PHILOSOPHICAL SOCIETY OP WASHINGTON. 39 



produce a retardation of leap year by one complete Julian cycle, 

 equivalent to a suppression of one intercalary day every 132 

 years ; in v\rluch period there would be by the calendar of Julius 

 Cajsar 33 leap years, and by the calendar of Omar Obey am only 

 32 leap years. I am under the impression that this calendar was 

 actually adopted by the Persian Sultan, but cannot speak with 

 confidence about it. 



The Julian year of 365 days and 6 hours gains over the tropi- 

 cal or equinoctial year of 365 days, 5 hours, 48 minutes, and 49 

 seconds, an excess of one day in about 129 years, and the beauti- 

 ful simplicity of Omar Cheyam's plan depends on the circum- 

 stance of the number 1 28 being happily divisible twice by 4. And 

 thus, while the error of our present calendar (the Gregorian), with 

 its inconveniently long cycle of 400 years, runs out into an excess 

 of one day in 3756 years, the error of Omar's calendar, with its 

 short cycle of 33 years, amounts to an excess of one day only in 

 6252 years. That is, it has a clear range of 52 centuries against 

 the 37 centuries of our Christian calendar. And by reason of the 

 shortness of the cycle, its evagation or intermediate range of 

 departure from the true equinoctial points is also, of course, very 

 much less. 



On the whole, I must confess a great admiration for this ex- 

 tremely simple adjustment, and a decided preference of it to the 

 system in common use, even when made still more accurate by 

 the suggestion of Delambre, that one of the Gregorian leap year 

 days should be omitted on every millennium divisible by 4000. 

 Even this third approximation, with its cumbrous cycle, would 

 still leave an error of a deficiency of one day in 216,000 years. 



The only consideration that occurs to me as likely to be suggested 

 in favor of the adopted calendar, is its mnemonic guide to the leap 

 year by means of the familiar divisor 4. This is a point which, 

 of course, would have a very different value with different minds. 

 To the Jew or the Mohammedan, employing a different chrono- 

 logical epoch, no such advantage is presented. If this mnemonic 

 have, however, any real importance, it could be equally well 

 secured by simply omitting one leap year every 128 years. Nor 

 would this period be practically any more arbitrary than the 400 

 year cycle; as is well illustrated by the general popular confusion 

 which took place in the year 1800, when very few persons were 

 able to say whether it was a leap year or not. The error of this 

 scheme would be a deficiency of one day in 18,776 years. 



The President communicated two letters from Mr. A. C. Ross, 

 of Zanesville, Ohio, 



ON LATENT IMPRESSIONS ON POLISHED GLASS PLATES PRODUCED BY 

 HEATING THE PLATES IN CLOSE CONNECTION WITH ENGRAVED 

 METALLIC PLATES : 



