SUPPLEMENTARY REPORT ON METEOROLOGY. 61 
ference of such instruments was therefore observed by Lam- 
bert*, Lesliet, and others. 
64. It is plain, however, that this indication can only be con- 
sidered to be comparable with itself so long as all external cir- 
cumstances besides solar radiation remain the same. ‘This 
Saussure well showed by one of his admirable experiments f, 
in which, by properly defending the thermometer from wind and 
common radiation, he raised its temperature in the sun to 190° 
Fabr. Sir John Herschel first pointed out that the momentary 
effect of the sun and other combined causes, in affecting the 
temperature of a thermometer, diminished by the momentary 
effect of those other causes acting separately, is the true relative 
measure of the force of the sun’s rays. The first notice I have 
met with substantiating Sir John Herschel’s claim to the ap- 
plication of this principle (which has since been rather un- 
scrupulously adopted abroad with a slight change of form), 
is in a paper by the late Dr. Ritchie, in the Edinburgh Journal 
of Science for 1825 §, where he gives an extract from a letter of 
Sir John Herschel, who, ina few words, sufficiently describes the 
instrument and its principle. Full instructions for its use have 
lately been printed by the Royal Society ||; it is called an ac- 
tinometer. 
65. The actinometer scale is an arbitrary one, obtained by 
direct comparison of one instrument with another; and so satis- 
factory is this kind of observation, that, from direct experiment, 
I am satisfied that the value of the actinometric degrees may be 
obtained within ;1,th of the amount of solar radiation. It is 
very singular, that Sir John Leslie, with his marked sagacity, 
should not have perceived that his mode of graduating photo- 
meters, by first converting them into hygrometers ], is radically 
erroneous; and accordingly I have found that, when his instru- 
ments are compared, after being constructed with the utmost 
care, they do not even approach to agreement except at 0°, 
unless they are of the same dimension, and in every respect 
similar ; but absolute identity in size, material, and arrange- 
ment, it is beyond the power of art to obtain. Careful experi- 
ments, which I have likewise made with this elegant instrument 
under different skies and in different climates, compel me to 
conclude, that though in certain very uniform circumstances 
its relative indications may be really of value, yet in a wider 
* Pyrométrie, p. 158. 4to. Berlin, 1779. 
+ Essay on Heat. Lond. 1804. + Voyages dans les Alpes, § 982. 
§ Vol. iii. p. 107. 
| Report of Committee of Physics, &c. 1840, p. 61. 
q Essay on Heat, p. 421. 
