X. DRAWINGS. 431 



The graphic representations drawn on the maps are called astronojnical webs, 

 and the rules contain scales. By the assistance of a diagram and a scale 

 any problem in spherical trigonometry can be solved without calculation, 

 a great advantage in navigation. It serves also on land for the determination 

 of time and azimuth by means of one observation of a star. 



In order to 'obtain the necessary accuracy in navigation, the drawing must 

 b$ made as exact as possible by a machine, so that the accuracy of the solu- 

 tion, so far as it depends 011 the accuracy of the observing instrument, will 

 be fully attained. Far greater precision will be arrived at by repetition, 

 namely, by the easy reading of dozens of results which depend upon as 

 many observations. Even the most extensive table for nautical calculation 

 cannot effect this ; besides which the inverse problem, often so difficult of 

 solution, becomes a pastime by means of this apparatus. 



1903. Specimen of Transparent Astronomical Chart, 



southern evening sky in winter, Central Europe. 



Prof. J. J. Oppel, Frankf or t-on- Maine* 



1904. Specimens of Astronomical Diagrams, for teaching. 

 White figures on black ground. J. J. Oppel. 



In both instruments the circular plate (white on one side and black on the 

 other) represents the plane of illumination, at a at the time of the equinoxes. 

 The arrangement demonstrates as a necessary effect of a secular revolution 

 from left to right of the plane of inclination of the earth's axis, A the in- 

 crease of the longitude of the stars ; JB the difference of the sidereal and 

 tropical year ; C the change of the pole star. The long wire at a can be 

 fixed either in the direction of the pole (for A) or of the earth's axis (for 

 C) . The plane of the ecliptic is supposed horizontal ; the appliance a must 

 b,e turned from left t<? right slowly on its pivot. 



1906. (1.) Diagram for Nautical Astronomy, engraved on 



stone ; with a printed explanation. (2.) Diagram of Nautical 

 Astronomy. Handbook of Practical Nautical Astronomy. 



Prof. Prestel, Emden. 



19O7* Orrery, by Cole; explanatory of eclipses. 



Royal Society, 



1907a. Working Model, showing the annual and diurnal 

 involutions of the earth, thus illustrating the changes of seasons 

 and the successions of day and night, designed by W. Adcock. 



Made by J. H. Adcock. Thomas Murly. 



1908. Celestial Globe, 34 cm. in diameter. 



Dietrich Reimer, Berlin (Reimer and Hoefer). 



1909. Celestial Globe of 80 cm. diameter, with complete 

 equipment. Dietrich Reimer, Berlin (Reimer and Hoefer). 







X. ASTRONOMICAL DRAWINGS. 



Drawing made by William Temple of the Nebula of Orion, 



observed by him in the first months of the present year, by means 

 of the great refractor of Amici. 



The Royal Institute of" Studii Superiori" Florence. 



