Jan. 23, 1879] 



NATURE 



169 



Perihelion Passage, April 25"99867, M.T. at Paris. 



Lcngilude of the perihelion 149 i 55*9) 



,, ,, ascending node 83 28 33*6 > 181 5*0 



Inclination of the orbit to ecliptic 44 29 54*6 ) 



Excentricity o 93121968 



Semi-axis major 17*63383 



Logarithm of perihelion distance 00838109 



Period of revolution 74*04913 years. 



Motion — direct. 



These elements represent the normals upon which they 

 are founded very closely, considering that observations of 

 comets in 18 15 did not pretend to the degree of precision 

 which is now sought to be attained, and, moreover, were 

 subject in the reductions to errors in the places of the 

 comparison stars. 



But Bessel's labours did not stop here. With a special 

 interest in the comet of 1815, not, it may be presumed, 

 alone due to its exceptional character, but in no small 

 degree to the circumstance of its having been detected 

 by his most intimate and revered friend, Olbers, Bessel 

 undertook, and in the year of its appearance accom- 

 plished, the laborious task of computing the perturbations 

 of the planets Jupiter, Saturn, and Uranus upon the 

 motion of the comet during the present revolution, and 

 so determining the epoch of the next perihelion passage. 

 The principal ^details of this work are comprised in the 

 memoir to which we have already referred. The masses 

 of Jupiter and Uranus were Laplace's, while the mass of 

 Saturn was taken from Bouvard's tables. The whole 

 period is divided into three sections, the first extending 

 from August 4, 18 15, to July 30, 1833 ; the second from 

 the latter date, with new values of the semi-axis and 

 excentricity to July 21, 1869, and the second from 

 July 21, 1S69, to the next perihelion passage. The action 

 of each of the three planets tends to accelerate the 

 comet's return, that of Jupiter by upwards of two years; 

 the final result indicating an acceleration of 824*51 days, 

 with reference to the period belonging to Bessel's defini- 

 tive ellipse for 181 5 ; it was thus found that the duration 

 of the actual revolution would extend to 26222'4 days, 

 and consequently the next perihelion passage is fixed to 

 February 9-4, 1887. This conclusion will be affected not 

 only by the imperfect values of the planetary- masses 

 which were available when Bessel undertook the investi- 

 gation, but in a greater degree by the uncertainty which 

 still remained as to the precise length of the revolution 

 at the last appearance ; this Bessel found to extend to 

 ± o'27657 of a year, or loi days. 



With such an amount of probable error attaching to 

 Bessel's result it must soon be a matter for the considera- 

 tion of the astronomer, whether a nearer approximation 

 may not be yet attained. We have mu?h more accurate 

 values of the masses of Jupiter, Saturn, and Uranus than 

 Bessel possessed, and are able to take into account the 

 influence of Neptune, though this is not likely to be very 

 material. Fortunately, in several series of observations', 

 the observed differences of right ascension and declina- 

 tion between the comet and the comparison stars are 

 preserved to us, and thus we can reduce the obser\-ations 

 anew, with much improved positions of many of the stars 

 and with modem elements of reduction. The series of 

 observations thus available include the long one of Olbers 

 {Berlitier astrotiomisches Jakrbuch, i8i8), and those of 

 Greenwich, Paris, and Dorpat. It is a wor'.c which, 

 together with the recalculation of the perturbations to 

 the next perihelion passage, may perhaps be made the 

 subject of a prize by one or other of our scientific 

 acadeniies ; on the last return of Halley's comet, the first 

 approximation to the epoch of arrival at perihelion was 

 due to action of this kind on the part of the Academy of 

 Turin, and though a much higher degree of interest 

 attached to the reappearance of that famous body, we do 

 not despair to see Olbers' comet deemed worthy of a new 

 and more refined calculation. 



If these cometary bodies wandering into the confines of 

 the solar system from the stellar spaces are fixed therein 

 by the action of one or other of the planets, it will have 

 been owing to a very close approach to the planet Mars 

 that Olbers' comet presented itself in 18 15, moving in an 

 ellipse of moderate dimensions. The nearest approach 

 of the two orbits in that year was 0*07 in 86'''4 heliocentric 

 longitude, but this distance must have varied in succes- 

 sive revolutions through the perturbations of the other 

 planets, and at some past time there may have been an 

 intersection of the orbits and a close encounter of the two 

 bodies. 



METEOROLOGICAL NOTES 



Before the commencement of the summer rains this 

 year Mr. Eliot, the officiating meteorological reporter to the 

 Goverrmient of India was called upon for a report on the 

 prospects of the season. His reply, to which we have 

 already referred in the "Notes," consisted of a short 

 resume of the most important characteristics of the south- 

 west monsoons of recent years, from which the following 

 conclusions were deduced : — " i. The persistent excessive 

 pressure over Northern India at the present time (June, 

 1878), tends to diminish the baric gradient between 

 Southern Asia and the Mid-Indian Ocean, and if this is 

 not compensated by increased pressure over the sea area 

 to the South of India, the monsoon current wiU be below 

 its average strength. 2. There appear to be no strongly- 

 marked abnormal variations of pressure over Northern 

 India. It is therefore probable that the rainfall will be 

 much more equally distributed than last year. 3. Com- 

 paring the present year with 1865, it is probable that the 

 heavy rainfall during the cold weather, and more espe- 

 cially in May, will slightly retard the advent of the mon- 

 soon in Upper India. 4. The probable effect of the low 

 pressure along the Bombay coast cannot be determined 

 except by comparison with last year. It appears to 

 promise fairly abundant rain over that portion of the 

 countr\-." These conclusions have now been subjected 

 to the test of experience and are found to have been veri- 

 fied in almost everj' particular. The southerly current 

 from the Indian Ocean has been decidedly below its 

 normal strength ; the rains set in from a fortnight to a 

 month after the usual time ; every district in the 

 country has received a moderate supply of rain, though 

 the average rainfall for the whole country has been 

 less than usual, and over the Bombay Presidencj', 

 from Belgarum to Kurrachee, the rainfall has been 

 in excess of the average for previous years. The 

 cnly peculiarity of the monsoon of 1878, that was not 

 predicted, was the frequent recurrence of heavy falls of 

 rain over a few small and well-defined areas ; but this 

 would seem to be the character of the rainfall of ever>' 

 year in which the monsoon current is of less than the 

 usual strength. The percentage of verifications reached 

 by Mr. Eliot has thus been as great as that attained by 

 the American obser\-ers, and the predictions in his case 

 were made months, not days or hours, in advance. The 

 same meteorologist has recently made a discovery which 

 promises to be of the greatest possible value in connec- 

 tion with the system of storm-warnings to the ports round 

 the Bay of Bengal. It is that a cyclonic vortex, when 

 generated in the middle of the Bay, always travels 

 towards that part of the coast where the wind velocity 

 for the time being is least in comparison with the average 

 velocity for the same place and time of year. This law 

 has been verified by almost all the cyclonic disturbances 

 that have occurred in the Bay since a chain of meteoro- 

 logical observatories was established round it, and it 

 lends a great deal of support to the theory that a cyclonic 

 vortex is developed through the accumulation, concentra- 

 tion, and condensation of aqueous vapour over a region 

 of comparative calm. All that appears now wanted to 



