1830 .] 
Of Geodesy. 
7 
ther permitting, and lunar distances repeatedly in the course of the same day or nmht 
The facility and rapidity given to these observations, by the employment of the°re- 
fleeting and repeating circle of Dollond, already mentioned, enhances verv much the 
value ot this method. In the first number of the Gleanings, a suggestion of mine 
was published, recommending a method of employing lunar distances, so as to lie 
independent of the tables, i. e. by successive or by corresponding observations The 
circle of Dollond adds great practical value to this suggestion 1 4, j n f act j 
persuaded that in this way, or by correcting the tables, a longitude may be determined 
by a proper series of distances to within 2' ; which in the mean parallel of India is 
equal to about two miles. Owing to the slowness of the moon’s motion, however it is 
scarcely to be expected that observation can come much within this quantity, unless 
in a very much prolonged senes. We may, therefore, proceed to the consideration 
of the next method, the transference of time. 
. 2 t (‘ f In re . a,it y» a11 methods of determining the longitude, may he said to consist 
n the transference of time; but that which I am now going to describe seems to 
have been considered to depend more immediately or directly on that principle. And 
Jiou_h I could easily show that this opinion is incorrect, and that the methods are 
all the same in principle, differing merely in the phenomenon or appearance by which 
Dm same absolute instant of time is indicated at two places, to be compared there 
' , th the account as derived from the appearance of the heavens, different to each 
observer ; though I say this be evident to anyone who will consider the subject vet 
refnt ♦ ”, sanctM ™ d & use generally intelligible and convenient! I S not 
no eiI,p °y. lt> tboi, £ b !t see,n to indicate a distinction without a difference 
I j'. * , a,e tvvo w ' A y s 111 wlll ch the results usually classed under this head mav 
; ta ' ne : ffl In , Principle is perfect, but the application limited In the 
? r ’ p ° s d,ffieu . lt > r occl ' rs ,n the application, but the result is rendered doubtful by 
£™ ary rel I HnC f Placed ° n the accurac y tbe work of man’s hand. Thedoc? 
•, l cbances teaches us to estimate the amount of this uncertainty and it is nns 
sible by increase of means to diminish it greatly, so as in fact to S a ouel inn l’ 
‘otbe comparative value of , be means, an?! tbe ^curacy t£y are [^^0^0 secure 
I, at instant; in other words, the appearance of tbe Heavens as refer red tort ®* 
ment, perhaps an unattainable height, if it be required to be visible , mPe ~ 
on account of the curvature of tbe earth vet it it , ', c t0 , an > distance 
ous body, which 1 1 owing G „ut ray ^ t/eo' ectoni ' 
of our atmosphere, so £ to be distinctly Tisible ev™ wben th^ obTct (L'T* 
below °ur horizon and actually concealed by tbe intervening svvell of the ulnb. R S 
e evatmg on a stage SOfeet high the luminous" signal, the sphere of ts v Mbim 1,^ 
and S ro ; ckt, b> 'r P, ° y i“ ? -’ ! ‘ S operations carried on b "creCich 
to he obseired es| ! Ioi T ° f wluc! ! shouI !' be considered the phenomenon 
i i ’ a greater distance might be given to the stations t,. 
in some measure, I mean the independence of results ; but it may be worth fntjuK 
° f ' th “« irele V Dollond. It 
load’s circle l/tK p Tbl * objection is completely obviated by Dal 
tioularlv for Cantiin e! j5° r f ou d lal . c r ? on ? f °r the substance of the pamphlet, and par- 
to manvbv Dubhshinc t l ) ,ine s remarks, in the Gleanings, I think he would do a service 
a hem ' ° Ur corres P ondent mlt fnd m Art. 11. the substance qfka 
