ON THE ELUCIDATION OF METEOROLOGICAL PHENOMENA. 175 
dark slide in position, sets the shutter and adjusts the camera, so that the 
image of the sun is in about the centre of the ground glass of the finder- 
B does the same with the other camera, and, when ready, puts up the 
yellow flag at that end, and stands ready to press the exposing key. A 
then watches for the best moment for exposure, and, when it arrives, holds 
up the blue flag, on seeing which B presses the key and holds up the other 
blue flag as a signal that the exposure is complete at that end of the line. 
The pink flag is used as an indication that something is wrong, and delay 
is inevitable ; but if pink and blue are shown simultaneously, it means 
that the opportunity for a good observation has passed, and that the dark 
slide must be closed while waiting for another chance. 
As soon as one exposure has been made the dark slide is turned, and 
preparations are made for a second exposure, leaving the drawing out of 
the slide until the signal is given. When A gives the signal for exposure 
he has his watch in his hand, and notes the time at which he hears the 
click made by the release of the shutter. This time is noted down and 
checked as soon as possible afterwards by comparing the watch with a 
trustworthy clock, and, if necessary, correcting the record. 
The exposures having been made, the cameras are replaced in their 
boxes, the relays are examined to see that the armatures have broken the 
local circuit, and the line wire is disconnected from the key, these precau- 
tions being taken to make sure that the batteries may not run down 
owing to the circuits being unbroken or remade by the operations of 
spiders or accumulations of earwigs, which find a welcome shelter in the 
camera-boxes, and which it seems impossible to entirely exclude. 
The plates are then taken into the dark room, and before opening the 
dark slides the shutter of each is pulled out a little way, while the date 
and time of exposure are written in pencil in the corner of each plate. The 
subsequent processes do not remove this. They are then developed with 
pyro and ammonia developer, and for the most useful results a fairly rapid 
development is best. It should be remembered that prints will not be 
required, and that, provided all the detail obtainable is on the plate, very 
great differences of density are permissible. Indeed, when the image of 
the sun is quite hidden in a black blur, as seen by transmitted light, it. 
can always be found on the glass side of the negative as a. white or pale 
disc. Sometimes it is reversed, and stands out clearly by transmitted 
_ light ; but this is exceptional with the exposures which have been used. 
In order to work out the negatives we have certain facts known. 
These are the latitude and longitude of the place of observation, the 
date and time at which the observation was made, and the relative 
positions of the image of the sun, and the selected point of the cloud in the 
two negatives. The first step is to determine the altitude and azimuth of the 
sun, since its image on the plate is the fixed point of reference from which 
the co-ordinates of the point of cloud in the image will be measured. 
From the declination of the sun corrected for variation, and from the 
known latitude, the meridian zenith distance can be calculated. From 
the Greenwich time observed, the longitude and the equation of time, the 
sun’s distance from the meridian is obtained. 
It should be remembered that the meridian zenith distance need only 
be determined once for a number of observations made within a few 
hours of each other, and the correction of time is practically constant for 
a day. Moreover, it is useless to attempt to do more than ascertain the 
altitude to the nearest minute of arc. 
