Great Astronomers - Robert Stawell Ball (the little red hen ebook .TXT) 📗

of Nature, it seems difficult for

a beginner to find a virgin territory in which to commence his

explorations. Halley may, however, be said to have enjoyed the

privilege of commencing to work in a magnificent region, the

contents of which were previously almost entirely unknown. Indeed

none of the stars which were so situated as to have been

invisible from Tycho Brahe’s observatory at Uraniborg, in

Denmark, could be said to have been properly observed. There was,

no doubt, a rumour that a Dutchman had observed southern stars

from the island of Sumatra, and certain stars were indicated in

the southern heavens on a celestial globe. On examination,

however, Halley found that no reliance could be placed on the

results which had been obtained, so that practically the field

before him may be said to have been unworked.

 

At the age of twenty, without having even waited to take that

degree at the university which the authorities would have been

glad to confer on so promising an undergraduate, this ardent

student of Nature sought his father’s permission to go to the

southern hemisphere for the purpose of studying the stars which

lie around the southern pole. His father possessed the necessary

means, and he had likewise the sagacity to encourage the young

astronomer. He was indeed most anxious to make every thing as

easy as possible for so hopeful a son. He provided him with an

allowance of 300 pounds a year, which was regarded as a very

munificent provision in those days. Halley was also furnished

with letters of recommendation from King Charles II., as well as

from the directors of the East India Company. He accordingly set

sail with his instruments in the year 1676, in one of the East

India Company’s ships, for the island of St. Helena, which he had

selected as the scene of his labours.

 

[PLATE: HALLEY.]

 

After an uneventful voyage of three months, the astronomer

landed on St. Helena, with his sextant of five and a half feet

radius, and a telescope 24 feet long, and forthwith plunged with

ardour into his investigation of the southern skies. He met,

however, with one very considerable disappointment. The climate

of this island had been represented to him as most favourable for

astronomical observation; but instead of the pure blue skies he

had been led to expect, he found that they were almost always more

or less clouded, and that rain was frequent, so that his

observations were very much interrupted. On this account he only

remained at St. Helena for a single year, having, during that

time, and in spite of many difficulties, accomplished a piece of

work which earned for him the title of “our southern Tycho.” Thus

did Halley establish his fame as an astronomer on the same lonely

rock in mid-Atlantic, which nearly a century and a-half later

became the scene of Napoleon’s imprisonment, when his star, in

which he believed so firmly, had irretrievably set.

 

On his return to England, Halley prepared a map which showed the

result of his labours, and he presented it to the king, in 1677.

Like his great predecessor Tycho, Halley did not altogether

disdain the arts of the courtier, for he endeavoured to squeeze

a new constellation into the group around the southern pole

which he styled “The Royal Oak,” adding a description to the

effect that the incidents of which “The Royal Oak” was a symbol

were of sufficient importance to be inscribed on the surface of

the heavens.

 

There is reason to think that Charles II. duly appreciated the

scientific renown which one of his subjects had achieved, and it

was probably through the influence of the king that Halley was

made a Master of Arts at Oxford on November 18th, 1678. Special

reference was made on the occasion to his observations at

St. Helena, as evidence of unusual attainments in mathematics and

astronomy. This degree was no small honour to such a young man,

who, as we have seen, quitted his university before he had the

opportunity of graduating in the ordinary manner.

 

On November 30th, in the same year, the astronomer

received a further distinction in being elected a Fellow of

the Royal Society. From this time forward he took a most active

part in the affairs of the Society, and the numerous papers which

he read before it form a very valuable part of that notable series

of volumes known as the “Philosophical Transactions.” He was

subsequently elected to the important office of secretary to the

Royal Society, and he discharged the duties of his post until his

appointment to Greenwich necessitated his resignation.

 

Within a year of Halley’s election as a Fellow of the Royal

Society, he was chosen by the Society to represent them in a

discussion which had arisen with Hevelius. The nature of this

discussion, or rather the fact that any discussion should have

been necessary, may seem strange to modern astronomers, for the

point is one on which it would now seem impossible for there to be

any difference of opinion. We must, however, remember that the

days of Halley were, comparatively speaking, the days of infancy

as regards the art of astronomical observation, and issues that now

seem obvious were often, in those early times, the occasions of

grave and anxious consideration. The particular question on which

Halley had to represent the Royal Society may be simply stated.

