MR JOHN AITKEN ON DEW. 35 



a large surface is colder at night than a small one, as the small surface receives 

 more heat, per unit of area, from the air than the large one. The absorbing 

 and radiating surface of these instruments is a large flat area, painted black, 

 and its temperature is taken by means of a thermometer, with its bulb placed 

 under the centre of the radiating surface."' 



The construction of these radiation instruments has been altered, and those 

 used in this investigation were made of metal in place of wood, as described in 

 the previous paper, the radiating surface being a thin plate of metal, 14 inches 

 (355 mm.) square. A thin metal tube is fixed close to and parallel with the 

 under surface of the plate. One end of the tube terminates at the centre 

 of the plate, and the other at the edge. The thermometer is placed in 

 this tube with its bulb under the centre of the plate, and to prevent heat 

 escaping or being absorbed at the back, a considerable thickness of cotton wool 

 is placed under it. The instrument is practically a shallow box, 14 inches 

 square by 2 inches (51 mm.) deep, packed with cotton wool. One of the 

 flat areas of the box is exposed to radiation, and its temperature is taken by 

 means of a thermometer placed under its surface. In the following I shall 

 refer to this instrument simply as the thermometer box. 



One of the advantages of this form of instrument for solar radiation 

 experiments is, that the readings given by different instruments agree with 

 each other, at least this is the case so far as my experience goes; and it is well 

 known that the vacuum radiation thermometers are unsatisfactory in this 

 respect, no two almost ever reading alike. For instance, the vacuum 

 radiation thermometers used at the Indian Stations, when compared with 

 another of the same pattern as standard, were in some cases found to 

 differ as much as 15°, though they were exact copies of each other, and 

 similarly exposed.t I find that when the different instruments of the kind 

 used by me are compared they agree very well when of the same size. It is of 

 course necessary that they be of the same size — this results from the principle 

 of their construction. It seems possible that we might make boxes of different 

 sizes, and from them determine the law of variation for size; so that, knowing 

 the size of the surface used in any particular set of observations, we could 

 determine what temperature its readings corresponded to in another instrument 

 of a different size, or all readings might be reduced to a standard size, say the 

 temperature of a very large surface. 



I may mention that the temperature given by an instrument of the size 

 here described when placed in sunshine is a good deal above that indicated 

 by a vacuum thermometer, which had been carefully prepared for me by 

 Casella of London. Generally the readings were about 12 per cent, higher. 



* Thermometer Screens, Proceedings of the Royal Society, Edinburgh, No. 117, 1883-84. 

 t Report of the Meteorology of India, 1879, hy H. P. Blanford, E.R.S. 



