Part 7 (2/2)
Professor J. C. ADAMS, Delegate of Great Britain, stated that he merely desired to refer to one subject touched on by the Delegate of France, Mr. JANSSEN, whose opinion he thought could hardly be supported, and that was that the question of longitude was purely one of geography. He desired to controvert that, and to hold that the question of longitude was purely one of astronomical observation. The difference of longitude between two places could not be determined by geodetic observations, because to do this you must take hypothesis as to the figure of the earth, and the figure of the earth is not a simple figure. You may take as hypothesis that the figure of the earth is spheroidal, and that the ratio of the axes is exactly defined. Now, in the first place, we are not agreed as to the exact ratio of the axes, nor are we agreed as to the exact figure of the earth. If an attempt is made to measure the difference of longitude between two points on the earth's surface, especially when they are a considerable distance from each other, it is necessary to depend upon astronomical observations. In attempting to deduce the difference of longitude from geodetic measures, you must a.s.sume that the true figure and dimensions of the earth are known, which is far from being the case. The theory that the prime meridian is a matter purely of a geographical nature is liable to the fatal objection that the determination of the difference of longitude between one place and the other is really the determination of the difference of time of the pa.s.sage of a star across the meridian of the two places concerned. That is very definite. You observe the transit of the star at one place, and you observe the transit of the star at the other place, and by means of telegraphic communications you are able to determine their difference of longitude independent of the figure of the earth. He said, in conclusion, that he thought the honorable Delegate of France was mistaken upon the main point which he had just referred to, if, indeed, he had rightly understood him.
M. JANSSEN, Delegate of France, replied as follows:
I think that M. ADAMS entirely misunderstands me. I agree with him absolutely in thinking that longitudes cannot be determined, especially of places far apart, except by astronomical methods.
Geodesy can only furnish it for short distances; in such cases, it is true, it supplies it with a degree of accuracy which meridia.n.a.l observations cannot attain. So, if the question be to determine rigorously the difference of longitude in time between two places on the earth at considerable distances apart, it becomes one of astronomy, because here it is astronomy which gives the quickest and most accurate solution. For these reasons if, for instance, we should wish to connect a given observatory with a point situated on the other side of the ocean which had been chosen as the starting point of longitudes, it would become a question of astronomy. Astronomy here is an admirable instrument for the solution, but it should only be the instrument.
On the contrary, the question becomes geographical, if it be that of determining where it will be most convenient to fix the origin of terrestrial longitudes. If the question be, for instance, to select one or another point, in some one or other ocean, astronomy has nothing to do with it, and when it wishes to impose upon us one of its observatories to fulfil such a function it tends to give an inaccurate solution.
At first sight it may seem that any point might become a starting point for terrestial longitudes, but when we study the question a little more we see there may be great advantages in choosing some one point in preference to some other. Hence it is that all geographers have agreed to place initial meridians, when possible, in the oceans.
The PRESIDENT stated that, in accordance with the decision of the Conference, he had sent to the scientists named by them invitations to a seat upon this floor. The Chair sees several of these gentlemen here to-day, notably one of the most eminent astronomers of this country, to whom his countrymen are always ready to do homage, Professor Newcomb, Superintendent of the United States Nautical Almanac. If it be the pleasure of the Congress, the Chair will now request Professor Newcomb to give us his views upon the resolution now under discussion.
No objection being made to the proposition of the President, Professor NEWCOMB arose and said:
That in reference to the remarks of the distinguished Delegate of France, Professor JANSSEN, he would prefer, if the Conference would consent, to study his arguments more carefully when they should be in print.
He remarked that some points raised by that argument have been already replied to, and he wished now more particularly to request that Professor JANSSEN would define precisely what he meant by ”a neutral meridian;” that he had partially answered this question in reply to Professor ABEe; but that there was a more fundamental point, one of practice, which must be brought in and kept in mind at every step, and which was raised by Commander SAMPSON'S paper, to which he had listened with great interest. Commander SAMPSON held that it would be necessary to have a fixed observatory on the chosen prime meridian, but he (Professor NEWCOMB) did not concur in that view, but rather agreed to a limited extent with what Professor JANSSEN had said on that question.
In choosing a meridian from which to count longitude, you meet a difficult problem. You have a point on the globe defined as the first meridian. This would be taken as the initial point of departure, and you are to determine the longitude of a certain place from that point.
Now, doubtless, there is no other way to do this than to have an astronomical instrument and telegraphic communication. And if they chose the Azores or Behring's Strait, in neither case could they mount a transit instrument or have a system of telegraphic communication.
Nor could we make a determination of longitude from a single fixed observatory in any case.
