ON THE EFFECTIVE TEMPERATURE OF THE SUN, 
365 
c = the ratio of the apparent areas of the platinum and the sun, 
and d = the ratio of the emissivity of bright platinum compared with that of 
lamp-black, 
then Ej, the radiation from the sun outside our atmosphere, will be 
K, = X c X A X 6 X d. 
The Meldometer. 
The meldometer in its original form was devised by Professor Joly,"^ for the 
purpose of finding the melting-points of minerals, hence its named As used by him, 
it consists of a strip of platinum, on which minute fragments of any mineral can be 
placed, while any alteration in its length can be determined by means of a micrometer 
screw which touches a lever connected wdth one end of the strip. 
The strip can be heated by an electric current, and is calibrated by observing 
the micrometer readings corresponding to the temperatures at which some substances 
of known melting-points melt. 
The first alteration which we made on the original form of instrument was to 
substitute an optical for a mechanical indication of the expansion of the strip, by 
means of which an alteration in length, due to a rise of 1° C. in temperature, could 
be detected. 
For purposes of calibration it is convenient to place the plane of the strip hori¬ 
zontal, so that the fragment of selected material may rest upon it, and this was the 
arrangement in our first instrument. 
But this introduces the necessity of a mirror at 45° to reflect the heat from the 
strip into the radio-micrometer—a serious source of error, as no good series of 
experiments on the reflecting power of speculum metal is to be found, and even if it 
were, tarnishing of the surface is bound to take place, and make the reflection 
irregular. 
We had, therefore, to solve the problem of keeping our thin strip in a vertical 
plane, while at the same time supporting fragments of our selected minerals upon it 
during the calibration experiments. The plan finally adopted was to turn up a very 
narrow ledge along one edge of the strip, at right angles to the remainder, this ledge 
serving with very careful handling, as a support for’the mineral fragments. A cross 
section of the strip was thus L-shaped, but with a very short horizontal arm, thus ; 
L 
* ‘ Proc. R. Irist Acad.,’ vol. 2, 3rd series, 1891, p. 38. 
t We Have thought it better to retain Professor Jolt’s name, although it no longer describes the 
function of the instrument as used in our -work. 
