277 
1895 - 96 .] Dr J. Halm on the Temperature of the Air. 
If, on the other hand, we take the mean of the six months for 
each of the three stations separately, we arrive at the values : 
Latitude. a m . 
59°*4 6°'45 Centigrade. 
51°-3 6°*21 
42°-9 6°*19 
There is apparently a distinct increase of the solar constant 
towards the north. But it is a well-known fact, from direct actino- 
metric observations, that indeed the solar radiation becomes more 
intense in northern latitudes, most probably a result of the lower 
mean state of atmospheric moisture. Thus we may undertake to 
assert that our result shows that a determination of the solar con- 
stant from observations of the daily change of temperature may be 
relied on with confidence. We may even go so far as to say 
that these observations may be used for testing the important dis- 
coveries previously made as to the periodic character of the solar 
radiation. 
After the elaborate investigations of this question by the late 
Astronomer-Royal for Scotland, Dr C. Piazzi Smyth , founded on 
the observations of the rock thermometers at Calton Hill, and 
similar researches of Prof. Stone at the Cape, and others, there can 
be no doubt that a cyclical wave of the sun’s heat must really exist, 
the period of which is synchronous with the eleven-year period of 
sun spots. 
Unfortunately, we have not yet at our disposal a sufficient 
number of stations with hourly observations. But we can easily 
show that all the places where only two or three observations a 
day were made — of which, indeed, an enormous number are avail- 
able — can also be used for determining the solar constant. 
We have seen how regular the annual curve of the quantity da 
has proved, and, at the same time, how constant is the angle u ; 
now, by introducing the above given values of both these para- 
meters and the value of the angle v determined by the equation 
1 - 7i 2 
tang v — — , we reduce the number of unknowns from 5 to 2, 
O/h 
that is to say, the constant temperature t 0 and the solar constant a n . 
After some particular considerations about the curve of temperature 
