of Edinburgh, Session 1883-84. 
663 
thus be formed, and will keep rushing up the tube while there is any 
sunshine. In this way a current of air will be kept flowing past the 
thermometer ; and the thermometer being protected from radiation, 
the correct temj)erature of the air will be obtained during calm and 
sunny weather. 
One of the principal difficulties met with in the construction of this 
screen is to prevent heat getting to the thermometer through the 
lower part of the tube. If any heat get conducted through the walls 
of this tube, it will heat the air inside, and the inside surface of 
the tube will also radiate heat to the thermometer, and the indicated 
temperature will be too high. 
In figs. 2, 3, and 4 are shown some of the different forms of this 
screen which have been made. It will be observed that they differ 
principally as to the manner in which the lower part of the tube is 
protected. In the screen represented in section in fig. 2, the 
thermometer t is placed near the bottom of the long tube ah, which 
is made of thin metal. To prevent heat getting to the thermometer, 
the lower part of the tube is surrounded by a second tube c, and 
the space between the tubes is filled with a non-conductor. But as 
all non-conductors are far from perfect, still greater protection is 
necessary, and the lower part of the draught tube is surrounded by 
two conical -shaped metal sunshades d, e. A space is left between 
the two shades for the free circulation of air to keep them cool. 
The screen is supported on three iron legs at the standard height of 
4 feet from the ground. 
In fig. 3 another form of construction is shown. In this instru- 
ment the lower end of the tube, as in fig. 2, is surrounded by a tube 
c, and non-conducting covering, but in place of further protecting the 
lower end with a shade, it is put into a circular box d, made of louvre 
boards similar to a Stevenson screen. The air which enters the draught 
tube of this screen comes from. the lower side of the box. As this 
air has been in contact with the surface of the under side of the box, 
and may thus have got heated, there are placed inside the tube, and 
surrounding the thermometer bulb, two concentric tubes / and g, 
made of non-conducting material, to prevent the heated air from 
touching the thermometer. By this arrangement the air which flows 
over the thermometer has the true temperature, as it has never 
touched the surface of any solid ; and as the only radiation to which 
