62 
Remarks on the recent Conference at Vienna on 
The Conference recognises the hours of observation in use in 
this country, 9 A.M. and 9 P.M., as sufficient, provided that 
observations of the maximum and minimum temperatures are 
made. These latter should always be taken at 9 P.M. Theo- 
retically they should be observed at midnight, but the incon- 
venience of such a practice is fatal to its adoption, and so the 
nearest reasonable hour has been selected for the observation. 
The old practice of reading both thermometers in the morning 
and putting down the maximum temperature to the preceding 
and the minimum to the current day must be abandoned, as it 
is illogical and may lead to error. 
E. 7. "Insolation (or the son's heat). As no thoroughly satisfactory 
actinometer is known, observations should in the first instance be taken 
only at large observatories, on the time, duration, and relative intensity of 
sunshine (the last-named by means of sensitised paper). 
" It is recommended to scientific institutions as an important inquiry to 
devise an actinometer which shall be applicable to the sums of temperature 
derived from solar heat." 
The correct measurement of the effect of sunshine is an ex- 
tremely difficult problem to solve, for no instrument has yet 
been invented, or at least been generally adopted in any country, 
which gives the total useful effect of the sun's heat during a 
day. The ordinary observation taken, with the black-bulb ther- 
mometer in vacuo, is merely of the highest temperature reached 
in the sun's rays during the day ; but this gives no idea of the 
length of time for which the sun has shone on that day, so that 
it in no way enables us to estimate the total useful effect of the 
sunshine. On the other hand, the sunshine recorders recently 
introduced in this country, and of which the results are printed 
in the Weekly Weather Report, give simply the epoch and 
duration of sunshine possessing sufficient power to scorch a blue 
card. The slightest film of cloud, barely perceptible to the eye, 
is sufficient to check the scorching entirely, but, once scorch- 
ing is effected, there is no appreciable difference between the 
comparatively weak sunshine of a winter's day and the intense 
heat of an August afternoon. This instrument, therefore, fails 
to give us a quantitative measure of the heat received from the 
sun, and as it has only been in use for a little more than a year, 
no conclusions of much value have been drawn from its indica- 
tions. 
In connection with the subject of the registration of sunshine 
it may be interesting to quote the most recent determinations of 
the difference between the intensity of the sun's heat at different 
levels in the Alps : — 
