poles. Therefore, the number of miles in a degree The following table shews how many geographical miles, and decimal parts of a mile, would be contained in a degree of longitude, at each degree of latitude from the equator to the poles, if the earth was a perfect sphere, and the circumference of its equinoctial line 360 degrees, and each degree 60 geographical miles. This table enables us to determine the velocity with which places upon the globe revolve eastward ; for the velocity is different, according to the distance of the places from the equator, being swiftest as passing through a greater space, and so, by degrees, slower towards the pole, as passing through a less space in the same time. Now, as every part of the earth is moved through the space of its circumference, or 360 degrees, in 24 hours; the space described in one hour, is found by dividing 360 by 24, which gives in the quotient 15 degrees; and so many degrees does every place on the earth move in an hour. The number of miles contained in so many degrees, in any latitude, is readily found from the table. " Thus, under the equator places revolve at the rate of more than 1000 miles in an hour; at London, at the rate of about 640 miles in an hour. 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 voy: 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. age: "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. PROBLEM XI. To find the angle of position of places. The angle of position, is that formed between the meridian of one of the places, and a great circle passing through the other place. P |