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Another circumstance which arises from this difference of meridians in time, must detain us a little before we quit this subject. For, from this difference it follows, that if a ship sails round the world, always directing her course eastward, she will, at her return home, find she has gained one whole day of those that stayed at home; that is, if they reckon it May 1, the ship’s company will reckon it May 2; if westward, a day less, or April 30.
This circumstance has been taken notice of by navigators. “ It was during our stay at Mindanao, (says Captain Dampier) that we were first made sensible of the change of time in the course of our voyage: for, having travelled so far westward, keeping the same course with the sun, we, consequently, have gained something insensibly in the length of the particular days, but have lost in the tale the bulk or number of the days or hours.
“ According to the different longitudes of England and Mindanao, this isle being about 210 degrees west from the Lizard, the difference of time, at our arrival at Mindanao, ought to have been about fourteen hours; and so much we should have anticipated our reckoning, having gained it by bearing the sun company.
" Now, the natural day, in every place, must be consonant to itself; but going about with, or against the sun's course, will, of necessity, make a difference in the calculation of the civil day, between any two places. Accordingly, at Mindanao, and other places
in the East Indies, we found both natives and Europeans reckoning a day before us. For the Europeans coming Eastward, by the Cape of Good Hope, in a course contrary to the sun and us, wherever we met, were a full day before us in their accounts.
“ So among the Indian Mahometans, their Friday was 'Thursday with us ; though it was Friday also with those that came eastward from Europe.
“Yet, at the Ladrone islands, we found the Spaniards at Guam keeping the same computation with ourselves; the reason of which I take to be, that they settled that colony by a course westward from Spain; the Spaniards going first to America, and thence to the Ladrone islands."
It is clear, from what has been said in the first part of this article, concerning both latitude and longitude, that if a person travels ever so far directly towards east or west, his latitude would be always the same, though his longitude would be continually changing
But if he went directly north or south, his longitude would continue the same, but his latitude would be perpetually varying.
If he went obliquely, he would change both his latitude and longitude.
The longitude and latitude of places give only their relative distances on the globe; to discover, therefore, their real distance, we have recourse to the following problem.
PROBLEM X. Any place being given, to find the distance of that place from another, in a great circle' of the earth.
I shall divide this problem into three cases.
Case 1. If the places lie under the same meridian. Bring them up to the meridian, and mark the number of degrees intercepted between them. Multiply the number of degrees thus found by 60, and they will give the number of geographical miles between the two places. But if we would have the number of English miles, the degrees before found must be multiplied by 69.
Case 2. If the places lie under the equator. Find their difference of longitude in degrees, and multiply, as in the preceding case, by 60, or 691.
Case 3. If the places lie neither under the same meridian, nor under the equator. Then lay the quadrant of altitude over the two places, and mark the number of degrees intercepted between them. These degrees, multiplied as above-mentioned, will give the required distance.
To find the angle of position of
The angle of position, is that formed between the meridian of one of the places, and a great circle passing through the other place,
Rectify the globe to the latitude and zenith of one of the places, bring that place to the strong brass meridian, set the graduated edge of the quadrant to the other place, and the number of degrees contained between it and the strong brass meridian, is the measure of the angle sought. Thus,
The angle of position between the meridian of Cape Clear, in Ireland, and St. Augustine, in Florida, is about 82 degrees north-westerly ; but the angle of position between St. Augustine and Cape Clear, is only about 46 degrees north-easterly.
Hence it is plain, that the line of position, or azimuth, is not the same from either place to the other, as the rhomb-lines are.
PROBLEM XII. To find the bearing of one place
The bearing of one sea-port to another is determined by a kind of spiral, called a rhomb-line, passing from one to the other, so as to make equal angles with all the meridians it passes by ; therefore, if both places are situated on the same parallel of latitude, their bearing is either east or west from each other ; if they are upon the same meridian, they bear north and south from one another ; if they lie upon a rhomb-line, their bearing is the same with it; if they do not, observe to which rhomb-line the two places are nearest parallel, and that will shew the bearing sought.
Example. Thus, the bearing of the Lizard point
from the island of Bermudas, is nearly E.N.E; and that of Bermudas from the Lizard is W.S.W. both nearly upon the same rhomb-line, but in contrary directions.
OF THE TWILIGHT.
That light which we have from the sun before it rises, and after it sets, is called the twilight.
The morning twilight, or day-break, commences when the sun comes within eighteen degrees of the horizon, and continues till sun-rising. The evening twilight begins at sun-setting, and continues till it is eighteen degrees below the horizon.
To illustrate the causes of the various lengths of twilight, in different places, a wire circle is fixed eighteen degrees below the surface of the broad paper circle; so that all those places which are above the wire circle will have twilight, but it will be dark to all those places below it.
I have already observed that it is owing to the atmosphere, that we are favoured with the light of the sun before he is above, and after he is below, our horizon. Hence, though after sun-setting we receive no direct light from the sun, yet we enjoy its reflected light for some time; so that the darkness of the night does not come on suddenly, but by degrees.
In a right position of the sphere, the twilights are quickly over, because the sun rises and sets nearly in a perpendicular ; but, in an oblique sphere, they last