142 SOME EEFINEMENTS OF MECHANICAL SCIENCE. 



understanding of the laws governing the nniverse, nearer and nearer 

 has been the a})])r()ach to perfection in the working out of these dif- 

 ficult problems, but the many limitations surrounding them have 

 always kept their full solution somewhere in the future. 



The diurnal revolution of the earth, which gives the solar day, and 

 the revolution of the earth around the sun, the solar year, are the 

 arbitrary divisions of time marked off with the utmost precision 

 by the celestial bodies; and while the length of the solar day has, 

 from before the Christian era, been fairly well defined, the length of 

 the solar year was but aj^proximately known until within a few hun- 

 dred years. 



The length of the year as counted by the Julian calendar was too 

 long by eleven minutes and fourteen seconds, and this error amounted 

 to ten full days in the sixteen hundred years from the time the 

 Julian Calendar went into eti'ect until the introduction of the Gre- 

 gorian calendar. 



A few years ago, when visiting the Vatican Observatory, I was 

 ]Darticularly interested in the (Iregorian tower, which forms a part of 

 the Vatican Library Building. After passing through a number of 

 ]"ooms which are used in connection with the observatory, Avhen near 

 the top of the tower, I was taken into the spacious and beautiful 

 calendar room, the walls of which are covered with paintings of the 

 liighest order, executed centuries ago, imder the direction of I^ope 

 Gregory XIII. In the center of the I'oom and forming a part of the 

 floor there w^as a large marble slab, on which was cut a fine line 

 exactly in the true meridian, and upon the line was a special mark 

 which indicated the altitude of the sun at noon of a certain day. 

 On the south wall, near the toj^ of the room, there was a small aper- 

 ture through which the direct rays of the sun passed at noon, ]:)roject- 

 ing a bright spot on the meridian line. 



All of this had been planned and executed by the astronomers in 

 order that they might demonstrate the necessity of reforming the 

 calendar, and when at noon on the 11th of March, 1582. Pope Greg- 

 ory saw that the altitude of the sun as shown by the beam of light 

 was not for that particular day, but for the day ten days later, he 

 directed that ten days be stricken from the calendar, and that day 

 should be the 21st of March instead of the 11th. 



With such precision had the astronomers determined the true 

 length of the year that our present calendar, with its intercalations, 

 will continue on for twenty thousand years with an error not to ex- 

 ceed a single day. 



The line on the marble slab and the aperture through the wall of 

 the calendar room were devices simple in the extreme, and in this day 

 of instruments such a method would hardly be considered, yet they 

 served their purposes admirably, and the placing of that line on the 



