mm 



► 



VOL. LV.] PHILOSOPHICAL TRANSACTIONS. 273 



in the islands of Mendoza, the morning ingress will be found to be retarded by 

 parallax 4'" 31'; and consequently a difference at this contact of 1 1"" 43' may be 

 obtained by comparing the observation of the first internal contact with the ob- 

 servations at Greenwich; or 11™ 24', if the same observation be compared with 

 that at Tornea". 



In order to see the beginning of egress accelerated by the greatest effect of 

 parallax possible, when the altitude of the sun is 5", an observer must be sta- 

 tioned in about 123° of w. longitude from London, and in about 19° of s. lati- 

 tude; or, as it does not appear at present that there is land there, the observer 

 may be stationed with considerable advantage either in the islands of Mendoza, 

 or in the island of St. Peter. The same contact will be as much retarded on ac- 

 count of parallax to an observer placed under the tropic of Cancer, in about 67^ 

 of E. longitude from London. This point indeed falls into the gulf of Sindi; 

 but as a difference of many degrees, either in longitude or latitude, will occasion 

 but a very inconsiderable difference in parallactic time, this contact may be very 

 advantageously observed on any part of the coast from the mouth of the Indus 

 to Cape Comorin, and thence along the coast of Coromandel and Golconda as 

 far as the mouth of the Ganges. It may naturally be expected that the end of 

 the transit will, if the weather be favourable, be observed at many of our own 

 settlements in these parts; Mr. H. therefore computed the effect of parallax at 

 the egress for Madras and Calcutta, at which places the last transit was observed; 

 and found that the time of the 2d internal contact will happen 6'"41» later on 

 account of parallax at the former, and 6"" 44' at the latter, than if seen from the 

 earth's centre. By comparing the observations made at either of the above places 

 with the corresponding observation at Mexico, we may obtain a difference of 

 11 •"44'; a difference greater than could be obtained by any observations that 

 could conveniently be made at the egress in the transit of 1761, 



On the whole, the necessity of sending an observer into the South seas suffi- 

 ciently appears, whether it be proposed to determine the sun's parallax by the 

 difference in the total duration of the transit, or by the observations of the in- 

 ternal contacts either at the ingress or egress. For if there should happen to be 

 no land in the meridian opposite to Tornea", and in about 2 1° of s. latitude; yet 

 if an observer can be stationed either in the islands of Mendoza, or in the islands 

 of Amsterdam and Rotterdam, a difference in parallactic time will be obtained 

 as in the following table. 



Places compared. Difference in total Difference Diffeience 



•^ duration. at uigress. at egress. 



Tornea" and Mendoza isles 23*" O* 1 1™ 24' 11™ 36* 



Tornea" and Amsterdam or Rotterdam . . 22 41 13 12 g 29 



Tornea" and the opp. merid. in 21 s. lat.. .23 56 13 3 10 53 



If the sky should prove favourable, the observations made at Tornea", and in 



VOL. XII. N N 



I 



