s they have more frequently their immediate cause from cold air,
inanition, or fatigue, than from the effects of lunations: whilst the cold
fits of hysteric patients, and those in nervous fevers, more frequently
occur twice a day, later by near half an hour each time, according to the
lunar day; whilst some fits of intermittents, which are undisturbed by
medicines, return at regular solar periods, and others at lunar ones; which
may, probably, be owing to the difference of the periods of those external
circumstances of cold, inanition, or lunation, which immediately caused
them.

We must, however, observe, that the periods of quiescence and exacerbation
in diseases do not always commence at the times of the syzygies or
quadratures of the moon and sun, or at the times of their passing the
zenith or nadir; but as it is probable, that the stimulus of the particles
of the circumfluent blood is gradually diminished from the time of the
quadratures to that of the syzygies, the quiescence may commence at any
hour, when co-operating with other causes of quiescence, it becomes great
enough to produce a disease: afterwards it will continue to recur at the
same period of the lunar or solar influence; the same cause operating
conjointly with the acquired habit, that is with the catenation of this new
motion with the dissevered links of the lunar or solar circles of animal
action.

In this manner the periods of menstruation obey the lunar month with great
exactness in healthy patients (and perhaps the venereal orgasm in brute
animals does the same), yet these periods do not commence either at the
syzygies or quadratures of the lunations, but at whatever time of the lunar
periods they begin, they observe the same in their returns till some
greater cause disturbs them.

Hence, though the best way to calculate the time of the expected returns of
the paroxysms of periodical diseases is to count the number of hours
between the commencement of the two preceding fits, yet the following
observations may be worth attending to, when we endeavour to prevent the
returns of maniacal or epileptic diseases; whose periods (at the beginning
of them especially) frequently observe the syzygies of the moon and sun,
and particularly about the equinox.

The greatest of the two tides happening in every revolution of the moon, is
that when the moon approaches nearest to the zenith or nadir; for this
reason, while the sun is in the northern signs, that i