THE YOUNG SCIENTIST. 
53 
chord H d, the length of c h becomes 
known. From c h subtract c w, and we 
have w h, which is really the object 
sought, and is the parallax of Yeniis. 
*' But this is not the parallax of the sun.'' 
True, but the parallax of the sun is ob- 
tainable from it. 
It is ivnown that the distance of Venus 
from the sun is 72-100 of the earth's dis- 
tance. Now let the earth's distance be 
represented by 100, then Yenus' distance | 
from the sun would be 72, and her dist ance i 
from the earth 28. Now% as the ratio of ! 
the parallaxes varies inversely as the dis- 
tances of the bodies, the parallax of the 
sun can be ascertained by the following 
proportion : e v : v w : : n s : (?) or, calling 
the eaith's ixidlus 1, we have 28: 72:: 2: 
(5), or as 1 is to 2?. In place of n s the ac- 
tual distance between the two observers 
will be substituted, which is their differ- 
ence of latitude. In other words, this 
proportion shows us that the displace- 
ment (parallax) of Venus is 25 times less 
when viewed from the points n and s in 
Fig. 3, than if viewed from the dis- 
tance of Venus. But the sun's liori- 
zontal parallax, the amount of displace- 
ment suffered by the sun when seen 
from the centre and circumference of 
the earth, must be still half of this or one- 
fifth of the parallax of Venus. Suppose 
the arc w n = 42.8", then must the sun's 
horizontal parallax be 8.56". The correct- 
ness of this method depends upon the ac- 
curacy with w^hich the angles are taken, 
but it will be seen that any error of obser- 
vation is reduced five times in the final re- 
sult. The extreme delicacy of the opera- 
tions required may be judged from the 
fact that the difference between the old 
parallax (8.73 ') and the latest (8.82") or 
.09", is equivalent to about 1,000,000 miles, 
and this arc (0.09' ' ) is n o greater than would 
be the apparent thickness of a liuTTian hair 
viewed at a distance of 100 feet. Notwith- 
standing the apparent insignificance of 
the figures, the latter estimate adds many 
millions of cubic miles to the calculated 
volume of the sun and many times the 
earth's mass to its weight; cha.nges the 
estimated distances, volumes, masses, 
and diameters of all the other planets. 
A still greater error is made in com- 
puting the distances of the fixed stars. 
The earth's distance from the sun is used 
as a base line to compute the distance of 
the stars. Thus, in Fig. 4, which, of 
course, is not drawn to scale, the better 
to illustrate, when the earth is at a in its 
OL'bit a certain star (a) is observed to be at 
d in the heavens s n, and six months from; 
the date of the first observation, when 
the earth is at c, the same star appears at 
h. Now it is a fact that the nearest fixed 
star shows no displacement when viewed 
from c and ff, 184,000,000 miles apart. But 
nevertheless this method is used indi- 
rectly to ascertain the depths of stellar- 
space, and the distances of over a dozen 
stars is known ; and hence it is that this, 
minute error in the sun's parallax of .09" 
will produce an error' of 200,000,000,000 
miles in the calculated distance of the 
nearest fixed star, and in some whose dis- 
tances have been computed this error 
would be mcr eased fivefold I 
"But how do you find the sun's dis- 
tance from the earth from the horizontal 
parallax?" 
In Fig. 5 let c D G represent the earth 
and s the sun ; then, according to what 
we have already shown, the sun's hori- 
zontal parallax is the angle e s d or e s c,. 
or, in other words, the earth's apxmrenf 
semi-diameter, e nor c e, as seen from the 
sun. Then, in the right-angled triangle- 
e d s, we have given the angle e s d and 
the side e d, to determine the hypoth- 
eneuse, e s, or the sun's distance, one of 
the simplest problems of plane trigono- 
metry. The earth's radius, divided by the- 
sine of the sun's horizontal parallax = the 
distance sought. Thus we find, by using 
a parallax of 8.82", that the sun's distance 
is 23409.4 times the earth's radius, or 
92,759,700 miles distant. 
— M. Tissandier, the French aeronaut, 
has devised an elliptical balloon, which is 
to be driven by a dynamo machine and 
storage batteries. The balloon will be 
131 feet long, and will have a capacity of 
more than 100,000 cubic feet, with a lift- 
ing power of 3t tons, which will allow for 
a ton of passengers and ballast. It does 
not require much knowledge to see that 
such a machine must prove a failure. 
