143 
Mr. Ritchie on a new photometer, &c. 
the form of the letter U, having small bulbs near its upper 
extremities, and containing a little sulphuric acid, tinged with 
carmine. The instrument is supported upon a pedestal, hav- 
ing a vertical opening through the stem to allow the glass- 
tube to pass along it, and thus secure it from accidents. 
A small scale divided into any number of equal parts, 
is attached to each branch of the tube. In the annexed 
figure, Plate XII. ABCD and'EFGH are the cylinders, 
AB and FG the plates of glass. CD, EFG the ends shut 
up by the circular tin plates, the blackened paper is repre- 
sented by the lines between AB, CD and EH, FG. The 
other parts will be obvious from the mere inspection of the 
figure. 
3. The accuracy of the instrument evidently depends upon 
the perfect equality of its two opposite ends. To ascertain, if it 
be accurately constructed, place it between two steady flames, 
and move it nearer the one or the other till the liquid in the 
tube remains stationary, at the division of the scale at which 
it formerly stood. Turn it half round without altering its 
distances from the flames, and if the liquid remains stationary 
at the same division, the instrument is correct. To show the 
extreme delicacy of the instrument, place it opposite a single 
candle, and it will be sensibly affected at the distance of 10, 
20, or 30 feet, provided it be of sufficient diameter, whilst it 
will not be sensibly acted upon at the same distance by a 
mass of heated iron affording twenty times the quantity of ^ 
heat. In order to cut off effectually the influence of mere 
radiant heat, I sometimes use screens composed of two plates 
of glass, placed parallel to each other, with a quantity of 
water interposed. 
