102 



Harmonometer. An instrument for measuring the harmonic 

 relations of sound. 



Harvey's Apparatus. For the extraction of gases dissolved in 

 water. See also Absorption Apparatus. 



Hastings' Triple Aplanat Magnifier. An improved and perfect 

 form of triple aplanatic magnifier, based on formulae by 

 Prof. C. S. Hastings. See also Magnifier. [13, 22.] 



Heath's Valvograph. See Valvograph. [13, 21.] 



Hedley Dial. A form of mine-surveying dial. See Dial. [17, 21, 

 24, 27.] 



Heeling Error Instrument. A specially designed dipping needle 

 (q.v.) controlled by an adjustable weight so that values of 

 the vertical component of the earth's magnetism may be 

 compared. Used in correcting the heeling error in ship's 

 compasses. [13, 15, 16.] 



Helicograph. An instrument for drawing helices or spiral lines 

 on a plane. [13, 17.] 



Helio-Chronometer. A form of sundial which shows true clock 

 time by simple direct observation, without reference to 

 equation tables. [2, 5, 15, 17, 20, 21.] 



Heliograph. (i) An instrument for obtaining photographs of 

 the sun. (2) An apparatus for telegraphing by means of 

 the sun's rays. See also Photo-Heliograph. [2, 8, 13, 15, 18, 

 20, 21, 24, 25, 26, 27.] 



Heliometer. An instrument, devised originally for measuring 

 the diameter of the sun, now employed for delicate measure- 

 ments of the distance and relative direction of two stars 

 too far apart to be easily measured in the field of view of an 

 ordinary telescope. [11.] 



Helioscope. A kind of telescope particularly adapted by means 

 of blackened or smoked glasses, which only partially reflect 

 the light, for viewing the sun. [26] 



Helios tat. (i) An instrument consisting of an adjustable mirror 

 or reflector worked in connection with a combination of 

 telescopes, used in trigonometrical surveys ; with it, 

 triangles with sides over 100 miles in length can be 

 measured. (2) A mirror moved by clockwork by which a 

 sunbeam is directed continuously to one spot. [2, 3, 11, 

 13, 14, 18, 20, 21, 25, 26, 27.] 



Heliotrope. (i) An astronomical instrument for showing when 

 the sun arrives at its farthest point north or south of the 

 equator as seen at that place. (2) A photographic instru- 

 ment used to illuminate negatives in the solar camera in 

 making enlarged pictures. (3) A plain flash signalling 

 instrument. [18, 21, 27.] 



Hemadynamometer. An instrument by which the pressure of 

 the blood in the arteries or veins is measured by the height 

 to which it raises a column of mercury. See Hsemodynamo- 

 [21, 26.] 



