464 ' Analyses of Boohs. [Dec. 



bibulous paper for the purpose of absorbing the light which 

 permeates tne glass, and instantly converting it into heat. 



" The two cylinders are connected by small pieces of thermo- 

 meter-tubes which keep them steady with their faces parallel to 

 each other, but turned in opposite directions, and also serve to 

 make the insulation as complete as possible. The chambers are 

 then connected by a small bent tube in 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, having a vertical opening through 

 the stem to allow the glass tube to pass along it, and thus secure 

 it from accidents. 



" 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 for- 

 merly 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 deli- 

 cacy 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 sutficient diameter, whilst it will not be sensi- 

 bly acted upon at the same distance by a mass of heated iron 

 aribrding 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. 



'^ Place the instrument between any number of steady lights 

 whose intensities are known, as for example, between four wax 

 candles opposite one end, and one candle opposite the other, 

 and move the photometer till the fluid remain stationary at the 

 division where it formerly stood, and it will be found that the 

 distances are directly as the square roots of the number of 

 candles ; or in other words, that the intensities of the lights will 

 be inversely as the squares of the distances. If gas lights be 

 employed, having burners capable of consuming known quanti- 

 ties of gas in equal times, and the photometer be placed between 

 them, so that the eflfect upon the air in each chamber shall be 

 the same ; it will be found that the quantities of gas consumed 

 by each will be exactly proportional to the squares of the 

 distances of their respective flames from the ends of the photo- 

 meter." 



VII. The Description of a Floating Collimator, By Capt. 

 Henry Kater, FRS. (See Anmlsiov Feb. 1825, p. 143.) 



VIII. Notice on the Jgua?iodon, a newii/ discovered Fossil 

 Reptile, from the Sandstone of Tilgaie Forest, in Sussex. By 

 Gideon Mantell, FL. & GS. : in a Letter to Davies Gilbert, 

 Esq. MP. VPRS. (See Annals for March, 1825, p. 228.) 



