Decline of Science in England by Charles Babbage (ebook reader with android os .TXT) 📕

those mentioned in the Act; and it recites the words of the Act,

that they shall be persons “WELL VERSED IN THE SCIENCES OF

MATHEMATICS ASTRONOMY, AND NAVIGATION.” Of the fitness of the

gentlemen who now hold those situations to pronounce judgment on

mathematical questions, the public will be better able to form an

opinion when they shall have communicated to the world any of

their own mathematical inquiries. Although it is the practice to

consider that acceptance of office is alone necessary to qualify

a man for a statesman, a similar doctrine has not yet prevailed

in the world of science. One of these gentlemen, who has

established his reputation as a chemist, stands in the same

predicament with respect to the other two sciences. It remains

then to consider Captain Sabine’s claims, which must rest on his

skill in “PRACTICAL ASTRONOMY AND NAVIGATION,”— a claim which

can only be allowed when the scientific world are set at rest

respecting the extraordinary nature of those observations

contained in his work on the Pendulum.

That volume, printed under the authority of the Board of

Longitude, excited at its appearance considerable attention. The

circumstance of the Government providing instruments and means of

transport for the purpose of these inquiries, placed at Captain

Sabine’s disposal means superior to those which amateurs can

generally afford, whilst the industry with which he availed

himself of these opportunities, enabled him to bring home

multitudes of observations from situations rarely visited with

such instruments, and for such purposes.

The remarkable agreement with each other, which was found to

exist amongst each class of observations, was as unexpected by

those most conversant with the respective processes, as it was

creditable to one who had devoted but a few years to the subject,

and who, in the course of those voyages, used some of the

instruments for the first time in his life.

This accordance amongst the results was such, that naval officers

of the greatest experience, confessed themselves unable to take

such lunars; whilst other observers, long versed in the use of

the transit instrument, avowed their inability to take such

transits. Those who were conversant with pendulums, were at a

loss how to make, even under more favourable circumstances,

similarly concordant observations. The same opinion prevailed on

the continent as well as in England. On whatever subject Captain

Sabine touched, the observations he published seemed by their

accuracy to leave former observers at a distance. The methods of

using the instruments scarcely differed in any important point

from those before adopted; and, but for a fortunate discovery,

which I shall presently relate, the world must have concluded

that Captain Sabine possessed some keenness of vision, or

acuteness of touch, which it would be hopeless for any to expect

to rival.

The Council of the Royal Society spared no pains to stamp the

accuracy of these observations with their testimony. They seem

to have thrust Captain Sabine’s name perpetually on their

minutes, and in a manner which must have been almost distressing:

they recommend him in a letter to the Admiralty, then in another

to the Ordnance; and several of the same persons, in their other

capacity, as members of the Board of Longitude, after voting him

a THOUSAND POUNDS for these observations, are said to have again

recommended him to the Master-General of the Ordnance. That an

officer, commencing his scientific career, should be misled by

such praises, was both natural and pardonable; but that the

Council of the Royal Society should adopt their opinion so

heedlessly, and maintain it so pertinaciously, was as cruel to

the observer as it was injurious to the interests of science.

It might have been imagined that such praises, together with the

Copley medal, presented to Captain Sabine by the Royal Society,

and the medal of Lalande, given to him by the Institute of

France, had arisen from such a complete investigation of his

observations, as should place them beyond the reach even of

criticism. But, alas! the Royal Society may write, and nobody

will attend; its medals have lost their lustre; and even the

Institute of France may find that theirs cannot confer

immortality. That learned body is in the habit of making most

interesting and profound reports on any memoirs communicated to

it; nothing escapes the penetration of their committees appointed

for such purposes. Surely, when they enter on the much more

important subject of the award of a medal, unusual pains must be

taken with the previous report, and it might, perhaps, be of some

advantage to science, and might furnish their admirers with

arguments in their defence, if they would publish that on which

the decree of their Lalande’s medal to Captain Sabine was

founded.

It is far from necessary to my present object, to state all that

has been written and said respecting these pendulum experiments:

I shall confine myself merely to two points; one, the transit

observations, I shall allude to, because I may perhaps show the

kind of feeling that exists respecting them, and possibly enable

Captain Sabine to explain them. The other point, the error in

the estimation of the division of the level, I shall discuss,

because it is an admitted fact.

Some opinion may be formed of transit observations, by taking the

difference of times of the passage of any star between the

several wires; supposing the distances of those wires equal, the

intervals of time occupied by the star in passing from one to the

other, ought to be precisely the same. As those times of passing

from one wire to another are usually given to seconds and tenths

of seconds, it rarely happens that the accordance is perfect.

