RECENT PROGRESS IN PHYSICAL SCIENCE. 109 
In all prior forms of apparatus accurate results require 
the careful determination of the rates of healing and cooling 
of the actinometer. These rates differ greatly, depending 
upon the difference between the temperature of the sensitive 
portion of the actinometer and the surroundings, and at the 
best are difficult to observe with necessary accuracy. 
The compensating pyrheliometer, on the contrary, is ob¬ 
served by what may be called a static method as distinguished 
from the dynamic method of observing rates of heating and 
cooling. This affords far greater facility of observation and 
serves to obviate numerous corrections incident to former 
methods. 
The instrument is shown in accompanying figures, taken 
from Angstrom’s paper giving an account of his observations 
at different altitudes on the peak of Teneriffe.* 
A general view of the instrument is shown in Fig. 2 ; de¬ 
tails in Figs. 3, 4, and 5. 
The metal strips are cut with great care by means of a 
dividing engine from platinum foil about .002 mm. thick, 
the foil being previously coated on one face with a layer of 
the thinnest silk paper, attached by shellac. Two strips are 
selected of equal size and resistance and fastened side by side 
to an ebonite frame, as shown in Fig. 5. 
The thermo-elements, consisting of a U-shaped thin, narrow 
strip of constantan or nickel, to the extremities of which 
copper strips of the same size are soldered, is fastened to the 
paper side of the platinum strips, so that the thermo-junc¬ 
tions J J lie about at the middle points of the strips. 
The backs—that is, the paper sides of the strips with the 
thermo-elements—are coated with black varnish. The fin¬ 
ishing of the strips is described as follows by Angstrom : 
“ In order to blacken the strips on the front sides they were firstcoated 
galvanically with a thin layer of zinc and then treated with a one per 
cent, solution of platinum chloride until the resistance, which had been 
slightly lessened through coating with zinc, reaches its former value. In 
* Intensity de la Radiation solaire d different Altitudes. Presente d la 
Societe Royale des Sciences d’Upsala, le 7 Avril, 1900. 
16-Bull. Phil. Soc., Wash., Vol. 14. 
