Page:The American Cyclopædia (1879) Volume VII.djvu/48

 EXPONENT EXPRESS of nitrogen. Some have so little stability that sound alone is sufficient to precipitate the ex- plosion, as iodide of nitrogen, which may he exploded by sounding a tuning fork of the proper pitch in its vicinity. When heat is re- quired, the rapidity of decomposition will de- pend also upon the rate of ignition throughout the m'ass. Thus in a charge of granular gun- powder, the flame from the vent passes be- tween the grains, progressively enveloping their surfaces, and through the pores of each into the mass, its progress being much hastened by the enormous tension produced when the ex- plosion is confined. Hence the rate of igni- tion (and consequently the intensity of the force at a given instant) may be varied by varying the size of pores and interstices in the mass; a fruitful field of experiment and im- provement, particularly in gunpowder. It is evident also that the tension is dependent upon the resistance to the expansion of the gases, and will rapidly increase unless the restraint is withdrawn in proportion to their progressive development. The increase of tension brings with it increased rapidity of ignition and decomposition, and this in 'turn augments the tension, which is thus a self-multiplying quantity. Restraint may be offered by an en- closing solid material, or by' the inertia of the gases themselves, and the surrounding air. If a block of compressed gun cotton is ignited in the open air by a flame of moderate tem- perature, it will often consume away very gradually ; but if ignited by an electric spark, or the impact of a bullet, it will explode with great violence ; the probable explanation being that in the former case the first ignition at lower temperature permitted the gases to ex- pand without producing a very high tension, this relation continuing to the end, while in the latter case the first ignition was violent, and the relief too slow to prevent a self-mul- tiplying tension. EXPONENT (Lat. expon&re, to manifest), in arithmetic and algebra, a small figure or letter, written to the right of and above a quantity or algebraic term, to show how often the quantity or term must be taken as a factor. Thus, 3 4 (which is read " the fourth power of 3," or " 3, fourth power ") signifies that 3 is to be taken as a factor four times, or multiplied into itself three times, as follows: 3x39; 3x9 =27 ; 3 x 27=81. In like manner (a + ~b) c sig- nifies that the sum of the numbers represented by a and & must be multiplied consecutively into itself as many times less one as there are units in c. (See ALGEBEA.) Exponential equa- tions and functions are those in which the ex- ponents contain unknown or variable quanti- ties; such as ?/=# z, in which a is the only known quantity. Exponential equations are usually reduced to logarithmic, and thus solved. EXPRESS, a messenger or conveyance sent on any special errand, particularly a courier despatched with important communications. In the United States the word is applied to a system organized for the transportation of mer- chandise or parcels of any kind. This system was originated March 4, 1839, when, agreeably to announcement published for several days in the newspapers, Mr. William F. Harn- den of Boston made a trip from that city to New York as a public messenger. His route was by the Boston and Providence railroad and the Long Island sound steamboat, which con- nected with that line. He had in charge a few booksellers' bundles and orders, and some bro- kers' parcels of New York and southern and western bank notes to deliver or exchange a service for which he charged an adequate com- pensation. Mr. Harnden proposed also to take the charge of freight, and attend to its early - delivery, for which purpose he had made a contract with the above named railroad and steamboat companies, and was to make four trips per week. The project recommended it- self to business men, especially those whose communications between the two cities were frequent. It was particularly acceptable to the press, to which Mr. Harnden made himself very useful in the voluntary transmission of the latest intelligence, in advance of the mail. A year later (1840) a competing express was started by P. B. Burke and Alvan Adams, the ownership and sole operation of which soon devolved upon the latter. In 1841 Mr. Adams associated with himself William B. Dinsmore of Boston as his partner, and gave him the charge of their New York office. Adams and co.'s express was carried by the Norwich and Worcester route. In 1840 D. Brigham, jr., Harnden's New York agent, became his part- ner, and soon after went to England, where he laid the foundation of Harnden and co.'s foreign business. He returned in 1841, and in that year their line was extended as far south as Philadelphia, and west to Albany. A year or two later Adams and co. established E. S. Sandford as their agent in Philadelphia, and he became a partner in their business there. He also became associated with S. M. Shoema- ker of Baltimore in an express from Philadel- phia to Washington, D. 0. About the same time Harnden and co.'s Boston, Springfield, and Albany express was purchased by Thomp- son and co., who gave it their name, which it still bears. About the same period Gay and co., afterward Gay and Kinsley, commenced what is now known as Kinsley and co.'s ex- press, running between New York and Boston, ma Newport and Fall River. The express lines from Albany to Buffalo, and thence to the remoter west, were established by Henry Wells. The first express west of Buffalo was com- menced in April, 1845, by Messrs. Wells, Far- go, and Dunning, under the style of Wells and co. It was disposed of two years afterward to William G. Fargo and William A. Livingston, who continued it, under the style of Living- ston and Fargo, till March 18, 1850, when it was consolidated with the expresses of Wells and co., and Butterfield, Wasson, and co. The