A KNOT OR NAUTICAL MILE.

New Zealand Herald, Volume XXVIII, Issue 8670, 12 September 1891, Page 2 (Supplement)

A KNOT OR NAUTICAL MILE.

How much is a knot ? The question is asked, we believe, in every sea-passage by some passenger or other, and never .nv«;.ts with a clear reply. Sailors themselves do not describe it distinctly, and books of reference differ as to its dimensions. Wo purpose to answer the question here.

A knot is one-sixtieth of a mean degree of the earth's meridian. This definition requires explanation, and also numerical computation. Tho earth's meridian is commonly described as any circle whose centre is the centre cf the earth, and whose circumfcrcncc passes through the Poles. •This is nob exact, because the meridian is not a true circle. Evidently, it would be a true circle if til 3 earth were a true sphere ; but the earth is not a true sphere—it is a spheroid, its diameter measured on the axis being less than its diameter at the equator. Hence the circumference of a section of the earth by a piano passing through its centre and the Poles, which circumference is a meridian, is not a true circle, but an oval. Bearing this in mind, it will be easy to understand the meaning of a mean degree of the earth's meridian.

If 300 separate decrees be pet off from the centre of a perfect circle, it is evident that the circular measure of each degree measured on the circumference of the circle will be the same. But if they be set off from the centre of an oval, the measurement on the circumference of the oval will not all bo the same. That this is the case anyone may demonstrate for himself by drawing an oval and its minor axis, and then, from the centre of the oval, with radius equal to its semi-minor axis, inscribing a circle in the oval, If now, degrees, or rather, for convenience, equimultiples of a degree, be set oil' from the common centre, the geometry of the figure will show at once the variation in the circular measurements on the circumference of the oval.

Now, a mean degree of the earth's meridian is the average length of these 300 unequal measurements, and it is obtained by dividing the length of the meridian by 300. Astronomers have measured the earth's meridian and found ib to be 131,259,257 English feet. Dividing this by MO we get 36-1,G09'13 feet as the length of a mean degree of the meridian. One-sixtieth of this then is a knot ; and thus, by division, a knot is found to be 6070 - 818 feet, or 2025'0 yards, or one mile 265 (J yards.

It will now be convenient to notice that a knot being CiO7G , SIB feet, and a mile being 5280 feet, the proportion of a knot to a mile is very nearly as 0076 is to 5250, or, dividing by four, as 1519 is to 13*20, which is very nearly as 15 to -13. So that, for ordinary purposes, knots may be converted into miles by taking 13 knots as equal to 15 miles, and vice versa.— Chambers' Journal.