It was long supposed that transits of Venus over the sun's disk afforded the only accurate method of determining the distance of the sun, one of the fundamental data of astronomy. Unfortunately, these phenomena are of the rarest. They come in pairs, with an interval of eight years between the transits of a pair. A pair occurred in 1761 and 1769, and again in 1874 and 1882. Now the whole of the twentieth century will pass without another recurrence of the phenomenon. Not until the years 2004 and 2012 will our posterity have the opportunity of witnessing it.

Much interesting history is associated with the adventures of the astronomers who took part in the expeditions to observe the transits of 1761 and 1769. In the almost chronic warfare which used to rage between France and England during that period, neither side was willing to regard as neutral even a scientific expedition sent out by the other. The French sent one of their astronomers, Le Gentil, to observe the transit at Pondicherry in the East Indies. As he was nearing his station, the presence of the enemy prevented him from making port, and he was still at sea on the day of the transit. When he at length landed, he determined to remain until the transit of 1769, and observe that.

In August, 1861, while I was passing my vacation on Cape Ann, I received a letter from Dr. Gould, then in Washington, informing me that a vacancy was to be filled in the corps of professors of mathematics attached to the Naval Observatory, and suggesting that I might like the place. I was at first indisposed to consider the proposition. Cambridge was to me the focus of the science and learning of our country. I feared that, so far as the world of learning was concerned, I should be burying myself by moving to Washington. The drudgery of night work at the observatory would also interfere with carrying on any regular investigation. But, on second thought, having nothing in view at the time, and the position being one from which I could escape should it prove uncongenial, I decided to try, and indited the following letter:—

Nautical Almanac Office,

Cambridge, Mass., August 22, 1861.

Sir, — I have the honor to apply to you for my appointment to the office of Professor of Mathematics in the United States Navy.

I would respectfully refer you to Commander Charles Henry Davis, U. S. N., Professor Benjamin Peirce, of Harvard University, Dr. Benjamin A. Gould, of Cambridge, and Professor Joseph Henry, Secretary of the Smithsonian Institution, for any information respecting me which will enable you to judge of the propriety of my appointment.

The term “Nautical Almanac” is an unfortunate misnomer for what is, properly speaking, the “Astronomical Ephemeris.” It is quite a large volume, from which the world draws all its knowledge of times and seasons, the motions of the heavenly bodies, the past and future positions of the stars and planets, eclipses, and celestial phenomena generally which admit of prediction. It is the basis on which the family almanac is to rest. It also contains the special data needed to enable the astronomer and navigator to determine their position on land or sea. The first British publication of the sort, prepared by Maskelyne, Astronomer Royal, a century ago, was intended especially for the use of navigators; hence the familiar appellation, which I call unfortunate because it leads to the impression that the work is simply an enlargement and improvement of the household almanac.

The leading nations publish ephemerides of this sort. The introductions and explanations are, of course, in the languages of the respective countries; but the contents of the volume are now so much alike that the duplication of work involved in preparing them seems quite unnecessary. Yet national pride and emulation will probably continue it for some time to come.

My first trip to Europe, mentioned in the last chapter, was made with my wife, when the oldest transatlantic line was still the fashionable one. The passenger on a Cunarder felt himself amply compensated for poor attendance, coarse food, and bad coffee by learning from the officers on the promenade deck how far the ships of their line were superior to all others in strength of hull, ability of captain, and discipline of crew. Things have changed on both sides since then. Although the Cunard line has completed its half century without having lost a passenger, other lines are also carefully navigated, and the Cunard passenger, so far as I know, fares as well as any other. Captain McMickan was as perfect a type of the old-fashioned captain of the best class as I ever saw. His face looked as if the gentlest zephyr that had ever fanned it was an Atlantic hurricane, and yet beamed with Hibernian good humor and friendliness. He read prayers so well on Sunday that a passenger assured him he was born to be a bishop. One day a ship of the North German Lloyd line was seen in the offing slowly gaining on us. A passenger called the captain's attention to the fact that we were being left behind.

