors will be insignificant. On July 15 and 16 the transits of the star and comet were made with an unchanged position of the polar axis, though the circles were read as on other evenings, and the results for these evenings are obtained by application of the index corrections obtained on other evenings.

All the observations are corrected for the effects of refraction and parallax.

The following is a list of the stars of comparison, together with the places assumed in the reduction of the observations.

The following are the notes which I made on the appearance of the comet :

1861, June 30. This splendid comet burst suddenly on the view this evening. I saw it first at about 10 o'clock, and immediately prepared to observe it. Its appearance, notwithstanding the strong daylight, was most brilliant. To the naked eye the condensation of light about the head was so great as to bear comparison with the Moon rather than with any other object. In fact the diameter of the coma was about 30' by measurement at some Observatories on this evening. The tail streamed vertically upwards, and near midnight could be traced considerably beyond Polaris, which it passed over. Its length on this evening has been variously estimated from 100° to 120° by the most careful observers.

In the telescope of the heliometer there appeared a nucleus of an elliptical shape, with its major axis a little inclined to the vertical, that is, directed nearly towards the Sun. It was about the size of the ball of Saturn, near opposition, or about 20" in diameter. A stream of light went off from the upper apparent part of the nucleus, and turned round towards the apparent west in the shape of a sickle. Another but fainter stream was seen on the apparent east side of the first stream, also turning round towards the west.

July 2. In appearance not very different from that on June 30. A sickle-shaped stream of light setting out westward (apparently), and seeming for a little distance parallel to the declination wire, and then turning rapidly downwards (that is, away from the Sun). The nucleus elliptical as before, with its major axis inclined at an angle of 15° to the declination wire. The second stream of light was visible, but not so well defined as before. Clouds rendered physical observations difficult. The tail at least 70° in length.

July 3. The evening was too cloudy for accurate physical

observations, but it appeared as if the formations round the nucleus were becoming more regular and concentric, and more like Donati's comet. At midnight the clouds cleared away, and the view of the comet with the naked eye was magnificent. The tail was about 60° in length, being visible beyond the zenith.

July 4. The comet is rapidly becoming fainter. The tail could not be traced farther than 40° from the nucleus, but the brilliancy of the nucleus is still very great. In the telescope the nucleus appeared pear-shaped, with the point (apparently westward) and the axis parallel to the declination axis. Two curved streams of light are still seen flowing from it, the lower one sickle-shaped, but not so well defined as previously, cutting the declination wire at an angle of about 60°; the upper arc ill defined and cutting the declination wire at an angle of 30°. The whole formed the sector of a circle of about 100°.

α

July 5. I examined the head of the comet to-night with a lower power. The nucleus is now very bright, and almost equal to a star of the first magnitude (I compared with « Ursa Majoris in the twilight). It is of the same shape as on the preceding night. The head of the comet is altogether like a fan expanded to about 120°. Round the nucleus is a very bright circle of light, scarcely separable from it, and the two diverging streams of light now pass symmetrically on each side of the nucleus.

July 9. The shape of the head is that of an expanded fan. The nucleus terminates in a bright stellar point towards the apparent east, and is very narrow and pear-shaped, running parallel to the declination wire (or to the equator). Round the nucleus is a bright envelope of a circular shape, and, beyond that, another very faint and diffused. The distinctness of the envelope is not at all comparable with Donati's comet of 1858. The whole of the field is very strongly illuminated with the light of the comet.

July 10. The appearance of the comet has not materially changed since last night, except that the shape of the nucleus is materially different, having become more obtuse.

By my directions, Mr. Quirling confined himself for several evenings after the first appearance of the comet to observations of the length, breadth, and peculiarities of the tail, and incorporated the results of his observations in a series of drawings, in which the position and magnitude of the tail are defined accurately by means of the stars which it passed over or near to. The following is an abstract of his observations :—

α

The

"At 11h 30m, the darkness being greater, the comet had increased considerably in apparent brightness. The northern border of the tail passed beyond & and Draconis, which were seen through it; it passed exactly over Draconis, and could be traced towards Herculis. The southern border was far less distinct; it passed north of d Draconis, of several stars of the 6th magnitude. Estimated length, about 44°.

"At 12h, the northern border could be traced beyond Herculis; the southern border still less distinct than the northern. In the vicinity of & Draconis there seemed to be faint traces of a second tail. Whole length of tail, 60° to 65°; breadth, 7° to 8°.

