October, 1902.] 



KNOWLEDGE 



229 



Just Vk'fore sunrise on the morning of the 17th the 

 moon is totally eclipsed, and althoutjh the following 

 method is applicable to any lunar eclipse, we will deal 

 more particularly with the one of this dale. An eclipse 

 of the moon takes place when the moon ](lunges into the 

 earth's shadow, and thus is deprived of the sun's rays 

 If then we can ascertain the size of the shadow at the 

 point where the moou is situated, and also know the 

 direction and rate of movement of the moon through it, 

 we have the material for predicting any particular phase 

 of the eclipse. 



From the Nautical Almanac we have given — 



Moon's equatorial horizoutal pamllax 

 Sun's „ „ 



Sun's radius 



59' 13" = 3553" 



9" = 9" 



16' 3" = 963" 



Adding the two former, and from the sum subtracting 

 the latter, we get 2599", which represents the radius of 

 the earth's shadow at the point of the moon's orbit.* 



The earth'satmosphere, however, tends to increase the size 

 of the shadow, and on this account it is usual to increase 

 the above radius by about -suth ; we will therefore take — 

 Eadius of shadow = 2650". 



Diagram showing path of moon tlirougli earth's shadow. 



Take a convenient scale of equal parts (say yi^ inch 

 = 10'), and with C as centre and radius C W equal to 

 265 in., describe the circle N E S W representing the 

 earth's shadow. 



The declination of the centre of the shadow C will be 

 the same as that of the sun but of contrary sign, thus 

 since the declination of the sun is south, that of the 



Shadow = N 8° 55' 21", 

 whilst the Moon's declination = N 9° 8' 53" 

 make C P = 13' 32" = 812", 



' Apparent semi-diameter of the earth's shadow at the moon ; that 

 this is the algebraic sum of the above quantities is shown as follows : — 

 S represents the sun. 

 E represents the eartli. 

 m >r m ' represents part of the moon's orbit. 



Semi- diameter of earth's shadow at moon 



». E M = E m A - E C m. 



= KmA - (AE.S - EAC). 



= E i« B -^ E A B - A E S. 

 But E m B =■ Moon's liorizontal parallax. 

 E A B = Sun's horizontal parallax. 

 A E 8 — Sun's apparent semi-diametei-. 



which is the amoimt the uioon is north of liie centre of tiie 

 shadow ; also make 



CD = 9' 11" = 551", 

 which is the hourly motion of the moon from the shadow 

 in declination, and since the direction of this motion is 

 northwards, the ]>oint D is taken north of the centre C ; 

 also make C K eciual to the hourly motion of the moon 

 from the sun in right ascension, this is 32' 14"' 7, which, 

 to be represented on the line C W must l)e reduced to the 

 arc of a small circle by multiplying by the cosine of the 

 declination, thus: — 32' 14"-7 X cos. 9° 9' 

 = 1934.-7 X 0-9875 

 .-. C R = 1910". 



Join R D; then by the parallelogram of velocities this re- 

 presents the direction and hourly rate of the moon's motion. 



Through P draw M T parallel to R D ; it is evident 

 then that M T represents the path of the moon through 

 the shadow. P is the position of the moon's centre 

 at the time of opposition = Gh. 10m. 131s., and 

 since 11 D represents the motion in one hour, a pro- 

 portional part, P V^I.. can be taken equal to the motion 

 in lOni. 131s., then VI will be the jiosition of the moou 

 at SIX o'clock. Make the distance from VI. to V., and 

 from VI. to Yll., &c., equal to K D, and we have a time 

 scale which may be sub-divided, and from which the time 

 of anv phase may be derived. 



The moon may now be drawn in at any required position, 

 such as the first contact at M, by taking the radius of the 

 circle to be described equal to the moon's semi-diameter. 

 In this case. 



Moon's semi-diameter = 16' 8"'-l, 

 or the points of first and last contact, M and T, may be 

 found by making C M or C T equal to the sum of the 

 •semi-diameters of the shadow and moon. 



The magnitude of the eclipse is measured by the ratio 

 of the amount the innermost limb of the moon is obscured 

 O B to the moon's diameter. 



Elements. 

 Eclipse of Moon, October 17th, 1902. 

 Greenwich mean time of opposition in R.A., 



October 16th 



18 10 131 



>('s Declination .. 



0*8 Declination,.. 



(C's Hourly motion in R.A. 



©'s Hourly motion in R.A. ... 



iC's Hourly motion in declination 



G's Hourly motion in declination 



i('s Equatorial horizontal parallax 



G's Equatorial liorizontal parallax 



(£'s Ti'ue semi-diameter 



0's True semi-diameter 

 Magnitude of Eclipse (Moon's 



diameter = 1) 1-463. 



RUDOLPH VIRCHOW. 

 By Sir Samuel Wilks, m.d., ll.d., f.r.s. 

 The death of Professor Virchow has removed from the world 

 of science one of its most devoted followers, and from the 

 medical profession one who held the most eminent place in its 

 ranks. Wherever scientific medicine was recognized the name 

 of Virchow was as familiar as a household word. Pathology 

 has as broad a basis as humanity itself, and .so has united its 

 cultivators in all parts of the world under one common master. 

 Nothing less than a volume could contain all the details of 

 the life of this great man, as he was not only closely alhed to 

 the scientific world, but he held an important place in politics, 

 possessing a seat in the Prussian Diet, as well as being a 

 member of the Municipal Council which imposed upon liini 

 important hygienic work. At the present time, therefore, we 

 can only regard the scientific aspect of the man, and even this 



