NA TURE 



[June 28, 1906 



Easter does not fall on April 25, as given by the formula, 

 but on April 18. 



These exceptions are caused by an inconsistency in the 

 Gregorian rule, caused by the adherence to the old custom, 

 that Easter should never fall later than April 25. 



Armagh Observatory, June 22. J. L. E. Dreyer. 



The discrepancy between the date of Easter, 1954, 

 April iS as given by the tables in the Book of Common 

 Prayer, and April 25 as given by the formula of Gauss, 

 arises from a purely artificial contrivance of Clavius, who 

 arranged the reformed calendar, which is thus described 

 on p. 55 of " The Prayer Book Interleaved," 1873, in an 

 account of the calendar founded on a paper by Prof. 

 De Morgan : — " It will never happen as to mean luna- 

 tions, and rarely as to real ones, that in the same cycle 

 there should be the lunation of a given month beginning 

 on the same day in two different years of the cycle ; and 

 such a thing never happened in the unreformed Calendar. 

 Clavius thought it desirable to imitate this in the new 

 Calendar ; and he observed that by taking the preceding 

 day whenever the Epact was xxv., and the year of the 

 cycle after the nth, he could avoid the reiteration, and 

 thus make the desired resemblance." " Whenever the 

 Epact should be xxv., the year of the cycle being upwards 

 of II, say that the Epact is 26. This is not an astro- 

 nomical correction, but a mere conventional mode of 

 reconciling the choice which Clavius made of the mode 

 of writing the Epacts with an essential peculiarfty of the 

 old cycle of 19 years which that mode of writing would 

 have otherwise destroyed." " In 1954 the Golden Number 

 is 17, the Sunday letter C, and the Epact according to the 

 ordinary rule, xxv. CaU il therefore xxvi. Thence 

 April 17 will be the 14th day of the Paschal Moon, 

 April i8, Easter Day. If the Epact xxv. were used April 25 

 would be Easter Day." The paper by Prof. De Morgan 

 will be found in the " Companion to the British Almanac " 

 for 1845. 



My copy of Nature for April 5 has long since gone to 

 Bolivia, but probably your correspondents will find that 

 Gauss did not take into account this artifice of Clavius. 

 If in this century golden number 6 and Sunday letter C 

 had coincided, Easter would have been set on April 25, 

 because 6 conies in the cycle before 11 instead of after it 

 as 17 does. An inspection of Table III. for finding Easter 

 will show in the two half-lines for April 17 and 18 the 

 arrangement made by Clavius. C. S. T.avlor. 



Banwell Vicarage, June 22. 



Musical Thunder. 



E.ARLV this morning a storm broke in this neighbourhood 

 accompanied by heavy thunder. During the storm I 

 noticed that two of the peals began with a musical note 

 of distinct and definite pitch. The " musical " portion of 

 the peal lasted for about two seconds in each case, and 

 the frequency of the note was both times about 400 per 

 second. 



This sound closely resembled a foot-fall in a narrow 

 alley between high walls, and was only heard in two 

 consecutive peals, separated by an interval of about a 

 minute, the first being much more definitely musical than 

 the second. In each case the interval between the flash 

 and the first sound of thunder was about five seconds. 



.^s is well known, a peal of thunder from lightning near 

 at hand frequently sounds like a quick succession of raps 

 or a volley of guns. Can the successive raps have followed 

 one another so rapidly in this case that they combined to 

 form a note? 



If so, and if this note was due to a special configuration 

 of reflecting surfaces in the clouds, possibly to others in 

 slightly different positions, considerably different frequencies 

 may have been observed. 



The fact that two peals only sounded in this manner 

 separated by the short interval of about one minute, and 

 that the second was not so decidedly musical as the first, 

 seems to indicate that they were due to some rapidly 

 changing source such as one might expect the reflecting 

 surfaces of a cloud to be. I listened carefullv to deter" 



NO. 19 1 3, VOL. 74] 



mine that the note had its origin outside and was not 

 due to resonance within the room, and in the second peal 

 it was certainly outside, and probably had the first had 

 its origin within the room 1 should have observed it. 



I should be very glad to hear if anyone has observed a 

 similar phenomenon. G. H. M.-vrtvn. 



I Marden Road, S. Tottenham, N., June 24. 



How do Inquiline Bees find the Nest of their Host ? 



The following observation may serve to throw light on 

 the above question, which has doubtless occurred to many 

 entomologists. Yesterday I saw a specimen of the 

 inquiline Coelioxys quadridentala enter the burrow of a 

 leaf-cutter bee, Mcgachile circumcincta. I dug the nest 

 out of the burrow, and in so doing scattered the sand over 

 an area of several square inches, completely destroying all 

 appearance of a burrow. I sat down to await the return 

 of the Megachile, in order to identify the species, and was 

 much astonished to see (and capture) in the course of the 

 next ten minutes two more specimens of Coelioxys, which 

 came hovering over the spot and alighted on the disturbed 

 soil. I can think of no other explanation than that these 

 " cuckoos " were attracted to the spot by the scent of the 

 excavated nest. I may add that during several hours spent 

 on the heath where this occurred I saw no other speci- 

 mens of Coelioxys, and, further, that there was a fresh 

 south-east breeze blowing at the time, and that the bees 

 came up against the wind. 



Osw.^LD H. Latter. 



Charterhouse, Godalming, June 24. 



THE DISTURBANCE OF GREENWICH 

 OBSERVATIONS. 



IN the House of Lords on Thursday last, June 21, 

 attention was directed to the threatened danger 

 to the continued efficiency of the Royal Observatory, 

 Greenwich, caused by the great electrical generating 

 station erected by the London County Council about 

 half a mile due north of the observatory. The danger 

 was referred to by the Astronomer Royal in his report 

 to the Board of \'isitors on May 30, a summary of 

 which appeared in N.ature of June 7 (p. 135). The 

 generating station is situated exactly in the Green- 

 wich meridian, as will be seen from the accompanying 

 photograph of a view looking north over the top of 

 the transit room ; and the tall chimneys shown in the 

 picture, as well as the heated air from them, will 

 obviously interfere with some observations of northern 

 stars, which are essential for latitude and refraction. 

 Moreover, from tests already made it appears that the 

 powerful engines which are being installed at the 

 generating station will cause vibrations that will 

 seriously affect the value of observations by reflection 

 from a mercury horizon, required for the fundamental 

 work of the observatory. 



This is not the first time that the effects of gener- 

 ating stations and electric tramway systems in the 

 neighbourhood of the observatory have been pointed 

 out. About six years ago the question of the possible 

 effect of disturbances from electric railways on the 

 magnetic work carried on at the observatory was 

 given careful consideration ; and the hope was then 

 expressed that in the event of future electric tramways 

 regulations would be laid down by the Board of 

 Trade to secure adequate protection for the magnetic 

 work. The records in this department of the observ- 

 atory have been obtained continuously on a general 

 system for sixty-five years, but the astronomical work 

 extends over more than two centuries and a quarter, 

 and it would be unfortunate if circumstances should 

 arise to break this chain of continuity. 



The generating station established at Deptford — 

 nearlv a mile from the observatory — to supply the 

 London Counft' Council Tramwavs with electric 