The transit instrument used by Captain Sabine was thirty inches

in length, and the wires are stated to be equi-distant. Out of

about 370 transits, there are eighty-seven, or nearly one-fourth,

which have the intervals between all the wires agreeing to the

same, the tenth of a second. At Sierra Leone, nineteen out of

seventy-two have the same accordance; and of the moon

culminating stars, p. 409, twelve out of twenty-four are equally

exact. With larger instruments, and in great observatories, this

is not always the case.

Captain Kater has given, in the Philosophical Transactions, 1819,

p. 427, a series of transits, with a three and a half foot

transit, in which about one-eleventh part of them only have this

degree of accuracy; and it should be observed that not merely the

instrument, but the stars selected, have, in this instance, an

advantage over Captain Sabine’s.

The transit of M. Bessel is five feet in length, made by

Frauenhofer, and the magnifying power employed is 182; yet, out

of some observations of his in January, 1826, only one-eleventh

have this degree of accordance. In thirty-three of the Greenwich

observations of January, 1828, fifteen have this agreement, or

five-elevenths; but this is with a ten-feet transit. Now in none

of these instances do the times agree within a tenth of a second

between all the wires; but I have accounted those as agreeing in

all the wires in which there is not more than four-tenths of a

second between the greatest and least.

This superior accuracy of the small instrument requires some

explanation. One which has been suggested is, that Captain

Sabine employs a chronometer to observe transits with; and that

since it beats five times in two seconds, each beat will give

four-tenths of a second; and this being the smallest quantity

registered, the agreement becomes more probable than if tenths

were the smallest quantities noticed. In general, the larger the

lowest unity employed the greater will be the apparent agreement

amongst the differences. Thus, if, in the transit of stars near

the pole, the times of passing the wires were only registered to

the nearest minute, the intervals would almost certainly be

equal. There is another circumstance, about which there is some

difficulty. It is understood that the same instrument,—the

thirty-inch transit, was employed by Lieutenant Foster; and it

has not been stated that the wires were changed, although this

has most probably been the case. Now, in the transits which the

later observer has given, he has found it necessary to correct

for a considerable inequality between the first and second wires

(See Phil. Trans. 1827). If an erroneous impression has gone

abroad on this subject, it is doing a service to science to

insure its correction, by drawing attention to it.

Should these observations be confirmed by other observers, it

would seem to follow that the use of a chronometer renders a

transit more exact, and therefore that it ought to be used in

observatories.

Among the instruments employed by Captain Sabine, was a repeating

circle of six inches diameter, made by order of the Board of

Longitude, for the express purpose of ascertaining how far

repeating instruments might be diminished in size:—a most

important subject, on which the Board seem to have entertained a

very commendable degree of anxiety.

The following extract from the “Pendulum Experiments” is

important:

“The repeating circle was made by the direction, and at the

expense of the Board of Longitude, for the purpose of

exemplifying the principle of repetition when applied to a circle

of so small a diameter as six inches, carrying a telescope of

seven inches focal length, and one inch aperture; and of

practically ascertaining the degree of accuracy which might be

retained, whilst the portability of the instrument should be

increased, by a reduction in the size to half the amount which had

been previously regarded by the most eminent artists as the

extreme limit of diminution to which repeating circles, designed

for astronomical purposes, ought to be carried.

“The practical value of the six-inch repeating circle may be

estimated, by comparing the differences of the partial results

from the mean at each station, with the correspondence of any

similar collection of observations made with a circle, on the

original construction, and of large dimensions; such, for

instance, as the latitudes of the stations of the French are,

recorded in the Base du Systeme Metrique: when, if due allowance

be made for the extensive experience and great skill of the

distinguished persons who conducted the French observations, the

comparison will scarcely appear to the disadvantage of the

smaller circle, even if extended generally through all the

stations of the present volume; but if it be particularly

directed to Maranham and Spitzbergen,—at which stations the

partial results were more numerous than elsewhere, and obtained

with especial regard to every circumstance by which their

accuracy might be affected, the performance of the six-inch

circle will appear fully equal to that of circles of the larger

dimension. The comparison with the two stations, at which a more

than usual attention was bestowed, is the more appropriate,

because it was essential to the purposes for which the latitudes

of the French stations were required, that the observations

should always be conducted with the utmost possible regard to

accuracy.

“It would appear, therefore, that in a repeating circle of six

inches, the disadvantages of a smaller image enabling a less

precise contact or bisection, and of an arch of less radius

admitting of a less minute subdivision, may be compensated by the

principle of repetition.”

Captain Sabine has pointed out Maranham and Spitzbergen as places

most favourable to the comparison. Let us take the former of

these places, and compare the observations made there with the

small repeating instrument of six inches diameter, with those

made by the French astronomers at Formentera, with a repeating

circle of forty-one centi-metres, or about sixteen inches in

diameter, made by Fortin. It is singular that this instrument

was directed, by the French Board of Longitude, to be made

expressly for this survey, and the French astronomers paid

particular attention to it, from the circumstance of