138 



Photometer, Fire Damp. An apparatus for the detection of the 

 presence of photocarbide of hydrogen in the air. It acts 

 by using an electric current to cause the combustion of 

 fire-damp and a photometer for observation of the luminous 

 intensity. 



Photometer, Flicker, Simmance-Abady. A photometer depending 

 upon the principle that, if two sources of light be viewed in 

 rapid alternations, a flickering effect is produced if the 

 intensities are unequal, but that when equality is obtained 

 all that is seen is an apparently motionless disc. By its 

 means light of any colour can be compared with any known 

 standard of light. 



Photometer, Prism. A photometer in which the light rays from 

 - two sources to be compared are brought together by means 

 of two total reflection prisms. 



Photometer, Richie's. A reflecting photometer in which the 

 two reflected images of the two sources of light are viewed 

 in the field of a sliding telescope. 



Photometer, Rumford's. An early simple form of photometer, 

 depending upon the comparison of the two shadows cast by 

 a single rod intercepting the light from the two sources to 

 be compared. 



Photometer Thermometer. See Thermometer. [13, 15.} 



Photometer, Wheatstone's. Based on duration of light. It 

 consists of a brilliant ball or globe fixed on a disc placed 

 eccentrically on a toothed wheel which is geared with the 

 interior circumference of a toothed crown. The apparatus 

 can be used in open daylight. 



Photophone. An instrument for communicating sounds by the 

 agency of a beam of light. 



Photoscope. An instrument for exhibiting photographs. [1.] 



Photo-survey Camera. A form of camera (q.v.) which, by giving 

 a record of compass bearing and horizontal level on the 

 plate when exposed, enables surveys to be made from a 

 series of photographs only. 



Photo-theodolite. A special form of theodolite (q.v.) incor- 

 porating a camera, in which the photographs made are used 

 to determine angles not taken by direct observation through 

 the telescope. 



Phrenograph. An instrument for registering the movements of 

 the diaphragm or midriff in respiration. [1.] 



Phthongometer. An instrument for measuring vocal sounds. 



Piche's Evaporimeter. See Evaporimeter. 



Pickering's Evaporimeter. See Evaporimeter. 



Picket Pole. A sighting pole used in surveying for marking off 

 points in the field being surveyed. [5, 13, 15, 17, 21, 24, 25, 

 27.] 



Pickets. See Picket Pole. [13, 15, 21, 27.] 



Picknohydrometer. A combination of picknometer and hydro- 

 meter (q.v.). [21.] 



