May 3, 1912] 
mometer stem from one to another fixed point. 
Provision is made whereby, after one time interval 
has been obtained, the mercury can be set back 
below the first mark, and then another interval 
read, and so on as long as necessary. 
The intensity of the insolation is inversely pro- 
portional, approximately, to the above time inter- 
vals—the times required for delivering substan- 
tially equal amounts of heat to the mercury. 
While capable of development as a standard it 
was designed as a secondary instrument, the chief 
features of which are: 
1. Essentially complete absorption of insoiation. 
2. Unchanging coefficient of absorption. 
3. Highest possible heat insulation. 
4, Freedom from calibration. 
5. Ease of manipulation. 
The Violle Actinometer as an Instrument of Pre- 
cision: EF. W. VERY. 
The principal objects of this research are to 
show that the Violle actinometer may be used 
either dynamically or statically with equal pre- 
cision, and to develop the theory of its static use. 
Hitherto the rates of cooling of a thermometer 
in a partial vacuum have been used to get an 
estimate of losses by convection in an actinometer 
in air, but these measures have not differed essen- 
tially from the experiments of Dulong and Petit, 
and are quite inadequate, since they entirely neg- 
lect the losses by penetration of gaseous molecules. 
It is very commonly assumed that the velocity of 
cooling in ‘‘vacuum,’’ obtained by Dulong and 
Petit, 
V = k( (1.0077) #8 — (1.0077) ¢), 
tepresents a law of radiation; but this is not the 
case, since the observations included both radiation 
and penetration, and the latter is by no means 
insignificant. 
By the use of Stefan’s law for pure radiation, 
I first separated the radiant component, and from 
experiments by Langley and myself, and by Kundt 
and Warburg, I derived a preliminary value of the 
penetration in C.G.S. units, 
P= 0.0001397 0. 
This, however, did not represent the observations 
so closely as could be wished, and it was evident 
that dimensions and form of both radiating body 
and enclosure must enter into a complete theory. 
Taking Winkelmann’s value of K (the constant 
of penetration for a plane surface in C.G.S. units), 
calling r the radius of the thermometer-bulb, 
SCIENCE 
707 
1 the distance to the enclosing surface, and 0 the 
temperature of excess, and reducing to minutes, 
4. 2 
P=4Ko x me x 60. 
With this formula the penetration was computed 
for two thermometers used by Langley on Mount 
Whitney in a Violle actinometer with the following 
result: 
6= 830° C., Green 4,572, cooling per minute by 
penetration, — 2°.248; observed, — 2°.08. 
6= 30° C., Baudin 8,737, cooling per minute by 
penetration, — 2°.588; observed, — 2°.84, 
The convection loss can be represented by the 
formula 
C=c X O83 (p/P) °*, 
where 
0.000,036,76 
a 
ce = 0.000,005,02 + (C.G.8.), 
a value determined by myself. p and p, are actual 
and normal barometric pressures, and the ex- 
ponents are those of Dulong and Petit. For the 
given excess and Baudin thermometer, 
Loss of temperature per minute by convection 
= — 4°.158 
Loss of temperature per minute by penetration 
= — 2°.588 
Loss of temperature per minute by radiation 
= — 2°.890 
Computed total loss =— 9°.636 
Found =— 9°.2 
The final difference of about 0°.4 includes errors 
of observation and also stem conduction which in 
this case was towards the bulb, or positive, the 
thermometer having been previously heated as a 
whole. 
With some minor emendations relating to the 
general theory of the instrument, which can not 
be described here, I obtained from Keeler’s ob- 
servation with the Violle actinometer on the sum- 
mit of Mt. Whitney: 
Solar radiation at noon from initial rate of heating 
== 1.995 cal. per sq. cm. per min. 
Solar radiation at noon from static temperature 
= 2.001 cal. per sq, cm. per min, 
The difference in the results by the two methods 
is insignificant. 
Improvements are suggested in the mounting 
and use of the instrument, and certain necessary 
precautions which have sometimes been neglected 
are described. 
