^{page 396 note *} For brevity, I use the word “transit” to denote a time of transit of the tide-generating body (whether sun or moon), or a time of transit of the point of the heavens opposite to the tide-generating body, across the meridian of the place; and the word 6 “o'clock,” to denote the middle instant of the interval of time between consecutive transits. If, to fix the ideas, we first think of the lunar tide alone as if there were no solar tide, 6 o'clock will mean 6 lunar hours before or after a lunar transit.

^{page 401 note *} In strong winds the barometer may stand sensibly above or below the proper value for the weight of the atmosphere over the place, according as the room containing the barometer is more exposed by openings on the windward or on the leeward side of the house in which it is placed. The error due to this cause may be sensible in the diurnal averages for one particular barometer, because of the daily periodic variations in the direction of the wind; but it is not probably large for any well-placed barometer, and, such as it is, it must be fairly well eliminated in the averages for different barometers in variously arranged buildings and in different parts of the world. In passing, it may be remarked, that it is probably not a matter of no importance that the barometer room of a well-appointed meteorological observatory should be as nearly as may be symmetrically arranged in respect to openings to the external air in different directions, and in respect to shelter against wind from other parts of the building.