Given a text passage, you need to generate a suitable title as the output. The output title should be one of the words/phrases used in the passage and must be no longer than five words. 

Paragraph:    The fourth stage involved remedying the problem of communicating between the two towers during the time of Pope Pius X. His plans were to make the Gregorian Tower into a historical tower and to record and carry out observations at the second tower by linking the two towers along the fortified wall with a 83 metres ( 272 ft ) iron bridge spanning the gap . At the west end of this bridge , a four @-@ inch equatorial was installed on semicircular bastion . The east end of the bridge , above the barracks of the gendarmes , had a heliograph , with a camera attached , used to photograph the Sun ( photoheliograph ) . A new 16 @-@ inch visual telescope , called Torre Pio X , was erected in the second tower . As a result of these modifications , the original library was moved to the Pontifical Academy Lincei , and the old meteorological and seismic instruments were shifted to the Valle di Pompei observatory . The new Astronomical Library was housed in two rooms of the building . The two new Repsold machines were used for recording on the astrographic plates . The recorded observations were published along with explanatory notes together with the last two series of the atlas of stars . Charts were printed on silver bromide paper .   Question: what is the suitable title of the passage ?
 Fourth stage