344 
On the Measurement of the 
[Nor. 
'1H27' 'll * SSUmei ’ ‘ hat f ° r day 0bswati0 “ s a ^-al -"ark should subtend 
Mack* ‘Csr z:;:?:z; e ; f p,ojected upon - sky - * ^ ba — 
T, d f Coh MudSe - BengaI %>“ s *» introduced ns signals, 
fitted fl it 7 u r Weat “ er - Re ™rberatio S lamps, or Argand lamp, 
afd lLI r n P “ mirr0rS ’ S °° n 8u PP la »‘ ad the troublesome fire-work, 
■ P Oved th T , :~ d ’ “ the lriSh “goao-etrical survey, has further in,! 
P o ed this kind of signal by the substitution, in lieu of the oil burner, of a frag- 
° Iu "<' «nited by a stream of mixed oxygen and hydrogen gases, which 
rie-ents an intense light, visible at an immense distance. 
belli? 0,11 by Captain Everest are of three kinds : one, the rever- 
lar nlZ. ? 1 ’ mgl,t ' WOrli ’ alread y Scribed ; the second, a heliotrope, or cim,- 
va ds th 8 aS5 ““ be adjusted over the centre of the station and to reflect 
‘ he theodolite a Potion of the solar rays : the light pro- 
of ndia w? f “““ is 8 ° Powerful as to penetrate the morning mists 
sislu l/intrlir;- The ,,rindpa ' diffiral ^ - «« use of the heliotrope con- 
cross wire pfaced M from on) n le<lected ra >' ; tllis is effected by means of a 
tant station, upon which the l'li't ' bC "''rtained' direction of the dis- 
placed another heliotrope throwing its ^7 7 ‘“"'“'V " ear the tl,eodolite /'* 
descripUorfof wlcht mLi lt'? " ^ 8 
Philosophical Transactions. P1CSLn ^ r6ader to tbe last voIume of tbe 
spherical excesf or The^tldity IfThe ^ih “ tm0 ' St aCC " raCy ’ and comcted for 
are deduced from the ordinal base ? “ ' 8 8l,rfaCe - the lengtlls of ,heir SK,CS 
ings taken at every station that is 7 C °" l ' erted lnto a m <fidionaI line by bear- 
triangle and the north-pole Azimuths of IT”? ‘ he anSle '° rmed ^ “ SWe ?' 
east and a , * . 1 s of the P olar star, at its greatest elongation, 
not be coinci le P omt > antl where the celestial and terrestrial sights can- 
referring nn ^ & S ™r &mp flXed at a COn venient distance is introduced as a 
- - *■ « - - - iiskt 
spl 6 fe?°r Pe - atI , 0 T’i he T S n0 ' V Supposed t0 fin™ . been carried over a sufficient 
precautions dea l t 7 T mea8l,reme " t °f a base of verification with all the 
pi ecautions described at the commencement of the work. 
to his terrestriT” UnS ^ ^ ffeodest to determine the celestial arc corresponding 
(and ifrequisite mea t SUre T nt: in ° ther wor ds, to find the latitude of his extreme 
tana it requisite, intermediate) stations. 
involmthTunre-f ne(l - fl0m the SUn ° r circun *-poIar stars of moderate elevation, 
refraction &e . r . aird ’ e8 of P ola r tables, of deflection of the instrument by heat, of 
employed - this is ^ ^ Slmeys > therefore a zenith sector has hitherto been 
lar-e radius fft ‘"f * alon & telescope fitted with a portion of a circle of very 
few de^re^of^he' eV6n ^ ^eet) for observing the altitude of stars within a very 
instrument is m- r k T, ‘ W16re the faction is nearly insensible. The zero of the 
require further descripUon.^^ dellCate plumb - Iine » hut it is too well known to 
prJcessilr and erV l d ^ ** Z6nith S6Ct0rhave *> undergo correction for nutation, 
observations* of a 3etrVltlon ’ Unless they can be compared with the simultaneous 
country such Lr e eStaMiShed ° bservato ry in the neighbourhood; in tbi, 
could never be the cane, aud even in England the catalogue, 
