would be 5.87 cwt. per knot; hence it appears that to increase the speed from 10 to 15 knots per hour, the power requires to be increased upwards of three times, and the consumption of coals per knot is more than doubled.

Again, let it be supposed that the weight of the hull of a ship of 5000 tons displacement fitted for sea amounts to 40 per cent. of the displacement, or 2000 tons, and suppose the weight of the engines and boilers to be one ton for each 10 indicated h. p., the vessel requiring, as shown by Table D, 1170, indicated h. p. to attain the speed of 10 knots per hour, with a consumption of coals at the rate of 261 cwt. per kuot; then on these data, the engines, to attain the speed of 10 knots per hour, would weigh 117 tons, and the weight of coals for a passage of, say 12,000 nautical miles, would be 12,000 × 261= 31,320 cwt., or 1566 tons weight, making together for hull, engines and coals 2000+117+1566=3683, and consequently the displacement available for cargo would be 5000-3683=1317 tons weight. But if it be purposed that the steaming speed shall be at the rate of 15 knots per hour, the required power, as appears by Table D, will be 3947 ind. h. p., consequently the weight of the engines will be 395 tons, and the maximum displacement available for coals will be 5000-2395=2605 tons weight, or 52,100 cwt., which, at the tabulated rate of consumption, 5.87 cwt. per knot, would be sufficient only for a passage of 8876 nautical miles, and this to the utter exclusion of all goods cargo, showing that the ship is inadequate for steaming 12,000 nautical miles at the required speed of 15 knots per hour, though the same ship, if duly fitted with engine-power for steaming at 10 knots per hour, would perform the whole passage of 12,000 nautical miles without re-coaling at any intermediate station, and carry 1317 tons of remunerating goods cargo.

These few examples will, it is hoped, sufficiently illustrate the application. and use of Tables C and D in facilitating mercantile inquiry into the capabilities of steam-ships with reference to the all-important question of consumption of coals; but in order still further to facilitate calculations on this subject, the diagram E has been prepared, whence, simply by inspection, the consumption of coals per knot, at any rate of speed, may be approximately ascertained for vessels of improved modern construction up to 25,000 tons, the data on which this diagram has been calculated being the same as that on which Tables C and D are based.

The use and application of this Diagram E is evident; it brings the Tables under ocular review, and generalizes their application. It is given as an example of a system that admits of being more fully and elaborately deve loped for the purposes of mercantile tabular reference, as is now being done for publication.

Having thus explained the use and application of Tables C and D and the Diagram E, it will be perceived that the task which I have undertaken on this occasion is to show palpably by comparison with these tabular statements, based on data within the limits of already realized results, taken as a standard, what is the relative character of steam-ships as respects their locomotive or dynamic capabilities, with reference to the economic performance of mercantile transportservice, so far as dependent on the consumption of fuel; thus affording an exposition whereby parties interested in steam-shipping, either as owners or directors, or agents, or as the charterers of shipping for government or for private service, though unacquainted with the details of marine engineering as a science, may be enabled to arrive at some definite appreciation of the capabilities that may be expected of steamers; that is, the weight of cargo they will carry, and the length of passage capable of being performed at any definite speed; for, as before observed, the dead weight of cargo that a ship 1859.

K

will carry is equal to the tons' weight of water displaced between the light and load water-lines of the ship, less the weight of coals and stores required for the voyage, and which for long voyages commonly amount to four times the weight of cargo chargeable as freight, and it constitutes the limitation of distance which the ship is able to run under steam at a given speed. This inquiry is therefore essential to a due appreciation of the economic consequences which are involved in progressive variations of steam-ship speed, especially as respects the high rates of speed, which are occasionally professed, but which are seldom realized, simply because there has been no recognized exposition, whereby such pretensions may be judged of with reference to the required consumption of fuel. In short, regarding this matter as a public cause, affecting as it does the pecuniary interest of the public to the extent of millions sterling per annum, my object is to promulgate, through the medium of the notoriety which every inquiry obtains upon its being brought before the "British Association for the Advancement of Science," a Mercantile Steam-ship Expositor, by reference to which as a standard of comparison the good or bad qualities of steam-shipping may be deterinined; and this surely is a public cause, for by the operation of the scrutiny which such a system of comparative exposition may be expected to inaugurate and popularize, steamers will soon become marketable, with reference, in great measure, to their capabilities for economic transport service, at the speed that may be required; under the influence of this scrutiny all bad types of form and vicious adaptation of mechanical system will be eradicated; incompetency in steam-ship management will become gradually eliminated, and the mercantile transport service of the country being then performed exclusively by good, well-appointed, and well-managed ships, would be performed at a minimum of cost to the shipping interests, and consequently to the best advantage for the interests of the public. Hitherto the dynamic character of steam-ships has been a mechanical problem enveloped in undefined and even delusive terms of shipping and engineering art; consequently its determination has not been based on any recognized principles of calculation. Hence the dynamical character of shipping has been a mysterya matter of mere assertion on the one hand, and of credulity on the other. But mystery being unveiled, commercial vision will be opened, and competition, in shipping as in any other well-understood and open field of public enterprise, will ensure the mercantile transport service of the country being performed to the best advantage, and it will gradually establish and preserve the just equilibrium of freight charges as between the carriers and consumers of all sea-borne productions.