66

July 5, 11h 30m. Sky very fine. The tail could be traced nearly to Herculis, the northern boundary being still the better defined. The southern border scarcely reached ♪ Draconis, and passed over and x Böotis, where there was

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again the faint beginning of a second tail, bending northwards, as on the night before. The tail did not reach & Herculis. Length, about 45°; greatest breadth, 10°.

"July 8, 11h 30m. The evening in general not favourable. The comet considerably fainter than on July 5, but still a conspicuous object in the heavens. The centre of the tail pointed midway between ę and x Herculis; but it could not be traced up to those stars. The fan or second tail, which was observed on July 4 and 5, had entirely disappeared. Length, about 27°.

"July 9. General appearance nearly the same as on the preceding night. Length, about 20° to 24°.

"July 14. The comet considerably smaller. The head, which had been up to this time always estimated as of the 1st and 2d magnitude, was to-night decidedly not brighter than a star of the 3d magnitude. Both borders of the tail well defined, though the northern could be traced farther than the southern. Length of tail, 15° or 16°.

"July 16. The appearance very similar to that on July 14. Length, 12° or 13°."

Progress of the Charts in course of execution at Bonn. Communicated by Mr. Carrington, as received from Dr. Krüger, by the direction of Dr. Argelander.

Seven sections, each comprising four sheets, are published. The first four sections were received several months since by the Observatories and Astronomers to whom they are presented. Sections 5, 6, and 7, were despatched, through Marcus of Bonn, at the end of last October; and vols. iii. and iv. of the Bonn Observations, being vols. i. and ii. of the present Survey, are about to follow. The charts comprising the Zone 2° S. to 20° N., and the next, 20° N. to 40° N., are fully published; and section 7, containing sheets 26, 27, 32, 33, will amount to half the next Zone, 40° to 60° N.

The observations by zones of 2 and 3 degrees broad with the 34-lines telescope were brought to a close at 81° N. on March 27, 1859, and amount to 1841 zones. By means of auxiliary tables, the positions of all the stars of Bessel's zones, from +15° to +45° from Weisse's catalogue, Oeltzen's catalogue, &c. were brought up to 18550, and entered in the same ledgers (about 190 in number), which record the original entries of times of transit in the Bonn zones. In order to see by inspection that each star was twice observed, a special catalogue was transcribed, in which the positions were arranged in zones of one hour each, and one degree broad. When the two observations of each star had been thus compared, the next step was the reduction to 1855 of all the stars contained in the former Catalogues of Lalande, Bessel, Argelander, Piazzi, Groombridge, Johnson, Rümker, Fedorenko, Schwerd, Struve (Pos. Med.), &c.; and the entering of the results alongside of those of the Bonn Survey; when, if the different authorities exhibited marked discrepancies, a further research was instituted. All cases of discrepancy were put aside, and entered in books for further examination or reobservation where the source of error was not easily detected.

The re-examination of the still dubious cases has been, for the most part, the special labour of Dr. Argelander himself, and a very laborious and important part of the undertaking, the necessary observations on the meridian having engaged his whole time from 1854 to 1860.

It has further been found necessary to repeat the survey in portions of the heavens unusually rich in stars, or where the previous ordinary survey had been taken during moonshine. For this purpose 476 additional Revision-Zones have been

58,59

observed with a 5-foot telescope, by taking 1° only at a time. in breadth, and on an average a much less extent in right ascension than those of the first survey.

In addition to the revision of doubtful stars, Professor Argelander has completely observed all stars to the 8th magnitude the positions whereof were found to be previously undetermined. The results of these observations will be embodied in a separate catalogue of about 30,000 stars observed with the meridian circle. The revision-observations of this class have been long in progress, so that the number of still undetermined stars is reduced to about 1000, and the whole is likely to be finished next summer. The reduction of these observations is somewhat in arrear, and a year or more may be required before the catalogue can be formed.

He

Dr. Krüger proceeds to remark, that after the correction. of all known errors, the next step was the preparation of the principal catalogue, the arrangement of which he describes, but which can be seen by inspection of the work itself. points out that the fourth volume contains a catalogue of all the errors detected at Bonn in the zones of Bessel, from -2° to +45° decl., and states that the result of the revision appears to be, that, excepting two cases of observations of planets, the whole of the observations of Bessel agree with positions of stars which still exist. He remarks that the result affords a weighty argument against those who hastily speak of vanished stars, and that the stability of the heavens has received so strong a support that suppositions of sudden change will in future be received with the greatest mistrust.

The preparation of the principal catalogue was assigned to Prof. Schonfeld, who has continued to forward the work since his removal to Mannheim; so that now the manuscript is complete.

