EXTENT OF SKY CLOUDED. ev 
EXTENT OF SKY CLOUDED. 
224. The Mean Extent of Sky Clouded, from 8 years’ observations, =6:98, totally clouded, being = 10-0. 
TABLE 98.—Monthly Means of the Estimated Extent of Clouded Sky, the whole Sky covered being 10, 
for the Years 1842-50. 
225. Annual Variation of the Extent of Clouded Sky.—In the mean of 8 years the sky was most clouded 
in July and least clouded in December ; the change from month to month is by no means regular ; on the whole, 
however, the extent of sky clouded is greatest for the 5 months April to August, and it is least in the 4 
months September to December. The means for these groups of months are as follow :— 
Jan.—Mar.=6-94. April—August=7-29. Sept.—Dec.=6°63. 
The means for the meteorological quarters are,— 
Winter, Dec.—Feb.=6-68. Spring, Mar—May=7'14. Summer, June—Aug.=7:31. Autumn, Sept—Nov.=6-79 
The least extent of sky clouded for any month in the 8 years occurred September 1843=5-26. 
The greatest extent of sky clouded for any month in the 8 years occurred July 1846=8-71. 
_ The mean for the month of December in each year was less than the mean for the year ; and the mean for the 
_ month of July in each year was greater than the mean for the year. 
“ 226. Variation of the extent of Clouded Sky, with the Moon?s Age.—lIt is well known that no heat has been 
detected in the moon-light even with the aid of the largest parabolic reflectors ; it was conceived possible, how- 
_ ever, that though no thermal indication could be obtained at the surface of the earth, yet there might be some 
_ found in the dissipation or formation of clouds in the upper regions of the atmosphere; the observations of 
the extent of clouded sky for 1843 were accordingly discussed for this purpose in the volume for that year. 
‘page 303: the result was very indefinite; it was remarked, however, that as no observations were made in 
_ that year between 9" p.m. and 5" a.m., the period when the moon’s heating effect must be greatest, little else 
could have been expected. In the volume for 1844, p. 443, the discussion was repeated ; from it the extent of 
clouded sky appeared on the whole greater about full moon than about new moon ;—thus, the daily mean for 
the 15 days about full moon = 7-05, whereas the daily mean for the 15 days about new moon = 7:14; and 
the daily mean for the 7 days about full moon = 6-94, and about new moon = 7:24. The difference of these 
numbers is still very small, and it was remarked (1844, p. 443), on account of the irregularities introduced 
_ by the sun’s cloud-forming power, that it might be desirable to limit the investigation to the hours of the night ; 
this has been done for the years 1844 and 1845 in the present volume, Table XXXIX., page 66, where the 
" extent of clouded sky is given for each day of the moon’s age and position in declination in each year, as de- 
duced from the 6-hourly observations between 9" p.m. and 25 a.m. It will be seen from No. 229 that the 
_ variation of the extent of clouded sky in the mean of the year is small for these hours, which include the epoch 
