_Mrs. B._ Besides the usual division of circles into degrees, the ecliptic is divided into twelve equal parts, called signs, which bear the name of the constellations through which this circle passes in the heavens. The degrees measured on the meridians from the equator, either towards the north, or towards the south, are called degrees of latitude, of which there may be 90; those measured from east to west, either on the equator, or any of the lesser circles, are called degrees of longitude, of which there may be 180; these lesser circles are also called parallels of latitude. Of these parallels there may be any number; a circle drawn from east to west, at any distance from the equator, will always be parallel to it, and is therefore called a parallel of latitude. _Emily._ The degrees of longitude must then vary in length, according to the dimensions of the circle on which they are reckoned; those, for instance, at the polar circles, will be considerably smaller than those at the equator? _Mrs. B._ Certainly; since the degrees of circles of different dimensions do not vary in number, they must necessarily vary in length. The degrees of latitude, you may observe, never vary in length; for the meridians on which they are reckoned are all of the same dimensions. _Emily._ And of what length is a degree of latitude? _Mrs. B._ Sixty geographical miles, which is equal to 69-1/2 English statute miles; or about one-sixth more than a common mile. _Emily._ The degrees of longitude at the equator, must then be of the same dimensions, with a degree of latitude. _Mrs. B._ They would, were the earth a perfect sphere; but it is not exactly such, being somewhat protuberant about the equator, and flattened towards the poles. This form proceeds from the superior action of the centrifugal power at the equator, and as this enlarges the circle, it must, in the same proportion, increase the length of the degrees of longitude measured on it. _Caroline._ I thought I had understood the centrifugal force perfectly, but I do not comprehend its effects in this instance.