266 
SCIENTIFIC SUMMARY. 
ASTRONOMY. 
Solar Parallax. — Mr. David Gill, her Majesty’s Astronomer at the Cape 
of Good Hope, has at length completed his investigation of the value of the 
solar parallax, and he communicates the result to the Royal Astronomical 
Society. It will he remembered that Mr. Gill went on an expedition to 
Ascension Island in the year 1877 in order to determine the solar parallax 
by measuring with a heliometer the distance and position of the planet 
Mat's from certain stars in the evening and morning. The expedition 
proved completely successful. Since this period Mr. Gill has been engaged 
in reducing these measures with the greatest care, taking every precaution to 
eliminate every source of systematic error. On twenty-five occasions he 
was able to obtain a complete series of measures of the distances of the planet 
Mars from the same star in both the evening and morning, this observation 
being superior to those in which the position of the planet had to be measured 
from different stars in the evening and morning. These twenty-five sets of 
measures give for the value of the solar parallax 
7r = 8"* 780 ± 0"-013 
When all the observations are reduced together, the result is 
7T = 8"-778 ± CT-026 
The larger probable error shows that this result is inferior in accuracy to 
the other. 
The separate results range between 8"'62 and 8"*90, so that they are 
fairly accordant, and would seem to render it probable that the resulting 
value, 
7r = 8"*780, 
is not so very far from the truth. This value is considerably smaller than is 
generally believed to be correct, most astronomers considering that the 
real value is not very far from 
= 8"*84 
The result of the last transit of Venus has given all kinds of value, from 
8"*76 to 8"*90, and up to the present time no very satisfactory result has 
been obtained. 
Lately an attempt has been made by Messrs. Campbell and Neison to 
determine the value of the solar parallax from the parallactic inequality in 
