348 On the Expansion and Contraction of Iron Bridges, 



4™ civil time, after 912 years eight days twenty-two hobrs 

 twenty-nine minutes ; but by reducing the calendar reckoning to 

 the old style, the same eclipse happened April 1, \6^ 21™ ; which 

 makes the difference two days one hour thirty-one minutes ; so 

 that allowing three days in each period for the lunar anticipation, 

 the corresponding eclipses of A. 1801 have been found to anti- 

 cipate three days in each period, or six whole days in two periods, 

 very nearly. 



3. Those eclipses set down in Ricciolus's catalogue are cor- 

 rected up to the Gregorian account; and therefore the several 

 corrections of the calendar since the Nicene Council must be re- 

 garded in making use of the corresponding dates. 



4. It seems possible to prove that 912 solar years constitute a 

 complete lunar period, in which the solar and lunar motions co- 

 incide in the same point of the ecliptic ; and that in this period 

 the whole system of solar and lunar eclipses is performed. 



5. That the variations of calendar time may possibly account 

 for the lunar anticipation of the old astronomy, and that such 

 may be accounted for on other principles than any real anticipa- 

 tion of the lunar motions, which principles we shall suggest in a 

 future communication. 



[To be continued.] 



XXXIX. On the Expansion and Contraction of Iron Bridges 

 by Changes of Temperature ; with a Svggestionfor counter- 

 acting these Effects. By A Correspondent. 



To Mr. Tilloch. 



Sir, — A HE effects of changes in the temperature of the at- 

 mosphere, in expanding and contracting metals in exposed situa- 

 tions — for example, iron bridges — does not seem to have excited 

 much attention until lately, when some of the effects became too 

 visible to remain longer unnoticed. Permit me to offer an idea 

 (through your very valuable Magazine) which may be of some 

 advantage in the construction of those very elegant and econo- 

 mical structures. As it is impossible to prevent heat and cold 

 from producing their natural effects, instead of the immense 

 abutments, that arc at present necessary to resist the increase 

 of thrust or pressure in warm weather with a constant change 

 of the curvature of the bridge, and necessarily a change in the 

 points of contact, in the parts forming the arch, I would plate 

 the ribs or segn^ciits on blocks or bars of zinc, placed in recesses 

 formed for that purpose in the abutments (and piers if consisting 

 of more than one arch) ; those blocks or bars in length should 

 be to the extent of the arch in the ratio that zinc expands to 



the 



