620 
PROFESSOR ROSCOE ON A METHOD OF METEOROLOGICAL 
Figs. 10 & 11, Plate XXVIII. exhibit the daily curve of chemical intensity thus deter- 
mined; the close agreement of the two curves for each day shows that the errors of 
graduation, exposure, and reading do not materially affect the accuracy of the measure- 
ments; whilst the values of the Daily Mean chemical intensities obtained from each 
curve, viz. 42 - 0 and 4P7 for fig. 11, July 15, 1864 ; and 74'3 and 70'0 for fig. 10, July 11, 
1864, confirm this conclusion. 
V. Application of the Method to actual Registration. 
A series of determinations of the varying intensity of the chemical action of total 
daylight, made at Manchester on more than forty days, at the most widely differing 
seasons of the year, extending from August 1863 to September 1864, serves to show, 
in the first place, that the daily determination of the varying chemical intensity can 
without difficulty be carried on ; whilst, secondly, they reveal a few of the many 
interesting results to which an extended series of such measurements must lead. The 
whole of the observations, with a few exceptions, were carried on in Manchester, upon 
the roof of the laboratory of Owens College. As a rule, one observation was made 
every half-hour ; frequently, however, when the object was either to control the measure- 
ments, or to record the great changes which suddenly occur when the sun is obscured 
or appears from behind a cloud, the determinations were made at intervals of a few 
minutes or even seconds. Sometimes, when the sky was overclouded, or when no 
great changes in the light occurred, the observations were made once every hour. On 
most of the days employed for observation, the temperature, atmospheric moisture, 
barometric pressure, varying amount of cloud, and the condition of the sun’s disk were 
noted. 
The curves given on Plate XXIX. serve to exhibit these same results graphically, the 
abscissae representing the hours of the day (solar time), and the ordinates giving corre- 
sponding chemical intensity expressed in terms of the unit above described. 
Consecutive observations were carried on each day for nearly a month, from June 16 to 
July 9, 1864 ; the labour thus incurred was found to be comparatively light, so that, when 
all the preliminary arrangements are made, the daily measurements take up but a small 
portion of the attention and time of one observer. From the results of these measurements 
the great difference becomes perceptible which often exists between the chemical inten- 
sity of neighbouring days ; examples of this variation are seen on PlateXXIX. figs. 12 
& 13, for June 27th and 28th, and on figs. 14 & 15, for June 29th and 30th. The tabular 
results show that the amount of chemical action generally corresponds to the degree of 
cloud or sunshine, as noted in the observation. Irregular changes in the chemical action 
are, however, observed on some days (as on March 19, 1864, fig. 16), on which the sun 
shone continuously, and these are to be mainly attributed to the variation in the amount 
of cloud passing at the time of observation. In several cases, when no apparent change 
in the amount of light as affecting the eye could be noticed, a considerable and sudden 
alteration in the chemical intensity occurred. This was clearly seen on September 26, 
