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into a circular form, we shall find it to be the arc of a large circle. If 

 we bend it more into the form of an arc, we find that it becomes the 

 part of a smaller circle, and the more we bend it into a circular form, 

 the smaller will be the radius of the circle with which it will correspond. 

 Thus we find that the larger circle deviates less from a straight line 

 than the arc of any of smaller radius ; so that, if the mariner sails over 

 the ocean by the route of an arc of the largest circle that can be drawn 

 on the surface of the globe, he may be said to sail directly to this port. 

 We may draw an unlimited number of circles on the surface of a globe, 

 each varying in its diameter ; but we cannot draw a circle on such a 

 surface, the radius of which is greater than that of the globe ; the arc 

 of a circle, the radius of which is equal to that of the globe, is what 

 we call the arc of a great circle. The arc of any larger circle than that 

 of a great circle, will, as is the case with a straight line, be a tangent 

 to the globe, touching at one point only. A great circle may also be 

 distinguished from any other circle drawn on the surface of a globe by 

 its dividing the surface into two equal parts ; thus, the equator is a 

 gi-eat circle, dividing the surface of the earth into two equal areas, 

 called the northern and southern hemispheres. But the tropics are not 

 great circles, and we consequently find that each divides the earth into 

 unequal parts. Thus, north of the Tropic of Cancer we have CO-J° of 

 latitude, whilst south there are 113|^°. On the north of the Tropic 

 of Cancer we have a temperate and a frigid zone ; on the south we 

 have three zones — a temperate, a frigid, and a torrid zone. There is 

 also a practical method of determining whether any arc on the surface 

 of a globe be the arc of a great circle. If we hold a piece of string 

 tightly by its ends, and press it down on the surface of the globe, it 

 will describe the arc of a great circle ; and this method of projecting 

 the arc of a great circle at once, is also a proof that a great circle is 

 the shortest possible track over the surface of a globe. The carpenter 

 draws liis straight line on a plane by a chalk-line. The principle is 

 this — he employs a tension which draws the line as short as the points 

 to which the ends are fastened will allow. Now, since a straiglit line 

 is the shortest track on a plane, he produces a straight line liy this 

 means ; but, when we stretch the line over the surface of a sphere, the 

 rotundity of the surface bends the line from a straight into a circular 

 form ; but, since it deviates as little as possible from the sti-aight line, 

 it forms the arc of a great circle, such being the shortest track across 

 the surface of a globe. There are, however, other circles connected 

 with the science of navigation, besides great circles. The parallels of 

 latitude are circles lessening in their diameters as we approach cither 



