HORSE-POWER is the name given to the unit in terms of which engineers measure the power of steam- engines, water-wheels, and other prime movers. It is detined to be the rate at which an engine works when it does 33,000 foot-pounds of work per minute, a foot-pound being the amount of work necessary to raise a pound weight a foot high. We must go back to the early history of the steam-engine to discover the reason why this number was adopted. The first steam-engines were employed to drive nulls, pumps, and other machinery which had previously been driven by hurses ; and it seemed natural to express their working-power in terms of the number of horses whose work tluy were got to accomplish. This led to experiments being made in order to get an estimate of the average working-power of a horse. Several such estimates have been given, all differing considerably from each other; but the one adopted whereby to express horse-power is that obtained by Boulton and Watt from observations on the strong dray horses employed at the London breweries work ing eight hours a day. They found that a horse was able to go at the rate of 2 J miles per hour and at the same time raise a weight of 150 Ib by means of a rope led over a pulley. This is easily seen to be equivalent to 33,000 Bb raised one foot per minute, and hence the number given above. In connexion with this s abject it is necessary to dis tinguish clearlybetween "horse-power indicated"and "horse power nominal" as applied to steam-engines. The horse power indicated is got from an examination of the indicator diagram (see Diagrams, vol. vii. p. 152). The area of the closed curve traced on the diagram, or " card " as it is technically called, gives the work done by the steam on the piston during each complete stroke. This divided by the difference between the extreme abscissoa gives the average pressure (j>) on the piston. If we multiply this by the area of the piston (A) and by the length of stroke (x), we get the number of foot-pounds of work done during each stroke, and this multiplied by the number (11) of strokes per minute and divided by 33,000 gives the indicated horse power. Thus—
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Nominal horse-power is a purely conventional term adopted by makers of steam-engines, and has no fixed relation to indicated horse-power. The method of calculat ing it dates from the time of Boulton and Watt. In their engines they supposed the average pressure on the piston to be 7 ft) on the square inch, and the velocity of the piston in feet per minute to be 128 times the cube root of the length of stroke in feet. Computing from these supposed data, we get the nominal horse-power. Thus—
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![{\displaystyle {\text{Nominal H.P.}}={\frac {7\times \mathrm {A} \times 128\times {\sqrt[{3}]{x}}}{33,000}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/12a68ca18db2feae150bf083928257c14f49a3ed)
{{indent|The British Admiralty rule for nominal horse-power differs from this in using the actual velocity of the piston instead of the above supposed velocity. These rules only apply to low-pressure engines ; for high-pressure engines it is usual, after Bourne, to assume 21 Ib as the average pressure on the piston, the other data remaining as before. See Rankine s Steam- Engine.
HORSE-RACING. See Horse, p. 199.
HORSERADISH (Ger., Meerrettig; Fr., raifort = ratine forte, cran de Bretagne; Swed., Peppar-rot ; Russ., chren), (Jochlearia Armoracia, L., a perennial plant of the natural order Cruciferce and tribe Alyssinece, having radical leaves on long stalks, ovate or oval-oblong, 4 to 6 inches broad, about a foot in length, subcoriaceous, crenate or serrate, and coarsely veined; stem-leaves short-stalked or sessile, elongate, and tapering to their attachment, the lower ones often deeply toothed ; flowers, which appear in May and June, ???-inch in width, in flat-topped panicles, with sepals purplish, and petals white; and fruit a small silicula, which in the climate of England seldom bears seed. The horse radish is indigenous to eastern Europe. Into western Europe and Great Britain, where it is to be met with on waste ground, it was probably introduced from Russia (De C.indolle, Geoyr. Botan., ii. 654, 1855). It was wild in various parts of England in Gerard s time. The root, the armoradce radix of pharmacy, is to 2 inches or more in diameter, and commonly a foot, sometimes 3 feet in length; the upper part is enlarged into a crown, which is annulated with the scars of fallen leaves; and from the numerous irregular lateral branches are produced vertical stolons, and also adventitious buds, which latter render the plant very difficult of extirpation. From the root of Aconite (q.v., vol. i. p. 98), which has occasionally been mistaken for it, horse radish root differs in being more or less cylindrical from a little below the crown, and in its pale yellowish (or brownish) white hue externally, acrid and penetrating odour when scraped or bruised, and pungent and either sweetish or bitter taste (see Bentley, Pharm. Journ., 1st ser., xv. 449, 1856). The fresh root yields on distillation with water about 05 per cent, of a volatile oil identical with that of black mustard, resulting from the mutual reaction of sinigrin (potassium myronate) and myrosin in the presence of the water. After drying, the root has been found to afford 11 15 per cent. of ash. Horseradish root is an ingredient in the spiritus armoradce cumpositus of the British Pharma copoeia. It possesses stomachic, diaphoretic, and diuretic properties, and hence is administered in atonic dyspepsia, chronic rheumatism, and dropsies. As a masticatory, or in the form of syrup or infusion, it is used for hoarseness. Gerard speaks of it as anthelmintic and emetic. Externally applied it acts as a rubefacient ; and the juice with vinegar is a popular remedy for freckles. In common with other species of Cochleana, the horseradish was formerly in high repute as an antiscorbutic. The root was, as well as the leaves, taken with food by the Germans in the Middle Ages, whence the old French name for it, moutarde des A llemands ; and Coles, writing in 1657, mentions its use with meat in England, where it is now chiefly employed as a condiment with beef. For the successful cultiva tion of the horseradish, a light and friable damp soil is the most suitable ; this having been trenched 3 feet deep in autumn, and the surface turned down with a liberal supply of farm-yard manure, a second dressing of decom posed manure should in the ensuing spring be dug in 2 feet deep, and pieces of the root 6 inches in length may then be planted a foot apart in narrow trenches. During summer the ground requires to be kept free of weeds ; and the application of liquid manure twice or thrice in sufficient quantity to reach the lowest roots is an advantage. When dug the root may be long preserved in good condition by placing it in sand. The horseradish tree is the Moringa pterygosperma of Gartner.
See Gerard, Hcrball, p. 240, ed. Johnson, 1636 ; Byrne, Soicerby s Eng. Hot., i. 183, pi. cxxix., 1863; Florist, 1875, p. 191 ; Floral World, 1879, p. 149 ; Fluekiger and Hfinbury, Pharmacogra- 2)hia, p. 71, 2d ed., 1879; Bentley and Trimen, Mcd. PL, i. 21, 1880.
