98 
wind blowing past C and the graduated stem D, which are shaded 
from the sunshine, or which may be kept immersed in melting ice, 
although this is not done at Montsouris. In order that the record of 
liquid condensed in C and D may be proportional to the heat received 
by A it is necessary that the volume of condensed liquid be inde- 
pendent of the temperature of the air and of the volume of liquid 
remaining in the bulb A, and be controlled only by the excess of 
radiant heat received by A over that received by C and D. 
The comparisons that have been made at Montsouris between this 
Bellani radiometer and the Marié-Davy actinometer, or the so-called 
conjugate thermometers, show that the Bellani apparatus does not 
perfectly satisfy the theoretical conditions, but as it is extremely 
sensitive, since it distills 16 or 17 centimeters daily, and as the appa- 
ratus is not.costly, Descroix has devised a formula for reducing its 
results to a standard. The following table gives the results of actual 
observations, showing the average results for the middle portion of 
perfectly clear days at Montsouris, near Paris, monthly during the 
growing seasons of 1885 and 1886. The column N shows the number 
of clear days; V, the volume of alcohol distilled over from 9 a. m. 
to 3 p. m. on these days in the Bellani apparatus; t, the average 
temperature of the air in the shade; T,, the average temperature 
of the black bulb thermometer in vacuum in the sunshine; T,, aver- 
age temperature of the bright-bulb thermometer in vacuum in the 
sunshine; T, — T, is the average difference of these conjugate ther- 
mometers at midday; R, total illumination from the sky at midday, 
expressed in Marié-Davy’s actinometric percentages or degrees. 
Comparison of actinometric results for perfectly clear days at Montsouris, near 
Paris, by the Bellani radiometer and by the Arago-Davy actinometer. 
| Belani. | Arago-Dayy at noon. 
Month. NES lS ak 
Vv. | 2 ta ee lena eRe 
1885. com. | oO | XO aGu lias 
Mar vee eee seen setae Stee ace cee ace 6 956°] * 1559)" 2559) 5 a5s0k) 1059 76.9 
TAC punted ef ed Rac: CNS Seb Fide De aR 2A 6| 104] 18.4] 88.5) 27.5] 11.0 70.2 
May lee 6 dogh ie ct ea he em erie eee 2| :10.2| 22.6] 442| 32.9] 11.3] 67.1 
STITT ee a es de rea ee dip esis Tee ae 10-| 10:%)|) 22/8) | 45.6) | 033.0 enn On eerie 
UUM ae ese eek be oe ee ene a ae 15} 116) 2407 | 4652") 84540) ies 73.9 
OAR UB tS eee na ue Ls es a Oe Bae bey CEM Bi 101%) | 1: 20200) 4y/39.9/0 ere hea Ony 73.2 
Septem per acates ai we Agee oe te 330 110: 1. 28.451, 43:01) Senlal edOng 73.2 
Octobe eee ate eae oe eee eae eee 2 | 7.2 12.1 29.5 4,1 | 8.4 | 72.0 
1886 | | 
Mae Tay ce ene a See go LI URaeal a gee Sa taues1 a9 WG ul 22) be enacted <O)) elias ae 
PArorilictint, ciel ty yi ace se has see eweiels 1), 12:3). 17.65) (36.7 ||, 28.8)| ese 74.1 
i VES al Ma a MMe cae an ON Coe Se 9| 12.9] 15.6] 98.7] 25.9] 124] 76.6 
A Rae aT MA cela ied 2s Man) aad NII ee CES a 2 12.4107 19.5-\) 4977.) 780-21 1022) eae 
Ar Saee p eaI A b t a | 40} 125} 9891 44) | $2.9) SI1's 72.0 
U NGr ee, Sh a OL AE od an Ueaa NEN ENCES jar aid!) ee oes 458s) St 3i) 10s5 68.6 
September it 12s eis pe eee Eee era ee as 9 7 ile 1.00 88887") -2B. Gils 1zO"S| lease 
Octovor eke ei ee eek tines aes | 4 | 10.1 | 19.3} 35.9| 26.6 | 93 | 71.2 
Notwithstanding the simplicity of Bellani’s apparatus and the 
ingenuity of the idea embodied therein it is evident that it needs an 
important modification, viz, the evaporation and condensation should 