The preceding account relates to north declination to 81°. It was considered impossible to continue the observations in zones beyond this point, on account of the extremely slow movement of the stars, and recourse was had to the first issue of Mr. Carrington's Charts (those printed by the anastatic process), which were compared anew with the heavens by means of a large comet-seeker, the magnitudes noted, and wanting stars supplied-the last by use of the heliometer.

.

To complete this work there now remains only the plotting out of certain charts, and the printing by lithography; and only three sheets besides the polar chart are incomplete. The rest are partly finished, partly in the press, and partly

in MSS.

The catalogue progresses simultaneously, and the third section is in the press. When this is done, there will follow the special catalogue of 30,000 stars observed on the meridian, and the correction of errors in the Histoire Celeste, Argelander's Northern Zones, and other catalogues.

The Fellows of the Society, and all to whom this account comes, cannot fail to admire the stately progress of this enormous work, which, simple in its conception, and free from insurmountable difficulties in execution, would appal many an astronomer by its vast extent.

The President communicated a letter from Mr. Basil Henry Cooper, "On the Egyptian Phoenix Period of 540 years, spoken of in a passage of Pliny." The letter partly relates to an eclipse of the Moon, recorded in a hieroglyphical inscription at Karnak, which the author concludes to have been that of the night of the 16 March, B.C. 851.

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Among the presents to the Society was included a volume of 111 Indian-ink drawings, by Mr. S. Gorton, of the planet Jupiter, radius about one inch, from observations made at Downs Road, Clapton, in the years 1859-60-61. The author remarks that as the observations were taken for the purpose of recording any great or marked changes in the principal belts, with an instrument of moderate size, the representation of those minute details for which great power was necessary was not attempted. The earlier drawings were made under some disadvantages-the telescope being used at a window, and not having an equatoreal mounting; this was added in April 1860, after which the planet was observed more regularly.

The power used was almost invariably 100; the telescope by Ross, 4 feet focal length, 3 inches aperture.

An Account of Observations on Solar Radiation.
By John James Waterston, Esq.

computations with reference to the Sun's heat, and suggesting
§ 1. In March of last year I submitted to the Society some
a mode of deducing the potential temperature of its radiating
surface.
method to the proof by a series of observations on solar
This last summer I have endeavoured to put this
radiation, supplemented with experiments on the rate of cooling
of thermometers in air and in vacuo with different kinds of
radiating surfaces; also by another series applying the method
of deducing the potential temperature of a radiating surface to
predicate the temperature of one that is maintained at a con-
stant known temperature.

yet been carried, encourages me to lay before the Society the The success of these last mentioned, so far as they have as

observations on solar radiation, with an account of the method employed to obtain the results, and of the mode of reducing them to a vacuum.

§ 2. It will be remarked, on inspecting the chart in which the observations are projected, that a simple law of atmospheric absorption is indicated, which, if confirmed by other similar observations in different climates, would perhaps lead to more exact ideas of the influence of the atmosphere on the Sun's rays. Unfortunately the best part of the summer had passed before I could begin to observe, and there was almost constant interruption with clouds and unsettled weather. In a tropical station, where the Sun rules in a cloudless sky, the presumed law might soon be put to the test, and the heating power of the Sun's rays before entering the atmosphere ascertained with precision. Having determined this for the Earth's mean distance from the Sun, its value for any other planetary distance may be deduced by the law of the inverse square.

§ 3. When a thermometer is exposed to the Sun with its bulb blackened, it is presumed to absorb all the heat that

61, 62

impinges on a plane surface equal to the transverse section of the bulb; it rises and is maintained at a certain temperature; and when this balanced condition is attained we can with certainty assert that the amount that issues from the bulb is precisely equal to the quantity that enters. The elevation of its temperature above surrounding bodies due to the Sun's radiant power (which is denoted by the symbol r) would be an exact measure of that power, if no heat issued from it except by radiation, and if the rate at which heat was emitted from it increased exactly in proportion to r. Now I find that if the

bulb of a thermometer is enclosed in a vacuum, the walls of which are brass, coated with lamp-black, the rate at which it cools is exactly proportional to the value of r, and this rate has exactly the same value, whether the glass bulb is uncoated or coated with lamp-black. When enclosed in air the rate of cooling increases faster than r. The mode of measuring the rate in both cases, and of reducing the values of r observed in air to what they would be in a vacuum, are described at the end of this paper.

