Given the task definition and input, reply with output. In this task, you will be given a passage to read. A fill in the blank question will be given to you.  Your answer should fit the blank appropriately.

 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 . fill in the blank  :  The fourth stage involved remedying the problem of communicating between the two towers during the time of __________________ Pius X. 

 Pope 