ON THE ELUCIDATION. OF METEOROLOGICAL PHENOMENA. Lv 
and the height / of the cloud point above X is given by the equation : 
Log h=log AX +L tan alt.—10. 
Thus in tlie case given above the angles a and b are 85° 45’ and 92° 53’, 
whence AXB must be 1° 22’. The altitude in angular measure is 67° 46’. 
Then : L sin } = 9:99945 
' Lsin AXB= 8:°37749 
1:62195 
Log AB) = 2:30103 
L tan alt. =10°38851 
10+logh =14:31150 
. Ah =20,488 yards =11°64 miles. 
If the angle AXB becomes much smaller than 1° less confidence can be 
placed in the result, and it is better to calculate from the different alti- 
tudes at the two stations, as such minute angles can only occur when the 
direction is nearly in line with the base. 
Now it is seen that in the above calculation the angle AXB is certainly 
small, but owing to the length of focus adopted an angle of 2’ may be 
certainly detected, and by taking the mean of three or four measurements 
there is little risk of error. The height determined above was that of 
some high cirriform clouds, and is confirmed, not only by other measure- 
ments on the same plates, but by other determinations made 35 and 47 
minutes later, the three determinations being 11°64, 11:2, and 11°45 
miles. 
A little later in the same day a still higher layer of cirrus appeared, 
and two measurements of this at a brief interval of time work out to 
16°83 and 17-02 miles. 
These are, of course, extreme altitudes, and are quoted in order to 
show that the results obtained by the method employed are suttficiently 
accurate even under such circumstances. With lower clouds the displace- 
ments of the image relatively to that of the sun are much larger, and the 
heights obtained are more uniform. 
_ It should be remembered that the base line adopted is only 200 yards 
long. This is not quite enough for very exact measurements of great 
heights, nor is it enough for the determination of heights of less magni- 
tude when the clouds under observation are either east or west, that is, 
in line with the base. But, on the other hand, the effects of perspective 
are quite sufficiently troublesome with low-level clouds, or when an upper 
layer is seen through gaps in a lower one, and many negatives have had to 
be rejected from the impossibility of identifying corresponding points. In 
some cases the corresponding negatives were so much unlike that it was 
difficult to believe they could really have been simultaneous exposures. 
The distance of 200 yards has therefore been chosen as a convenient 
mean. For low stratus and cumulus 100 yards would be better, and for 
high cirrus about 400 yards would give more precise results. 
The orientation of the base line again simplifies the angular measure- 
ments, but for observations in the afternoon later than about 3.30 to 4 p.m. 
the horizontal projection of the base is reduced to a very trifling amount, 
and a complete installation should certainly consist of three stations, the 
acres placed either due north or due south of one of the others, so 
6. N 
