MARCH, 1820. I THINK, that I have made it appear very clearly, that with the two publications, the Nautical Almanack, and the Connaissance des Tems, a mariner may go to sea with a considerable degree of confidence. But he must not rely entirely on this advantage; for the case may occur, that an errour is in the Nautical Almanack, when the Connaissance des Tems does not afford him that assistance, which is pointed out by the example noticed in the last month. For there are distances of the Moon from a fixed star in the Nautical Almanack, which have no corresponding distances in the Connaissance des Tems. It will be of use therefore to show the mariner, how he may detect and correct the errour in the Nautical Almanack, without applying to the Parisian observatory; and as the sub ject is of so much importance to the navigation' of our vessels, our time will not be ill bestowed in making this clear to the reader. We will take the instance given last month, as thus it will be shown how nearly we come to the same conclusion, by means of the Nautical Almanack alone; and this will be of use, not only in this case, peculiarly belonging to navigation, but to many other cases in which the astronomer only is concerned. The distances of the Moon from Antares, on part of the 6th and 7th of January were as follows; At three in morning of 7th... 68 52 29 At three in the morning...... 68 52 29 Difference...... At six........... At nine........ Difference At nine........ At noon................ Difference 67 22 19 Here we observe, that the difference in the distances, taken by three hours, is continually increasing; but it is between one degree twentynine minutes, and one degree and a half, till after three in the morning of the 7th. Con sequently we may take it for granted, that one degree and a half must be near the difference in the two last instances: and as we find the differences in these last instances at so great a variance from all the rest, we must be convinced that there is an errour in them. It is evident also, that the errour must be in the first of the two, namely, 1° 16′ 20′′, and this is to be corrected. To do this we must consider the nature of the preceding differences, and as we have noticed that they increase, we will now try in what manner they increase; and to do this we will write them down in their order, and take the differences between them. The differences between the differences lead us naturally to imagine, that the difference be tween the Nos. 6 and 7, must be either ten or eleven seconds, and either of these latter numbers will be sufficient for us. Let us take ten seconds for the first, which, being added to No. 6, will give us for No. 7, 1° 30′ 20′′; and eleven for the other, i. e. No. 7, 1° 30′ 31′′. Instead therefore of the difference between the distances, at six and nine in the morning of the 7th, being 1° 16′ 20′′, we will suppose it to be 1° 30′ 20′′; and instead of that between nine in the morning and noon being 0° 44′ 31′′, we will suppose it to be 1° 30′ 31′′. Hence we correct the two distances in the following manner: Distance at six in morning of the 7th. 67 22 19 Difference between this and distance at nine........ Distance at nine 1 30 20 65 51 59 Supposing then our distance at nine to be properly corrected, we will in the same manner. find the distance at noon thus: |