[1] See reference No. 2 on p. 181.

[2] The telescope with which these observations have been made is a 6·8-inch refractor equatorially mounted.

[3] Ferricyanide of potassium and hyposulphite of soda.

[4] From the declination of the sun corrected for variation and from the known latitude, the meridian zenith distance is calculated.

From the Greenwich time, the longitude, and the equation of time, the hour angle is obtained.

Now, if H be the hour angle, D the reduced declination, and M the meridian zenith distance, the sun’s altitude may be calculated by the formula—

log versin H + L cos lat. + L cos D-20 = log n,

where n is a natural number, and

n + vers M = covers alt.

Again, to find the azimuth—

vers sup. (lat. + alt.)-vers polar dist. = m,

where m is another natural number, and

log m + L sec. lat. + L sec. alt.-20 = log vers azim.,

reckoned from the south.

Hence the position of the sun is ascertained for both negatives.

By actual measurements on the plates and reference to a previously constructed scale the position of the cloud as seen from each camera is next determined, and the angle subtended by the base line at a point X vertically beneath the cloud is calculated. If A and B are the stations, and a and b the angles from them respectively, the distance AX is given thus—

log AX = L sin b-L sin AXB + log AB,

and the height h of the cloud above X is given by—

log h = log AX + L tan alt.-10.