The observations required may be made in the open air and completed, in clear and favourable weather, in about three ... the instrument without twisting the suspension thread. average of thirty or forty minutes ; and for the between each seriesanbsp;...
Title | : | Supplement to the Magnetical Instructions for the Use of Portable Instrumensts, &c,. |
Author | : | Charles James Buchanan Riddell |
Publisher | : | - 1844 |
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