296 GEOGRAPEY AND AST$ONOMY. sure) you will find that you have travelled about 70 English miles; though geographers often reckon 60 geographical miles to a degree for greater ease in computation, as I have said before. Problem XIII. "To find the circumference, the diameter, the surface and solid contents of the earth. Having found the value of one degree to be 70 miles, mul. tiply that by 360, and it produces 25,200 miles for the circum- ference. The diameter is in proportion to the circumference as 113 to 355, or as 50 to 157, or in more brief and vulgar account as 7 is to 22, which will make the diameter of the earth to be about 8000 miles. Multiply thecircumference by the diameter, and that pro- duct shall be the square feet, furlongs, miles, &c. of the sur- face. Multiply the surface by the sixth part of the diameter, and that will give the solid content. Note, That geographers differ a little in the computation of these measures, because they differ in the measure of a single degree: And that is because of the crookedness and inequality of any road that you can travel for 70 miles together. The justest measures have made 691miles go to a degree, or the round number of 70 miles. Problem XIV. " To find the value of a degree of a lesser circle on the earth, i. e. the value ora degreeof longitude on the lesser parallels of latitude." I have mentioned it before under the IIId Problem of the 19thSection that all the degrees marked on the equator, or on any of meridians are70 miles, because all those lines are great circles ; yet in the parallels of latitude, the further you go from the equator, the circle grows less and less, and consequently each degree of it must be less also ; and for this reason the whole circle of 360 degrees near the pole will not make above 360miles ; and as you approach still nearer to the pole, it will not make so many furlongs or feet. To find therefore the true value of a degree suppose in the parallel of latitude of London 51 1 degrees, use this method, figure xxii. Make a straight line A s to represent one degree in the equator, divide it into 60 geographical miles, or into 70 English miles, all equal. Set the foot of your compasses in A, describe an arch from B to e of 511 degrees, then from the point c let fall a perpendicular to n, and a n is the measure of a degree of longitude in Me parallel of London, (viz.) about 431 mtles. The demonstration of it may thus be explained. Prolong the arch B c and complete the quadrant E A B, Then E shall represent the nortlt pole ; a n the northern half of the axis of
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