A few features of Washington as it appeared during the civil war are indelibly fixed in my memory. An endless train of army wagons ploughed its streets with their heavy wheels. Almost the entire southwestern region, between the War Department and the Potomac, extending west on the river to the neighborhood of the observatory, was occupied by the Quartermaster's and Subsistence Departments for storehouses. Among these the astronomers had to walk by day and night, in going to and from their work. After a rain, especially during winter and spring, some of the streets were much like shallow canals. Under the attrition of the iron-bound wheels the water and clay were ground into mud, which was at first almost liquid. It grew thicker as it dried up, until perhaps another rainstorm reduced it once more to a liquid condition. In trying first one street and then another to see which offered the fewest obstacles to his passage, the wayfarer was reminded of the assurance given by a bright boy to a traveler who wanted to know the best road to a certain place: “Whichever road you take, before you get halfway there you 'll wish you had taken t' other.” By night swarms of rats, of a size proportional to their ample food supply, disputed the right of way with the pedestrian.

One hardly knows where, in the history of science, to look for an important movement that had its effective start in so pure and simple an accident as that which led to the building of the great Washington telescope, and went on to the discovery of the satellites of Mars. Very different might have been a chapter of astronomical history, but for the accident of Mr. Cyrus Field, of Atlantic cable fame, having a small dinner party at the Arlington Hotel, Washington, in the winter of 1870. Among the guests were Senators Hamlin and Casserly, Mr. J. E. Hilgard of the Coast Survey, and a young son of Mr. Field, who had spent the day in seeing the sights of Washington. Being called upon for a recital of his experiences, the youth described his visit to the observatory, and expressed his surprise at finding no large telescope. The only instrument they could show him was much smaller and more antiquated than that of Mr. Rutherfurd in New York.

The guests listened to this statement with incredulity, and applied to Mr. Hilgard to know whether the visitor was not mistaken, through a failure to find the great telescope of the observatory. Mr. Hilgard replied that the statement was quite correct, the observatory having been equipped at a time when the construction of great refracting telescopes had not been commenced, and even their possibility was doubted.

It is sometimes said that no man, in passing away, leaves a place which cannot be equally well filled by another. This is doubtless true in all ordinary cases. But scientific research, and scientific affairs generally at the national capital, form an exception to many of the rules drawn from experience in other fields.

Professor Joseph Henry, first secretary of the Smithsonian Institution, was a man of whom it may be said, without any reflection on men of our generation, that he held a place which has never been filled. I do not mean his official place, but his position as the recognized leader and exponent of scientific interests at the national capital. A worldwide reputation as a scientific investigator, exalted character and inspiring presence, broad views of men and things, the love and esteem of all, combined to make him the man to whom all who knew him looked for counsel and guidance in matters affecting the interests of science. Whether any one could since have assumed this position, I will not venture to say; but the fact seems to be that no one has been at the same time able and willing to assume it.

On coming to Washington I soon became very intimate with Professor Henry, and I do not think there was any one here to whom he set forth his personal wishes and convictions respecting the policy of the Smithsonian Institution and its relations to the government more freely than he did to me.

The questions involved in the heading have engaged the attention of astronomers in the past, but does the photographic method contribute evidence more reliable in character than that formerly available?

Let us consider the evidence which has up to the present time been obtained by photography, remembering that the last dozen years covers the whole interval during which it has been accumulated:–

(A) Eleven years ago photographs of the Great Nebula in Andromeda were taken with the 20-inch reflector and exposures of the plates during intervals up to four hours; and upon some of them were depicted stars to the faintness of 17th to 18th magnitude, and nebulosity to an equal degree of faintness. The films of the plates obtainable in those days were less sensitive than those that have been available during the past five years, and during this period photographs of the nebula with exposures up to four hours have been taken with the 20-inch reflector. No extensions of the nebulosity, however, nor increase in the number of the stars, can be seen on the later rapid plates than were depicted upon the earlier slower ones, though the star-images and the nebulosity have greater density on the later plates.

The plates are arranged in classes or groups so as to indicate apparent physical relationship between them, and the Right Ascensions are, as far as practicable, given in the order of time within each group.

