Description of a differential Thermometer. 329 
ee ee Se 
Heat in the rays of the Moon. 
Extract of a leties eo Dr. Howard, dated August me 1820, to 
the Editor heal American Journal of Science, &e. 
eating 
thermometer, have been unsuccessful. Indeed this instru- 
ths however skilfully constructed, is not sufficiently deli- 
to be affected by the heat of the rays of the moon, 
mu as 
(Optics, Vol. L.) that the light of ihe full moon is to than. of 
our day only as 1 to . 
Having blackened the upper ball of my Silferpatial ticks, 
mometer, I placed it in the focus of a thirteen inch reflect- 
mg mirror, which was opposed to the light of a bright full 
e li He nee ee to sink, and: jn half 
| to be present, chat hard, was no falacy in the coche , 
of i Its being a positive proof of the calorific power of the lu- 
ar rays, and at the same time sirens an evidence of the 
great ces sf the instrument.’ 
stmosphe: ric air ; ‘itis also convenient to Mure the ether ofa red saioak: by 
2 dition of a drop of tincture of co 
Thave constructed upon the same eancioke ea photometer, and an ethrio 
scope, both of which, though liable to some obje ections, are most tecinaaiy 
ments upon the light of ag lpa ee For this purpose. the instrument 
is constructed as shewn in the engraving, by Dr. Howard ; the upper 
; covered by a thin eauaing of Tuan sik, and the other. “ gold leat, 
specs by adilute spirit-varnish ; it is then covered by a thin glass shade 
from the flame of a wax candle, it fe as 
pee ie I had previonsly ascertained, by a comparison of shadows, te 
give the light of ei x candles, caused a depression ad 
t ‘ 
‘the | 
Placed at the same ¢ distance frem the instramen 
