Easterly Deviation of the Astronomical Zenith.
| Name. | Longitude. | ||
| ° | ′ | ″ | |
| Feaghmain | −10 | 21 | −3.3 |
| Killorglin | −9 | 47 | +2.8 |
| Haverfordwest | −4 | 58 | +1.6 |
| Greenwich | 0 | 0 | +1.5 |
| Rosendaël-Nieuport | +2 | 35 | −1.7 |
| Bonn | +7 | 6 | −4.4 |
| Göttingen | +9 | 57 | −2.4 |
| Brocken | +10 | 37 | +2.3 |
| Leipzig | +12 | 23 | +2.7 |
| Rauenberg-Berlin | +13 | 23 | +1.7 |
| Grossenhain | +13 | 33 | −2.9 |
| Schneekoppe | +15 | 45 | +0.1 |
| Springberg | +16 | 37 | +0.8 |
| Breslau-Rosenthal | +17 | 2 | +3.5 |
| Trockenberg | +18 | 53 | −0.5 |
| Schönsee | +18 | 54 | −2.9 |
| Mirov | +19 | 18 | +2.2 |
| Warsaw | +21 | 2 | +1.9 |
| Grodno | +23 | 50 | −2.8 |
| Bobruisk | +29 | 14 | +0.5 |
| Orel | +36 | 4 | +4.4 |
| Lipetsk | +39 | 36 | +0.2 |
| Saratov | +46 | 3 | +6.4 |
| Samara | +50 | 5 | −2.6 |
| Orenburg | +55 | 7 | +1.7 |
| Orsk | +58 | 34 | −8.0 |
These deviations of the plumb-line correspond to an ellipsoid having an equatorial radius (a) of nearly 6,378,000 metres (prob. error ± 70 metres) and an ellipticity 1/299.15. The latter was taken for granted; it is nearly equal to the result from the gravity-measurements; the value for a then gives Ση² a minimum (nearly). The astronomical values of the geographical longitudes (with regard to Greenwich) are assumed, according to the compensation of longitude differences carried out by van de Sande Bakhuyzen (Comp. rend, des séances de la commission permanente de l’Association Géod. Internationale à Genève, 1893, annexe A.I.). Recent determinations (Albrecht, Astr. Nach., 3993/4) have introduced only small alterations in the deviations, a being slightly increased.
Of considerable importance in the investigation of the great arc was the representation of the linear lengths found in different countries, in terms of the same unit. The necessity for this had previously occurred in the computation of the figure of the earth from latitude-degree-measurements. A.R. Clarke instituted an extensive series of comparisons at Southampton (see Comparisons of Standards of Length of England, France, Belgium, Prussia, Russia, India and Australia, made at the Ordnance Survey Office, Southampton, 1866, and a paper in the Philosophical Transactions for 1873, by Lieut.-Col. A.R. Clarke, C.B., R.E., on the further comparisons of the standards of Austria, Spain, the United States, Cape of Good Hope and Russia) and found that 1 toise = 6.39453348 ft., 1 metre = 3.28086933 ft.
In 1875 a number of European states concluded the metre convention, and in 1877 an international weights-and-measures bureau was established at Breteuil. Until this time the metre was determined by the end-surfaces of a platinum rod (mètre des archives); subsequently, rods of platinum-iridium, of cross-section
, were constructed, having engraved lines at both ends of the bridge, which determine the distance of a metre. There were thirty of the rods which gave as accurately as possible the length of the metre; and these were distributed among the different states (see [Weights and Measures]). Careful comparisons with several standard toises showed that the metre was not exactly equal to 443,296 lines of the toise, but, in round numbers, 1/75000 of the length smaller. The metre according to the older relation is called the “legal metre,” according to the new relation the “international metre.” The values are (see Europ. Längengradmessung, i. p. 230):—
Legal metre = 3.28086933 ft., International metre = 3.2808257 ft.
The values of a given above are in terms of the international metre; the earlier ones in legal metres, while the gravity formulae are in international metres.
The International Geodetic Association (Internationale Erdmessung).