©. 7020 approximately 
The double circles, instead of one for each star, have another 
Advantage, for if they do not subtend the precise angles sta 
the angles they actually subtend may be found by testing, and 
the stars placed accordingly. ) 
__Thelines cut on the glass diaphragm by Messrs. T. Cooke & Sons, 
imation and other instrumental errors are eliminated by 
tn a telescope and taking the mean of the readings. The 
mes “sie has yet to be determined accurately. Speaking from 
be co. » 1 think, with ordinary care, the maximum error will 
-surveyj to be less than 15”, which for all practical purposes of 
ra is sufficiently precise. 
Medic! here be mentioned that, to find the stars, the altitude 
should be set to the approximate latitude, the needle floated, 
of tru horizontal circle set to the approximate magnetic bearing 
®south ; the two stars are then visible in the field of view. 
iste experimenting to adapt this method to the ordinary 
be .. € of a 5-inch theodolite. The same precision will not 
The nog but it will still be enough for all practical purposes. 
its cost, + i of carry ing about the additional telescope, as well as 
Sur “ill, however, be saved. 
and | bin who have not the required knowledge of astronomy 
Usual pratt trigonomet cy. 
. hitherto adopted will be enabled to determine it as 
rT on n riation in Nev th We 
Society in July, 1871, no attention was paid to 
the needle. : 
: of 