Report on the present state of Celestial Photography in England. By WARREN DE LA RUE, Ph.D., F.R.S., Sec. R.A.S., &c.

IN bringing before the Association the present Report it will be only necessary, after referring briefly to the labours of others, to confine myself to an account of my personal experience; for, although other observers have occasionally made experiments in Celestial Photography, there has not been any systematic pursuit of this branch of Astronomy in England, except in my Observatory, and under my immediate superintendence in the Kew Observatory.

PART 1.

Historical Outline.

The late Professor Bond of Cambridge, in conjunction with Messrs. Whipple and Black of Boston in the United States, was the first to make a photographic picture of any celestial body. By placing a daguerreotype plate in the focus of the great refractor of the Harvard Observatory, of 15 inches aperture, he obtained a daguerreotype of our satellite. This was, I believe, about the year 1850, for I remember seeing one of these pictures in the Exhibition of 1851, and some were exhibited at the meeting of the Royal Astronomical Society in May 1851. The experiments were discontinued after a time in consequence of irregularities in the going of the clock-work driver, and were not resumed again till 1857, when new clock machinery was attached to the telescope*.

At the latter end of 1852, I made some successful positive lunar photographs in from ten to thirty seconds on a collodion film, by means of an equatorially mounted reflecting telescope of 13 inches aperture, and 10 feet focal length, made in my workshop, the optical portion with my own hands; and I believe I was the first to use the then recently discovered collodion in celestial photography+. In taking these early photographs, I was assisted by my friend Mr. Thornthwaite, who was familiar with the employment of that new medium ‡. At that period, I had not applied any mechanical driving motion to the telescope, so that I was constrained to contrive some other means of following the moon's apparent motion; this was accomplished by hand; in the first instance, by keeping a lunar crater always on the wire of the finder by means of the ordinary hand-gear of the telescope, but afterwards by means of a sliding frame fixed in the eye-piece holder, the motion of the slide being adjustable to suit the apparent motion of our satellite; the pictorial image of the moon could be seen through the collodion film, and could be rendered immoveable in relation to the collodion plate, by causing one of the craters to remain always in apparent contact with a broad wire placed in the focus of a compound microscope, affixed at the back of the little camera box, which held the plate. Although these photographs were taken under the disadvantage referred to, namely, the want of an automatic driving motion, excellent results were nevertheless obtained, which proved how perfectly the hand may be made to obey the eye. I could not take photographs of the moon in this way alone, but required always the aid of an experienced coadjutor, willing to lose the greater portion of a night's rest, often to be disappointed by failures resulting from the state of the weather, and numberless impediments sufficient to damp the ardour of the most enthusiastic. For some months Mr. Thornthwaite was so kind as to continue his valuable aid, and several good positive pictures were obtained; but the difficulties we had to encounter were so great that it was at last resolved to discontinue the experiments until such time as a driving motion could be applied to the telescope. This was done early in 1857 §, since which period I have unremittingly followed up the subject of celestial photography whenever my occupations and the state of the atmosphere have permitted me to

* Astronomische Nachrichten, No. 1105, p.

†These pictures were exhibited in the early part of 1853 at the Royal Astronomical Society.

Mr. Archer applied the solution of gun cotton (collodion) to photography in 1851, and suggested pyrogallic acid for developing the latent image.

§ Monthly Notices of the Roy. Ast. Soc. vol. xviii. p. 16.

do so. With what result, the Association will have an opportunity of judging by the examples exhibited*.

Professor Phillips, aided by Mr. Bates, obtained some lunar photographs in July 1853, and communicated the results of his experience in a valuable paper at the Hull meeting of the Associationt. Mr Hartnup of Liverpool, aided by Mr. J. A. Forrest, Mr. McInnes, Mr. Crooke, and other photographers, took some good pictures of the moon in 1854; Father Secchi, at Rome, and more recently Mr. Fry, in Mr. Howell's observatory at Brighton, and Mr. Huggins, near London, have also produced lunar pictures: these experiments were in all cases made with refracting telescopes, corrected for the visual ray. Professor Bond, in April 1857, applied the process with promise of a fruitful future, in measuring the distance and angle of position of double stars §, and also in the determination of their magnitudes; just previous to his decease, this new application of the art appears to have engaged his attention more than lunar photography. He succeeded in obtaining pictures of fixed stars down to the 6-7th magnitude.

The Photographic Picture compared with the Optical Image.

It will render what I shall hereafter have to say more easily understood if I commence by bringing under notice what happens in applying photography to sidereal astronomy. The optical image of a fixed star, it will be remembered, is not a mathematical but an optical point, which, in consequence of the properties of light, is seen with the telescope as a very minute disc, surrounded by rings, which become fainter and wider apart as they enlarge, these rings being always more or less broken up, according to the state of the atmosphere. The photographic image must, therefore, be of a certain size, but it is after all a mere speck, difficult to find among other specks which are seen in the most perfect collodion film, when it is viewed with a magnifying power.

