112 Mr. Parker on the Skull of the Ostrich Tribe. [Mar. 9, 



one in the sun, and the other in the shade, disturbing influences being as 

 much as possible avoided in both cases, and the observations being confined 

 to those days on which the sun shone sufficiently clearly to cast a distinct 

 shadow during some part of the interval between noon and 4 P.M. Com- 

 paring the differences of insolation in different parts of India and in different 

 seasons, he is led to regard insolation as dependent greatly, on relative 

 humidity. Thus, generally speaking, it is greater on the seaboard than in 

 the interior of India. At individual stations, the maxima of insolation 

 occur on days of great relative moisture, i. e. on days in the rainy season 

 when the clouds are temporarily broken, or in the months immediately 

 following the rainy season, when the atmosphere is still very humid. Cal- 

 cutta and Columbo are taken as types of a sea-climate, Konagheri and 

 Bellori as types of an interior or very dry climate. In the one type the 

 relative humidity is from 88 to 93, the insolation 50 ; whilst in the other 

 type the relative humidity is from 60 to 65, and the insolation from 8 

 to 1 1. Still more striking results are obtained by comparing the moun- 

 tain climates of Sikkim and Ladak, nearly at similar absolute altitudes. At 

 Ladak the relative humidity is about 30, and the insolation about 1 8 ; 

 whilst in Sikkim the relative humidity is estimated at from 81 to 84, 

 and the insolation from 60 to 75. The contrasts in these comparisons 

 are very great, and, with other examples, which are cited, appear to sub- 

 stantiate a connexion between the presence of aqueous vapour in its 

 transparent state, and insolation as measured by the differences of ther- 

 mometers in the sun and shade. The connexion is shown to be in perfect 

 harmony with the results obtained by Professor Tyndall, and is explained 

 by considering simultaneously the gain of heat which the thermometer 

 experiences by direct radiation from the sun, and its loss of heat by ra- 

 diation to the surrounding air. The opacity of the air for the invisible 

 heat radiating from the thermometer rapidly increases with the amount of 

 vapour of water which the air contains, while its transparency for the heat 

 directly radiated from the sun is comparatively little affected. Thus when 

 the air is highly charged with moisture, free radiation from the thermo- 

 meter is much impeded; or rather, what the thermometer loses by radiation 

 into the air is in some measure restored by radiation back again from 

 the air. 



II. " On the Structure and Development of the Skull of the Ostrich 

 Tribe." By WILLIAM KITCHEN PAKKER, Esq. Communi- 

 cated by Prof. T. H. HUXLEY. Received February 23, 1865. 



(Abstract.) 



The earliest condition of the struthious skull described by the author is 

 that of a " pullus " of the African species, at about the end of the first 

 third of the period of incubation. There are two individuals in this stage 



