HOR 



HOR 



HORIZONTAL parallax. See PARALLAX. 



HoiuzoNTAL/>/arce, that which is paral- 

 lel to the horizon of the place, or nothing 

 inclined thereto. The business of levell- 

 ing is to find whether two points be in the 

 horizontal plane, or how much the devia- 

 tion is. 



HORIZONTAL plane, in perspective, is 

 a plane parallel to the horizon, passing 

 through the eye, and cutting the per- 

 t spective plane at right angles. 



HORIZONTAL range, of a piece of ord- 

 nance, is the distance at which a ball falls 

 on, or strikes a horizontal plane, whatever 

 be the angle of elevation or direction of 

 the piece. When the piece is pointed pa- 

 rallel to the horizon, the range is then 

 called the point-blank, or point-blank 

 range. The greatest horizontal range, 

 in the parabolic theory, or in a vacuum, 

 is that made with the piece elevated to 

 45 degrees, and is equal to double the 

 height from which a heavy body must 

 freely fall, to acquire the velocity with 

 which the shot is discharged. Thus, a 

 shot being discharged with the velocity 

 of v feet per second ; because gravity 

 "generates the velocity "2 g, or 32^ feet, in 

 the first second of time, by falling 16_L. 

 or g feet, and because the spaces descend- 

 ed are as the squares of he velocities, 



v z 



therefore as 4 a : v* :: g :T the space a 

 4g 



body must descend to acquire the velocity 

 v of the shot, or the space due to the ve- 

 locity -v ; consequently the double of this, 



or =S;^Y is the greatest horizontal 



range with the velocity v, or at an eleva- 

 tion of 45 degrees, which is nearly half 

 the square of a quarter of the velocity. 

 In other elevations, the horizontal range 

 is as the sine of double the angle pf ele- 

 vation : so that, any other elevation being 

 e, it will be, 



As radius 1 : sin. 2 e :: !Ly^. X sin. 2-e, 



the range at the elevation e, with the ve- 

 locity TV But in a resisting medium, like 

 the atmosphere, the actual ranges fall far 

 short of the above theqrems, in so much 

 that with the great velocities the actual 

 or real ranges may be less than the 

 tenth part of the potential ranges; so that 

 some balls, which actually range but a mile 

 or two, would in vacuo range twenty or 

 thirty miles. And hence also it happens, 

 that the elevation of the piece', to shoot 



farthest in the resisting medium, is always 

 below 45, and gradually the more below 

 it as the velocity is greater, so that the 

 greater velocities with which balls are 

 discharged from cannon with gunpowder 

 require an elevation of the gun equal to 

 but about 30, or even less. And the less 

 the size of the balls is too, the less must 

 this angle of elevation be, to shoot the 

 farthest with a given velocity. See GUN- 

 NERY and PROJECTILES. 



HORN, in physiology, a tough, flexible, 

 semitransparent substance, intended for 

 - the defence or covering of animals. The 

 hollow horns of the ox, goat, &c.; the 

 hoof, the horny claw and nail, 'and the 

 scale of certain insects, ,as the shell of 

 the tortoise, resemble each other in chemi- 

 cal characters; but they differ very 

 widely from stag's horn, ivory, &c. Horn 

 is distinguished from bone, in being 

 softened very completely by heat, either 

 naked, or through the mediunvof water, 

 so as to be readily bent to any shape, 

 and to adhere to other pieces of horn in 

 the same state. Horn contains but a 

 small portion of gelatine, and in this it 

 differs from bone, which contains a great 

 deal. Horn consists chiefly of condens- 

 ed albumen, combined with a small and 

 varying portion of gelatine, with a small 

 part of phosphate of lime. The fixed al- 

 kalies readily and totally dissolve horn 

 into a yellow saponaceous liquor. 



Horn and tortoise-shell are applied to 

 mechanical purposes, which require them 

 to be bent and united in various V ays ; 

 this is performed by the aid of heat, ap- 

 plied either dry, with warmed irons or 

 burning charcoal ; or by softening the 

 horn in boiling water, or in a weak so- 

 lution of alkali : when thus softened, 

 they will easily adhere. Mr. Aiken gives 

 the following- process for making the 

 horn-ring that surrounds a common ope- 

 ra-glass : " A flat piece of horn is cut 

 out, of the requisite shape, the ends to 

 be joined are thinned down by a file, the 

 piece is then put into boiling water till 

 sufficiently Supple, and is then rolled 

 round a warm iron cylinder, and held in 

 that position by a vice, so that the ends 

 over-lap each other : another piece ot" 

 iron, -heated and grooved, is then laid 

 upon the seam of the joined ends, and 

 pressed upon the cylinder, and there con- 

 fined by an iron wire; and the heat of 

 the two partially melts that portion 

 of the horn, and cements the ends so 

 completely, that no seam or joining can 

 be observed wheji cold." For the manner 