*

§ 4. The instrument employed was designed so that the thermometer exposed to the Sun's rays should radiate against an enclosing metallic surface coated with lamp-black, and so that the temperature of that surface should always be known. Fig. 1, with the description that accompanies it, gives the details. The rays of the Sun were admitted to strike upon the bulb of the thermometer, x, through a hole but little larger than its diameter, and were entirely screened off the brass tube against the blackened inner surface of which the radiation of the bulb took place. The thickness of the sides of the brass tube was inch, and the thermometer, Y, that indicated its temperature, was lodged in a hole cut in the upper side. The circumference of the bulb touched the brass, and its upper side was enclosed with cork, while the lower was exposed to the air within the tube, but was untouched by the rays of the Sun that passed through. The internal diameter of the tube was 0.9 inch and length 6 inches. The bulb of the solar thermometer was 0.42 inch in diameter, spherical in shape, and fixed in the centre, as shown in the figure. Its shadow was an easy guide in moving the tube in altitude and azimuth to keep pace with the Sun. It is difficult, if not impossible, to demonstrate that the thermometer, Y, shows the exact temperature of the inner surface of the tube. It was subjected to three tests.

1. The instrument being out of the Sun's rays, and x and y showing the same temperature, it was removed to a place where the atmospheric temperature was 10° lower. Both thermometers descended and showed a difference equal to about one-tenth the amount they had to fall to arrive at the atmospheric temperature, y being so much in advance of x.

2. A bat's-wing flame of gas was brought within 3 inches fronting the middle of the tube; both x and y rose together, keeping pace exactly.

3. While taking observations, the heat absorbed by x from the Sun, and again emitted from it and transferred to the tube, gradually raised its temperature until a maximum was obtained. Now, comparing x and y while both are rising, and after having obtained their maximum, a difference of o°3 was remarked, and this difference, no doubt, affected isolated observations when this maximum was not attained, in consequence of interruption by clouds passing, when it was usual to heat the solar thermometer artificially to near the stationary point, in order to save time; the great inconvenience of the apparatus in this climate being the slowness with which r obtained its final value. An arrangement with a differential air-thermometer would, no doubt, be preferable in this respect, but the absolute value of the degrees indicated does not seem capable of being exactly determined.

* See Plate; and for the description, post pp. 69, 70.-ED.

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The thermometers were carefully graduated and compared by myself, and the divisions between two fixed points, 60° and 100° (which included all the observed temperatures), were drawn as nearly equal as a good ivory scale and magnifying lens would admit. The length of a degree on the scale of Y was about 05 inch, and upon x 067 inch; with practised eye it was easy to read off the temperature to th of a degree with lens; but such accuracy was unattainable for other reasons, and chiefly a sensible difference was caused by the varying amount of the stem that was under the influence of the Sun's rays as it moved.

The observations taken on the morning of the 21st August continuously, during 22 hours of uninterrupted sunshine, were graphically equalised, the curve drawn and ordinates measured off at every 20 minutes. This was the only opportunity that

occurred of continuous observation between such favourable limits of altitude as to indicate the direction of a line with some precision.

In the table of observations, given in the Appendix, the date and apparent time are given in the first column. The time-piece was regulated daily by the one o'clock signal-gun. The second column contains the values of r, the observed difference between thermometers x and y. The film of talc that was interposed between the Sun and x was found to reflect th of the incident rays. This ratio was determined by observations taken with the film off during calm weather. The value of r without the film to r with the film on, was as 118 to 100; the sun's power not sensibly varying during the interval. This proportion was maintained at low values of r, and even when the source of radiation was a gas-flame. The third column contains the observed values of r increased in this ratio. The fourth column contains the corrections required to reduce the values in the third to a vacuum. The correction is taken from a scale that was constructed by means of an empirical formula derived from observations on the cooling of x, as detailed in the Appendix. The fifth column is the final value of r as it would appear in a naked vacuum, that is, a vacuum without any interposed transparent solid between the Sun and the bulb of the thermometer. The numbers in this column represent the quantity of heat-force supplied from the Sun to the bulb of the thermometer in a constant element of time, or the quantity that emanates from the bulb in a unit of time.

The experiments on the cooling of the thermometer, x, in a vacuum, show that from r = 30° to r = 15°, the time of cooling was 294 beats of a time-piece, of which 77 were equal to 60 seconds; also from r= = 15° to r = 71°, the time was the same, and generally from r = 2 m to rm the elapsed time is the constant 294, which thus represents the logarithm of 2 in the logarithmic curve, of which the ordinates are r and the abscissæ the time of cooling, t. The equation of the curve being e log = t1 to, in which e log 2 = 294, or to reduce to seconds, e log 2 = 294 × and c =

60

771 in which

756.1. Let hyp. log of 10;