A manual of spherical and practical astronomy, embracing the general problems of spherical astronomy, the special applications to nautical astronomy, and the theory and use of fixed and portable astronomical instruments, with an appendix on the method of least squares . the sides of the angle is the required measure.*To give this mode precision when the angle is found by linesdrawn to two distant points, the aid of the telescope is is connected with the circle in various ways, according tothe nature of the instru-ment of which it forms ^ a part; but, in general,we may conceive it
A manual of spherical and practical astronomy, embracing the general problems of spherical astronomy, the special applications to nautical astronomy, and the theory and use of fixed and portable astronomical instruments, with an appendix on the method of least squares . the sides of the angle is the required measure.*To give this mode precision when the angle is found by linesdrawn to two distant points, the aid of the telescope is is connected with the circle in various ways, according tothe nature of the instru-ment of which it forms ^ a part; but, in general,we may conceive it to beessentially as the tube of the tele-scope, AB, Fig. 11, isattached a pivot, C, atright angles to the op-tical axis, which turnsin a circular hole in thecentre of the graduatedcircle MN. An arm aCh, extending from the centre Cto thegraduations on the limb, is permanently attached to the telescope,and revolves with it. To measure an angle subtended by twodistant objects at the point C, the circle is to be brought into theplane of the objects and firmly fixed. Then the telescope is * In the sextant and other instruments of double reflection, the vertex of theangle to be measured is not in the centre of the arc used to measure it. See 30 MEASUREMENT OF ANGLES. directed successively upon the two objects, and in each casethe number of degrees indicated by a mark on either extremityof the arm ab is to be read oft; the difference of the two readings,which is the number of degrees passed over by the arm, and,consequently, also by the telescope, will be the required measureof the angle. The same result is reached by permanently con-necting the circle and telescope, which then revolve together,while a fixed mark near the limb of the circle serves to indicatethe number of degrees through which the telescope revolves. In order to point the telescope with ease and accuracy uponan object, a clamp and tangent screw are commonly contrivance, which
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Keywords: ., bookcentury1800, bookdecade1, booksubjectastronomicalinstruments
