Description of a differential Thermometer. 329 
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Heat in the rays of the Moon. 
Extract of a letter from Dr. Howard, dated August 29, 1820, to 
the Editor of the American Journal of Science, sc. 
‘¢ All attempts, as far as I am aware, to discover any 
heating power in the lunar rays, by means of a common 
thermometer, have been unsuccessful. Indeed this instru- 
ment, however skilfully constructed, is not sufficiently deli- 
cate to be affected by the heat of the rays of the moon, 
which, if it bears any proportion to the light of the same 
rays, must be extremely small; as Dr. Smith concludes 
(Optics, Vol. I.) that the light of the full moon is to that of 
our day only as 1 to 90.000. 
Having blackened the upper ball of my differential ther- 
mometer, I placed it in the focus of a thirteen inch reflect- 
ing mirror, which was opposed to the light of a bright full 
moon. ‘The liquid began immediately to sink, and in half 
a minute was depressed 8°, where it became stationary. 
On placing a skreen between the mirror and the moon, it 
rose again to the first level, and was again depressed on re- 
moving this obstacle. I repeated this experiment several 
times to satisfy myself and some of my friends who happen- 
ed to be present, that there was no fallacy in the conclusion 
of its being a positive proof of the calorific power of the lu- 
nar rays, and at the same time affording an evidence of the 
great delicacy of the instrument.” 
atmospheric air ; it is also convenient to tinge the ether ofa red colour, by 
the addition of a drop of tincture of cochineal. 
I have constructed upon the same principle a photometer, and an ethrio- 
scope, both of which, though liable to some objections, are most curiously 
sensible to the impression of light, and to the frigorific emanations of the 
heavens. I have also employed a modification of the same instrument as 
photometric thermometer, which I have found useful in comparative experi- 
ments upon the light of different flames. For this purpose, the instrument 
is constructed as shewn in the engraving, by Dr. Howard ; the upper ball is 
then covered by a thin coating of Indian ink, and the other with gold leat, 
applied by a dilute spirit-varnish ; it is then covered by athin glass shade. 
Upon bringing a candle near the black, or sentient ball, that is within the, 
distance of fourteen inches, or one foot, it produces an instantaneous de- 
pression of the column of liquid. Placing this instrument at the distance 
of sixteen inches from the flame of a wax candle, it fell 1° in 1’. A gas 
flame which { had previously ascertained, by a comparison of shadows, te 
give the light of eight wax candles, caused a depression of 10° in 1’, when 
placed at the same distance from the instrument. We oy 
