644 Tr ansae tion s . — Miscellaneous . 



Abt, LXIII. — Skeleton revealed hy Deio. 



By Major-General Schaw, C.B., R.E. 



\_Rcad before the Wellington Philosophical Society, 27th July, 1893.] 



The cold nights and fine days which we have experienced 

 lately, with clear skies, and a consequent very copious deposi- 

 tion of dew, have produced a phenomenon which, probably, 

 many may have remarked under varying conditions, and 

 which at first was somewhat puzzling to me. A blank wall 

 of a wooden house is visible from my window. The wall 

 faces west, and is therefore in shade in the morning, and I 

 have observed in it every morning, marked out in clear 

 lines of moisture, the skeleton of the framing of the house 

 — the vertical studs, diagonal braces, and floor -line. The 

 effect was evidently due to inequalities of temperature 

 caused by the contact of the various parts of the frame- 

 work with the outer planks : where these planks were in 

 contact with a part of the framing they were practically 

 thicker, and therefore the surface lost, or acquired, heat more 

 slowly than the unsupported and thinner parts. As the outer 

 surface cooled down at night by radiation, the heat lost would 

 be supplied again from the mass of the framework where the 

 plank was in contact with it ; and so these parts would 

 remain warmer than the intervening portions, and on them the 

 dew would therefore not be deposited so soon or so freely as 

 on the intermediate parts. This seemed to me the natural 

 course of events ; and yet the effect observed in the morning 

 was just the contrary. The portions attached to the frame- 

 work were w^et, and the intermediate parts dry. Eeflectiou, 

 however, showed that this was quite in accordance with the 

 first impression that inequalities of temperature caused the 

 inequalities of moisture. There can be no doubt that the 

 deposition of dew takes place first on the thin unsupported 

 portions of tha wall, but soon the w^hole surface cools down 

 by radiation below the temperature of the surrounding air, and 

 dew is deposited all over the surface during the night. In the 

 morning, as the air is warmed by the sun, it expands, and 

 its capacity for holding vapour of water increases ; it com- 

 municates its increased heat to the surface of the wall, and 

 the thin unsupported parts are the soonest to take up the 

 increased temperature, and the dew on those parts is dried^off 

 first. The thicker supported parts require a longer time to 

 assimilate their temperature to that of the warmer air, and so 

 continue for some time to retain their deposit of dew, which 

 remains until the whole surface of the wall has taken up the 

 day-temperature of the air. 