When Tycho Brahe made his memorable investigations into the places

of the stars, he had no telescopes to help him. The famous

instruments at Uraniborg were merely provided with sights, by

which the telescope was pointed to a star on the same principle as

a rifle is sighted for a target. Shortly after Tycho’s time,

Galileo invented the telescope. Of course every one admitted at

once the extraordinary advantages which the telescope had to

offer, so far as the mere question of the visibility of objects

was concerned. But the bearing of Galileo’s invention upon what

we may describe as the measuring part of astronomy was not so

immediately obvious. If a star be visible to the unaided eye, we

can determine its place by such instruments as those which Tycho

used, in which no telescope is employed. We can, however, also

avail ourselves of an instrument in which we view the star not

directly but through the intervention of the telescope. Can the

place of the star be determined more accurately by the latter

method than it can when the telescope is dispensed with? With our

present knowledge, of course, there is no doubt about the answer;

every one conversant with instruments knows that we can determine

the place of a star far more accurately with the telescope than is

possible by any mere sighting apparatus. In fact an observer

would be as likely to make an error of a minute with the sighting

apparatus in Tycho’s instrument, as he would be to make an error

of a second with the modern telescope, or, to express the matter

somewhat differently, we may say, speaking quite generally, that

the telescopic method of determining the places of the stars does

not lead to errors more than one-sixtieth part as great as which

are unavoidable when we make use of Tycho’s method.

 

But though this is so apparent to the modern astronomer, it

was not at all apparent in the days of Halley, and accordingly he

was sent off to discuss the question with the Continental

astronomers. Hevelius, as the representative of the older method,

which Tycho had employed with such success, maintained that an

instrument could be pointed more accurately at a star by the use

of sights than by the use of a telescope, and vigorously disputed

the claims put forward by those who believed that the latter

method was the more suitable. On May 14th, 1679, Halley started

for Dantzig, and the energetic character of the man may be judged

from the fact that on the very night of his arrival he commenced

to make the necessary observations. In those days astronomical

telescopes had only obtained a fractional part of the perfection

possessed by the instruments in our modern observatories, and

therefore it may not be surprising that the results of the trial

were not immediately conclusive. Halley appears to have devoted

much time to the investigation; indeed, he remained at Dantzig for

more than a twelvemonth. On his return to England, he spoke

highly of the skill which Hevelius exhibited in the use of his

antiquated methods, but Halley was nevertheless too sagacious an

observer to be shaken in his preference for the telescopic method

of observation.

 

The next year we find our young astronomer starting for a

Continental tour, and we, who complain if the Channel passage

lasts more than an hour or two, may note Halley’s remark in

writing to Hooke on June 15th, 1680: “Having fallen in with bad

weather we took forty hours in the journey from Dover to Calais.”

The scientific distinction which he had already attained was such

that he was received in Paris with marked attention. A great deal

of his time seems to have been passed in the Paris observatory,

where Cassini, the presiding genius, himself an astronomer of

well-deserved repute, had extended a hearty welcome to his English

visitor. They made observations together of the place of the

splendid comet which was then attracting universal attention, and

Halley found the work thus done of much use when he subsequently

came to investigate the path pursued by this body. Halley was

wise enough to spare no pains to derive all possible advantages

from his intercourse with the distinguished savants of the

French capital. In the further progress of his tour he visited

the principal cities of the Continent, leaving behind him

everywhere the memory of an amiable disposition and of a rare

intelligence.

 

After Halley’s return to England, in 1682, he married a young lady

named Mary Tooke, with whom he lived happily, till her death

fifty-five years later. On his marriage, he took up his abode in

Islington, where he erected his instruments and recommenced his

observations.

 

It has often been the good fortune of astronomers to render

practical services to humanity by their investigations, and

Halley’s achievements in this respect deserve to be noted. A few

years after he had settled in England, he published an important

paper on the variation of the magnetic compass, for so the

departure of the needle from the true north is termed. This

subject had indeed early engaged his attention, and he continued

to feel much interest in it up to the end of his life. With

respect to his labours in this direction, Sir John Herschel says:

“To Halley we owe the first appreciation of the real complexity of

the subject of magnetism. It is wonderful indeed, and a striking

proof of the penetration and sagacity of this extraordinary

man, that with his means of information he should have been able

to draw such conclusions, and to take so large and comprehensive a

view of the subject as he appears to have done.” In 1692, Halley

explained his theory of terrestrial magnetism, and begged captains

of ships to take observations of the variations of the compass in

all parts of the world, and to communicate them to the Royal

Society, “in order that all the facts may be readily available to

those who are hereafter to complete this difficult and complicated

subject.”

 

The extent to which Halley was in advance of his contemporaries,

in the study of terrestrial magnetism, may be judged from the fact

that the subject was scarcely touched after his time till the year

1811. The interest which he felt in it was not of a merely

theoretical kind, nor was it one which could be cultivated in an

easy-chair. Like all true investigators, he longed to submit his

theory to the