726 REPORT—1896. 
9. The Effect of Atmospheric Retraction on the Apparent Diurnal Move- 
ment of Stars, and a Method of allowing for i in Astronomical 
Photography. By Professor A. A. Rampaut, J/.4., Se.D, 
The variation in the degree of refraction which the light of a star undergoes in 
passing through the earth’s atmosphere, apart from irregularities which arise from 
local disturbances in the strata of air, affects the apparent movement of a star, so 
that the angular motion depends upon its position in the sky. 
When approaching its upper culmination the hour angle of a star is diminished 
by refraction, but to a continually diminishing extent, and consequently the motion 
of a star at this part of its course appears slower than it actually is. After 
culmination the result is similar, the refraction in this case throwing the apparent, 
more and more to the following side of the true, image as the distance from the 
meridian increases. 
When the observer's object is merely to obtain pictures of star groups the work 
can be so arranged that each group is photographed when it arrives at or near the 
meridian. It is different, however, when it is intended to utilise the plates for the 
detection of stellar parallax. In connection with this research, it is desirable that 
a large proportion of the photographs should be taken when the stars are near the 
apses of their parallactic ellipses, and this condition often necessitates the photo- 
graphing of stars at very large hour angles. 
If the apparent western hour angle of a star at any moment be denoted by /, 
the effect of refraction in hour angle by Af, the right ascension by a, and the 
sidereal time by 6; then 
h=6-—a+Ah 
NSS PAIN 
and 7+ Gee 
Hence the expression = measures the rate at which the apparent movement 
dé 
gains on sidereal time. 
If @ denotes the latitude and 6 the declination, and if we assume m, n, p, v, 
such that 
tan m=cot ¢ cos h, cot »=tan ¢ cos hk, 
cot n=sin m tan h, cot v=cos p» tan h, 
then we may write 
dh _1_8 cos ¢ sin y sin (1 +4) i : \ P (a) 
in which 8 is the refraction constant. 
If the telescope were required to follow the star with absolute precision it 
would be necessary to construct a clockwork system which would drive the 
instrument at a rate varying continually with the hour angle according to the law 
expressed by thisformula. In practice, however, it is sufficient to alter the rate at 
intervals, the length of which will depend upon the rapidity of the refraction 
changes, provided always that the error thus introduced does not exceed a certain 
definite limit. 
A description of the method of making this alteration, a full account of how 
formula (a) is deduced, and diagrams showing the appropriate rate for any given 
hour angle and declination, and the length of exposure for which a uniform rate 
is permissible, will be found in the ‘ Monthly Notices’ of the Royal Astronomical 
Society. 
10. On the Sailing Flight of Birds. By G. H. Bryan, Se.D., LRS. 
That birds are capable, under certain circumstances, of supporting themselves 
indefinitely in the air without expending energy by flapping their wings is a matter 
of common observation. To account for this apparent realisation of ‘perpetual - 