He then stated that it was impracticable under any circ.u.mstances to have an absolutely neutral prime meridian; that the definition of the prime meridian must practically depend upon subsidiary considerations, no matter where it might be located. In the practical work of determining longitudes a connection with the prime meridian cannot be made in each case. What is really determined is the longitude from some intermediate point, generally in the same country, and in telegraphic communication with the place whose longitude we wish to know. This intermediate point would, for the time, be the practical prime meridian. But the longitude of this point itself must always be uncertain. Science is continually advancing in accuracy, and we find that we continually need to correct the longitude of our intermediate meridian, and hence of all points determined from it. How can this difficulty of constantly changing longitudes be avoided? He replied that each system of connected longitudes must rest upon its own basis.
It must be referred to an a.s.sumed prime meridian, and the measurements must be made from that, even if it be found to be somewhat in error. If some such system had been adopted thirty or forty years ago, we would have avoided the confusion arising from the fact that the longitudes given on many maps do not refer at all to any absolute meridian. All that is known is that the astronomers determined the longitude of the place, and then the maps had to be corrected accordingly. The longitude of one place would be determined from Cambridge, and perhaps in the neighborhood is another place determined from the observatory at Was.h.i.+ngton. In either case we know nothing of the longitude of Cambridge or Was.h.i.+ngton which the observer a.s.sumed in his calculations.
Generally, in determining longitude, the country adopts the princ.i.p.al place within its confines as a subsidiary prime meridian, and the a.s.sumed longitude of this place is necessarily selected somewhat arbitrarily. The longitude, for instance, of Was.h.i.+ngton was, thirty years ago, known to be nearly 5 hours 8 minutes and 12 seconds west from Greenwich. Had we adopted this difference by law, it would have amounted to choosing for our prime meridian a point 5 hours 8 minutes and 12 seconds east of Was.h.i.+ngton, whether we happened to strike the transit instrument at Greenwich or not. This would have fixed an a.s.sumed longitude for the Cambridge observatory and for all points within our telegraphic net-work. We should have had a practical system, which might, however, require to be corrected from time to time, if some slight error were found in the a.s.sumed longitude of Was.h.i.+ngton.
In the present state of astronomical observation these little errors are of no consequence except in some very refined astronomical discussions. For all geographical and perhaps geodetical purposes the error may be regarded as zero, and it may be said, in regard to astronomical work, that it will always be independent of any meridian that might be chosen.
But even if this difficulty were avoided, he could not see how they could have any place which would come within the definition of a neutral meridian. Supposing they took the Azores, they belong to Portugal; then certainly they would have a Portuguese prime meridian, belonging to the Portuguese nation. Thus they would no longer have a neutral point, if he (Professor NEWCOMB) rightly understood the meaning of Professor JANSSEN.
He said that the Delegate of Great Britain, Professor ADAMS, had expressed very clearly his (Professor NEWCOMB'S) ideas, and the difficulty we have in meeting the propositions of the French Delegates; that what he had said would apply very properly to any neutral meridian that might be chosen in accordance with the plans of Professor JANSSEN. Whatever that meridian might be, we must always a.s.sume for it a certain number of degrees from the capital of the country, where the place to be determined is located, and then take that imaginary meridian instead of a real point on the surface of the globe.
It is true that this is perfectly practicable, and on that theory there might not be any necessity of having an astronomical observatory. But why we should go to this trouble and expense Mr.
JANSSEN did not make very clear; his considerations were purely sentimental, as was remarked by the Delegate of Great Britain, Professor ADAMS, and he (Prof. NEWCOMB) did not see what advantage would be gained by a neutral meridian in preference to one fixed by convenience.
In order that a discussion may proceed, it is necessary to agree on a given basis from which to start, and it is extremely difficult to agree upon a basis if there are considerations of sentiment introduced, because such considerations are peculiar to each person.
He therefore wished to propose this question again to the Delegate of France, namely, what advantages can we derive from fixing upon a neutral meridian?
Mr. JANSSEN, Delegate of France, said:
Professor NEWCOMB asks me to point out the advantages of a neutral meridian. These advantages are of two kinds--they are of a geographical nature and a moral nature. Let us examine the first. By placing the initial meridian between Asia and America, we get away from the centres of population, which is almost indispensable in view of the change of dates. We divide the world into two parts, the Old World and the New. The advantage of drawing the prime meridian through the ocean has always been understood, and it was precisely for this reason that Marinus of Tyre, during the first century, placed it at the Fortunate Isles, west of the African Continent. It is idle to urge the difficulty of fixing such a meridian as an objection.
Astronomy is so far advanced in our day as to enable us to make this calculation with all desirable accuracy.
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