HOU 



H Y A 



one horse would produce during his ordi- 

 nary work. Various values have been 

 affixed to this unit of comparison ; but it 

 is now generally estimated as a force 

 which would be capable of raising a 

 weight of 33,000 lbs. a height of one foot 

 per minute, or 550 lbs. per second; and, 

 on a railway, as a force capable of trans- 

 porting ,400 tons one mile per day. 



HORTUS SICCUS. Literally, a dry 

 garden; an emphatic appellation given 

 to a collection of specimens of plants, 

 carefully dried and preserved : a more 

 general term is herbarium. 



HOUR. The twenty-fourth part of a 

 natural day, answering to fifteen degrees 

 of the equator. 



1. Hour, Sidereal. The sidereal day 

 is four minutes shorter than the mean 

 solar day ; it is the actual revolution of 

 the heavens (see Day). The astronomer 

 always makes it begin when the vernal 

 equinox is on his own meridian ; he di- 

 vides it into twenty-four sidereal hours 

 (each a little shorter than the hour of the 

 common clock) of 60 sidereal minutes, of 

 60 sidereal seconds each, and he mea- 

 sures it by a sidereal clock, with a pen- 

 dulum a very little shorter than that of 

 the common clock. He rejects the sub- 

 division into two periods of 12 hours 

 each, and speaks of 15 o'clock, or 16 

 o'clock, under the phrase 15 hours or 

 16 hours, meaning 15 or 16 hours from 

 the commencement of the sidereal day, 

 from the time when the vernal equinox 

 was last on the meridian. 



2. Hour of Angular Measure. The 

 twenty-fourth part of a revolution which 

 is made in twenty-four hours, and divided 

 in reference to the division of time. Thus, 

 an arc of a circle is said to be a certain 

 number of hours, minutes, and seconds, 

 of time ; meaning, that the arc in ques- 

 tion would be described in that number 

 of hours, minutes, and seconds, if the 

 whole circle were described in twenty- 

 four hours. At this rate, a revolution 

 being divided into 360 equal parts or 

 degrees, 15° make one hour, 15' one 

 minute of time, 15" one second of time ; 

 also, 1° is four minutes of time, 1' is four 

 seconds of time, and l"is one-fifteenth 

 of a second of time. 



3. Hour-circle. In Astronomy, the 

 equator is the principal circle employed ; 

 it is the circle of progression and regres- 

 sion most commonly used. All second- 

 aries to the equator are called hour- 

 circles ; or rather, each half of a secondary 

 is an hour semi-ciicle, the two halves of 



168 



the same secondary belonging to different 

 hours ; in fact, a meridian on the earth 

 always answers to an hour-circle in the 

 heavens. Every star is on one hour- 

 circle, and on one only ; unless it be at 

 one of the poles of the equator, and then 

 it is on all hour-circles. 



4. Hour-angle. The hour-angle of a 

 star is the angle which its hour-circle 

 makes with the meridian of the place 

 (represented by the brazen hour circle 

 when the globe is properly elevated). 

 This hour-angle is nothing when the star 

 is on the meridian, and is an hour of 

 angular measure for every sidereal hour 

 to or from the time of the transit, being 

 eastward before transit and westward 

 after it. 



HUM'BOLDTITE. A rare mineral, 

 consisting of a boro-silicate of iron, and 

 found in chalcedonic geodes in trap rocks 

 in the Tyrol. From this must be distin- 

 guished Humboldtine, which is a native 

 oxalate of the protoxide of iron. 



HU'MITE. A reddish-brown mineral 

 found near Naples in a rock of granular 

 topaz, and named from Sir Abraham 

 Hume. 



HUMMOCK. A sheet of ice, which 

 presents a surface generally level, but 

 here and there diversified by projections, 

 arising from the ice having been thrown 

 up by some pressure or force to which it 

 has been subject. (See Iceberg.) Also, a 

 term applied by navigators to a circular 

 and elevated mount appearing at a dis- 

 tance. 



HUMUS. Vegetable mould; woody 

 fibre in a state of decay. The various 

 names of ulmin, humic acid, coal of hu- 

 mus, and humin, are applied to modifi- 

 cations of humus. 



Humic acid of chemists. A product 

 of the decomposition of humus by al- 

 kalies ; it does not exist in the humus of 

 vegetable physiologists. — Liebig. 



HURAU'LITE. A new mineral found 

 in the Haute Vienne, consisting of a 

 phosphate of iron and manganese. 



HURRICANE. A phenomenon sup- 

 posed to be of electric origin. A large 

 vacuum is suddenly created in the atmo- 

 sphere, into which vacuum the surround- 

 ing air rushes with immense rapidity, 

 sometimes from opposite points of the 

 compass, spreading frightful devastation 

 along its track. 



HY'ACINTH. A sub-species of pyra- 

 midal zircon; a red mineral found in 

 volcanic sand in Ceylon, &c. 



HY'ADES (uo), to rain). The name 



