36 MR JOHN AITKEN ON DEW. 



One objection to these large-surface radiation thermometers is that they are 

 more affected by wind than the vacuum ones. If it is a question of solar 

 energy we are considering, this certainly is an objection, but if it is one of 

 climate it will scarcely be so. I need not say that for questions of terrestrial 

 radiation at night the vacuum thermometer is of no use. 



In using these thermometer boxes for determining the radiating powers of 

 different surfaces at night the following method was employed : — Two precisely 

 similar boxes were prepared, and their upper surfaces painted black. They 

 were placed in an elevated position in the open air, commanding a clear 

 view of the sky all round. They were first exposed without anything on their 

 surfaces, to see if their readings were exactly alike. In constructing them care 

 was taken to put the same amount of cotton wool in each, in order that their 

 non-conducting powers and heat capacities might be the same, so that both 

 might take the same amount of heat to warm them, and both lose the same 

 amount of heat at the back. On trial both instruments were found to read 

 alike wheu similarly exposed. 



As the sky radiation is a rather variable quantity, it would not do, on most 

 nights, to leave one of these test surfaces bare, and use it as a standard with 

 which to compare the other, over which we have put the substance to be 

 tested, because the uncovered surface will follow the changes in the radiation 

 more easily than the other, and will change more, and sooner, than the 

 one covered with the substance to be tested, particularly if the substance is 

 a bad conductor. The method generally adopted was to place both surfaces as 

 nearly as possible under the same conditions. For instance, the first substances 

 tested were black and white cloths of different materials; of each kind a black 

 and a white was selected, each pair being as much alike as possible, of the 

 same material, of the same weight, and of the same texture. A black one 

 was placed over one thermometer box, and the corresponding white one 

 over the other. After a time the readings were taken, and the position of 

 the cloths reversed, the black being placed over the box where the white 

 was, and vice versa, and readings again taken. Then if radiation remained 

 constant one of the cloths was removed, and the other compared with the 

 black surface. 



The following table shows the results of some experiments made on the 

 radiating power of black and white cloths tried in this way. The readings 

 were taken on the evening of the 14th November. The sky on the occasion 

 was quite cloudless. The air was very dry, and had scarcely any movement — 

 an unusually favourable condition for conducting experiments of this kind. 

 The radiating surfaces were placed at a height of about one metre from the 

 ground, and a protected thermometer for taking the temperature of the air was 

 placed alongside at the same height. 