For example, let it be supposed that a telescope of sufficient aperture is turned upon a Lyræ; a star conveniently situated from its great meridional altitude for photography, and moreover sufficiently brilliant to give a nearly instantaneous picture: if the telescope be steadily supported at rest, the star will, in consequence of the earth's rotation, course along the field of the telescope, in a line parallel to the earth's equator, and, as it produces an instantaneous picture, the image obtained is a streak, representing the path of the star. We might be led to expect, a priori, that this line, for a short distance, would appear straight; but, so far from this being the case, it is broken up and distorted, and consists of a great number of undulating points, crowded in some places, and scattered in others. This distortion arises from the disturbances in our atmosphere which cause the star to flicker.

In the foregoing remarks, the telescope was supposed to be at rest; now *The photographs exhibited at the Aberdeen Meeting were the following:-Two original negatives which would bear considerable magnifying power; two positive enlarged copies of other negatives, eight inches in diameter, which would bear still further enlargement with a lens of low power; twelve enlarged positives of the Moon in different phases, 3 inches in diameter, among which were three, showing the progress of the lunar eclipse on February 27, 1858; enlarged positive copies of Jupiter, exhibiting his belts and satellites; lastly, a photograph of Saturn and the Moon taken together at the recent occultation of that planet just after the planet had emerged from the moon's bright limb (May 8, 1859). The lastnamed photograph was produced in 15 seconds;-a remarkably rapid result for so faint an object as Saturn. The planet on this occasion was seen to be of about the same brilliancy as the Mare Crisium situated near the moon's western limb, with which the planet could be readily compared, from its proximity to that lunar district.

+ Report of Brit. Assoc. 1853, Trans. Sect. A, p. 14. 1 Report of Brit. Assoc. 1854, Trans. Sect. B, p. 66. § Astronomische Nachrichten, No. 1105.

let it be assumed that the telescope is mounted on an axis parallel with the earth's axis, and provided with a driving apparatus, capable of carrying the telescope round in the direction of the star's apparent path so equably, that, if viewed with a micrometer eye-piece, the image of the star would remain always in contact with one of the wires of the eye-piece. The photographic picture of a star, obtained by a telescope under these conditions after some seconds' exposure, is not one single clear disc or point, but a conglomeration of points, extending over a greater or less area, according as the atmosphere has during the interval produced more or less flickering.

If a mere speck, like a fixed star, acquires comparatively large dimensions on a sensitized plate in consequence of atmospheric disturbances, every optical point in an image of other celestial objects must, from the same cause, occupy a space of greater dimensions than it would if no disturbing influences existed. When the telescope is employed optically, the mind can make out the proper figure of the object, although its image dances before the eye several times in a second, and is able to select for remembrance only the states of most perfect definition; on the other hand, a photographic plate registers all the disturbances. The photographic picture will consequently never be so perfect as the optical image with the same telescope, until we can produce photographs of celestial objects instantaneously: we are still a long way from this desirable end.

Relative Advantages of Reflecting and Refracting Telescopes for Photography. With refracting telescopes, the photographic focus of a point of light occupies a larger area than with reflectors; this is especially the case with Astronomical Telescopes, because they are corrected so as to produce the best optical image, and the outstanding chemical rays are dispersed around the luminous focus*. The reflecting telescope has, therefore, considerable advantage over the refracting telescope for celestial photography, on account of all rays coming to focus in the same plane; hence, the focus having been adjusted for the luminous image, it is correct for the chemical image, and has not to be disturbed, as with a refractor. In the telescope employed by Professor Phillips, of 64 inches aperture and 11 feet focal length, the actinic focus was found to be 0.75 inch beyond the visual focus; and in the Liverpool Equatorial of 12 feet focal length the actinic focus was 0.8 inch beyond the visual focus. With my telescope the focusing is critically effected with the aid of a magnifier, the image being received on a piece of ground glass placed temporarily in the actual slide destined to contain the sensitized plate; a second piece of ground glass fixed in a frame is put into the camera just previous to each operation, for the purpose of placing the telescope in position; but the focusing is always effected in the manner described, for the goodness of the picture depends greatly on the accuracy of this adjustment. I attribute much of my success to the employment of a reflector, while my fellow-labourers in the same field have used refractors.

Actual Process employed at the Cranford Observatory.

With the view of facilitating the labours of others desirous of entering the field of photography, I will now describe, with all necessary minuteness, the process finally adopted after many trials and failures; I would remark at the same time that it is quite impossible to give such directions as will enable another operator to ensure perfect results, as this can only be attained by perseverance, long practice, and a strong determination to overcome obstacle after obstacle as it arises, therefore, no one need hope for * Refracting telescopes can be specially corrected for the chemical focus in the same way as Camera lenses.