Astrolabes of the World

t h e R o y a l I t a l i a n N a v y takes the form of a precise handbook of observations by means of the astrolabe. I t gives the description and method of use of the J o b in instru­ ment and of the instrument constructed by the S. O. M. (So c ié t é d ’O p t iq u e e t d e M é c a n iq u e d e h a u t e p r é c is io n , Paris). For the computation of observations by prismatic astrolabe the Italian Hydrographie Institute has adopted the J o r d a n formula by means of which the computations of distance from the estimated position to the position line given by the astrolabe obser­ vation, may be easily estimated. An example of computation is given in the work. For the preparation of the programme of observations the Italian Hydrographic Institute has adopted a graphic procedure worked out by Admiral T o n t a ; this procedure, which makes use of stellar plans of the northern and southern hemispheres in stereo­ graph-polar projection and of a graph representing the almucantar of 30° zenithal dis­ tance corresponding to the station position, is described on pp. 40-45 of Professor F o r n i ’s publication ; by its means the list of sidereal times and azimuths of the different stars observable passing at the zenithal distance of 30° is very readily established.