Gentlemen who have outlets might contrive to make ornament subservient to
utility: a pleasing eye-trap might also contribute to promote science: an
obelisk in a garden or park might be both an embellishment and an
heliotrope.
Any person that is curious, and enjoys the advantage of a good horizon,
might, with little trouble, make two heliotropes; the one for the winter,
the other for the summer solstice: and the two erections might be
constructed with very little expense; for two pieces of timber
frame-work, about ten or twelve feet high, and four feet broad at the
base, and close lined with plank, would answer the purpose.
The erection for the former should, if possible, be placed within sight
of some window in the common sitting-parlour; because men, at that dead
season of the year, are usually within doors at the close of the day;
while that for the latter might be fixed for any given spot in the garden
or outlet; whence the owner might contemplate, in a fine summer's
evening, the utmost extent that the sun makes to the northward at the
season of the longest days. Now nothing would be necessary but to place
these two objects with so much exactness, that the westerly limb of the
sun, at setting, might but just clear the winter heliotrope to the west
of it on the shortest day; and that the whole disc of the sun, at the
longest day, might exactly at setting also clear the summer heliotrope to
the north of it.
By this simple expedient it would soon appear that there is no such
thing, strictly speaking, as a solstice; for, from the shortest day, the
owner would, every clear evening, see the disc advancing at its setting,
to the westward of the object; and, from the longest day, observe the sun
retiring backwards every evening at its setting, towards the object
westward, till, in a few nights, it would set quite behind it, and so by
degrees, to the west of it; for when the sun comes near the summer
solstice, the whole disc of it would at first set behind the object;
after a time the northern limb would first appear, and so every night
gradually more, till at length the whole diameter would set northward of
it for about three nights; but on the middle night of the three, sensibly
more remote than the former or following. When beginning its recess from
the summer tropic, it would continue more and more to be hidden every
night, till at length it would descend quite behind the object again; and
so nightly more and more to the westward.