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tides are raised must be calculated by adding together the attractions of the sun and moon; and in the latter, it must be estimated by the difference of these attractions.

Tutor. You are right. When the sun and moon are both vertical to the equator of the earth, and the moon at her least distance from the earth, then the tides are highest.

Charles. Do the highest tides happen at the equinoxes?

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Tutor. Strictly speaking, these tides do not happen till some little time after, because in this, as in other cases, the actions do not produce the greatest effect when they are at the strongest, but some time afterwards: thus the hottest part of the day is not when the sun is on the meridian, but between two and four o'clock in the afternoon.-Another

circumstance must be taken into consideration: the sun being nearer to the earth in winter than in summer, it is of course nearer to it in February and October, than in March and September; and therefore, all these things being put together, it will be found that the greatest tides happen a little before the vernal, and some time after the autumnal Equinoxes. The probable times of the greatest tides in each year are given in White's Ephemeris; a most useful almanac for all young astronomers.

James. Since the attraction of the moon has a greater effect in producing the tides than that of the sun, it is natural to conceive, that the magnitude of the tides varies with the distance of the moon from the earth.

Tutor. You are perfectly right in that conjecture. The moon's attrac

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tion upon the waters is greatest when she is in her perigee, or nearest the earth; and it is least, when she is in her apogee, or the point farthest from the earth. The tides are proportionally greater in the former case, than in the latter. The moon's attraction is also greatest, all other things being the same, when she is in the equator. The moon's declination always has an effect, more or less, in retarding the actual time of high water. Tables are given in several books of astronomy and navigation, by means of which the time of high water may be accurately computed for any assigned place, for any particular declination, and for the various positions with regard to the moon's distance from the earth.

CONVERSATION XVII.

Of the Harvest Moon.

TUTOR. From what we said yesterday, you will easily understand the reason why the moon rises about three quarters of an hour later every day than on the one preceding,

Charles. It is owing to the daily progress which the moon is making in her orbit, on which account any meridian on the earth must make more than one complete rotation on its axis, before it comes again into the same situation with respect to the moon that it had before. And you told us that this occasioned a difference of about 50 minutes.

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Tutor. At the equator this is generally the difference of time between the rising of the moon on one day and the preceding. But in places of considerable latitude, as that in which we live, there is a remarkable difference about the time of harvest, when at the season of full moon she rises for several nights together only about 20 minutes later on the one day than on that immediately preceding. By thus succeeding the sun before the twilight is ended, the moon prolongs the light, to the great benefit of those who are engaged in gathering in the fruits of the earth; and hence the full moon at this season is called the harvest moon. It is believed that this was observed by persons engaged in agriculture, at a much earlier period than it was noticed by astronomers; the former

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