The edge next to the printed heading on each plate is the south, and the lower edge the north; the right is the following, and the left the preceding edge.

The scales of the photographs, which are given in the letterpress, are such that by eye alignments of the stars, without the application of measuring instruments, changes which have taken place in their positions or in the structures of the nebulosities, if these changes should not be less than about five seconds of arc in extent, could be detected by comparing corresponding dual plates in this simple manner. The examination and comparison of stars, both as regards their positions and magnitudes, could thus be made in a single day though they should number several thousands on the dual photographs.

Besides this alignment method, measurements by scale and compasses, or by a réseau on glass or other transparent substance, or by a rectangular L-shaped metal rule divided into millimètres on both limbs, or by the superposition of the plates upon each other, are obvious methods available for detecting changes in the position angles and magnitudes of the stars shown on the photographs.

The catalogues of stars are numerous, as will be seen by referring to the list given in Chambers' “Handbook of Descriptive Astronomy,” where 170 are enumerated between that by Hipparchus in the year b.c. 128 and the year a.d. 1876; besides these there are catalogues of nebulæ, and atlases or charts of stars.

In considering these records one is impelled to ask the questions–Have these vast stores of computative and descriptive literature, the product of great energy expended in physical and mechanical operations, and of much thought, been utilized in the advancement of astronomical knowledge to a degree commensurate with the labour and cost of their production?

Or, have the astronomers been deterred from undertaking the work of correlation, on a comprehensive scale, because they know that there is a considerable margin of probable error in all these records–particularly in those of earlier date than the middle of the present century–they therefore judge it would be unprofitable to devote their time to making the necessary examinations. They know that when they find differences to exist in the records, it would be uncertain whether they were objective, or were only the result of human errors.

In my own experience I have found differences to exist between photographs and carefully prepared modern charts of stars–differences in the position angles, in distances, in the magnitudes of the stars and in the structure and extent of nebulosities, which were most probably due to errors in charting.

It is a general opinion that the longer the time a sensitive film is exposed, in a photographic instrument, under clear atmospheric conditions, the greater will be the number of stars and the extent of nebulosity imprinted upon the film. But so far as my experience enables me to judge, after twelve years' use of the 20-inch reflector, and more than two years' use of an excellent and specially-made portrait lens combination of 5-inches aperture and 19-inches focus, the limit of photographic effect is reached sometime within ten to twelve hours on clear nights, and with very sensitive films, in the 20-inch reflector. With the 5-inch lens very much longer exposures may be given before the darkening of the films, by atmospheric glare and diffraction effects, reach the same degree of density as in the reflector.

The photographic effect produced by the 5-inch lens with an exposure of two or three hours and upwards is about two stellar magnitudes less than that given by the reflector in the same time and with films of equal sensitiveness. It would, therefore, appear that, given sufficient time, the atmospheric glare would, in both instruments, mask or extinguish the light of faint stars and faint nebulosity, which is provisionally assumed to be equal to that of 18th magnitude stars. When that limit has been reached no fainter light-effect than this would be imprinted on the films; and upon these premises the questions in the following section require consideration.

N.G.C. 7089. G.C. 4678. h 2125.

Sir J. Herschel, in the G.C., describes it as partly resolvable; a globular cluster; bright; very large; gradually, pretty much brighter in the middle; resolvable into stars; stars extremely faint. A drawing of it is given in the Phil. Trans., 1833, pl. XVI., fig. 88, and 1844, pl. XVIII., fig. 88.

Lord Kosse (Obs. of Neb. and Cl., p. 162) describes the cluster, and gives measurements of the position angles, and distances of some of the stars.

The photograph shows the cluster with a large central mass of nebulosity so dense as to obliterate the star-images; but the faint stars surrounding it are arranged in a manner suggestive of their origin from a spiral nebula.

The three clusters depicted on Plate XXVIII., and there are others of a similar character but not yet published, are strongly suggestive of, if they do not indisputably prove, that the same principle of aggregation has been in operation to cause the origin and development of each of these clusters, and I have not been able to detect any clearer evidence of their origin than that of development from spiral nebulæ.