Page:EB1911 - Volume 18.djvu/942

 told the story of a stirring period in the history of the world with full attention to the character of the actors and strict fidelity to the vivid details of the action. But it may safely be said that his tale is best where most unvarnished, and probably no writer of the same rank has owed less to the mere sparkle of highly polished literary style.

An excellent edition of his historical works was published in nine volumes in London in 1903–1904. See the Correspondence of John Lothrop Motley, edited by G. W. Curtis (New York, 1889); O. W. Holmes, John Lothrop Motley, a Memoir (Boston, 1878); M. D. Conway, Biographical Introduction to The Rise of the Dutch Republic (London, 1896); and John Lothrop Motley and his Family: Further Letters and Records (1910), edited by his daughter, Mrs Susan St John Mildmay.

 MOTLEY, i.e. of many colours, a term particularly used of the parti-coloured dress of the professional “” (q.v.) of the middle and later ages. The origin of the word is probably to be found in “mote” (O. Eng. mot), a particle of dust, &c., hence a spot or patch. “Mottle,” a blotch or spot, is probably a back formation of motley.  MOTMOT. According to Hernandez in his Historia avium Novae Hispaniae (p. 52), published at Rome in 1651, this is the Mexican name of a bird which he described well enough to leave no doubt as to what he meant; but the word being soon after printed Momot by Nieremberg and others gave rise to the Latinized Momotus, invented by M. J. Brisson as a generic term, which has since been generally adopted by ornithologists, though motmot has been retained as the English form. Linnaeus knew of only one species of motmot, and referred it to his genus Ramphastos (properly Rhamphastus) under the name of R. momota. This is the Momotus brasiliensis of modern ornithologists, and from its geographical range cannot be the original Motmot of Hernandez, but is most likely the Guira guainumbi of Marcgrave.

The motmots form the sub-family Momotinae, which with the Todinae (see ) form the family Momotidae of Coraciiform birds, the nearest allies being (q.v.) and  (q.v.). In outward appearance the motmots have an undoubted resemblance to bee-eaters, but, though beautiful birds, various shades of blue and green predominating in their plumage, they do not exhibit such decided and brilliant colours; and, while the bee-eaters are only found in the Old World, the motmots are a purely Neotropical form, extending from southern Mexico to Paraguay, and the majority of species inhabit Central America. Their ordinary food is small reptiles and fruits, and insects caught on the wing. The nest of one species, as observed by Robert Owen, is at the end of a hole bored in the bank of a watercourse, and the eggs are pure white and glossy (Ibis, 1861, p. 65). Little else has been recorded of their ways.

The Momotidae form but a small group, containing about six genera, of which the best known are: Momotus, Baryphthengus, Hylomanes, Eumomota, Aspatha and Prionorhynchus, and the number of species is very small. While all have a general resemblance in the serrated edges of the bill and many other characters, Momotus has the normal number of twelve rectrices, while the rest have only ten, which in Hylomanes have the ordinary configuration, but in adult examples of all the others the shaft of the median pair is devoid of barbs for the space of about an inch a little above the extremity, so as to produce a spatulate appearance, such as is afforded by certain humming-birds known as “racquet-tails” (see ), kingfishers of the genus Tanysiptera (see ), and parrots of the group Prioniturus. C. Waterton (Wanderings, Journey 2, chap. iii.), mentioning the species M. brasiliensis by its native name “houtou,” long ago asserted that this peculiarity was produced by the motmot itself nibbling off the barbs, and this extraordinary statement, though for a while doubted, has since been shown by O. Salvin (Proc. Zool. Society, 1873, pp. 429–433), on A. Bartlett’s authority, to be perfectly true.

 MOTORS, ELECTRIC. Fundamentally, electric motors are electric generators reversed in function: they convert into mechanical energy the continued stresses between two electromagnetic fields relatively movable, just as generators convert into electromagnetic stresses the mechanical energy applied to them. Since no transformation of energy is ever absolutely quantitative, the conversions just considered are not accomplished without loss of energy to about the same extent in both cases. The sources of this loss are ohmic loss in the conductors, hysteresis, friction of bearings and brushes, air friction and eddy currents; the sum of these losses in large modern machines does not exceed 5 or 6%. The torque of the motor is the dynamical result of the electromagnetic stresses between the magnetic field of the motor and that due to the armature currents, the latter field being proportional to the strength of the current sheet due to the numerical strength of the current and the number of its effective convolutions. This applies to all types of motors, if one remembers that whenever either of these two stress factors is a periodic variable, as in the case of alternating motors, the torque is proportional to their geometrical co-directed product and not merely to their numerical product. At this point it will be convenient to distinguish between the various types of motors. The first broad distinction is between continuous-current and alternating-current motors, a distinction rather of convenience than of necessity, for in point of fact the two depend upon the same broad principles and can be considered on precisely the same lines.

Electric motors may be conveniently divided as follows:— (A) Continuous Current. 1. Separately excited. 2. Series-wound constant current. 3. Series-wound constant potential. 4. Series-wound interdependent current and potential. 5. Shunt-wound constant potential. (B) Alternating Current. 1. Synchronous constant potential. 2. Induction-polyphase constant potential. 3. Induction-monophase constant potential. 4. Repulsion-commutating. 5. Series-commutating.

Of these, the series-wound constant potential, shunt-wound constant potential, and polyphase induction motors do a very large proportion of the active work of power transmission: the first mentioned furnish power for electric railways; the second chiefly power distribution from public electric supply stations; while the third are mainly relied upon in long-distance transmission systems. The fourth and fifth groups of class (B) are old in principle but have been slow in practical development. They include many modifications and transition forms not involving radical changes in the principles or properties of the machines. Their chief use has been for electrical traction, with reference to which they have, in the main, been developed, and their performance is best at low frequency, 15 to 25 cycles per second.

In class (A) in general, for a certain value of the torque current must be forced through the armature against the motor electromotive force which results from the rotation of the armature in a given field. This demands a certain greater applied electromotive force to produce the current required, which is determined by the effective electromotive force, equal to the geometrical difference between the applied and motor electromotive forces, and by the impedance of the armature. For steady currents this last is of course the same as the ohmic resistance, just as for steady electromotive force the geometrical and the numerical difference of the applied and motor electromotive forces are coincident. The torque depends, as heretofore noted, on the field strength and the strength of the current sheet due to the current thus determined. For small values of the torque the speed practically depends upon the applied electromotive force and the field, so that if the former and the latter be constant the speed is also sensibly constant. This is likewise the case if the armature resistance be very small; and in general the variations of speed at constant potential are determined by the product of this resistance and the torque, while the absolute speed depends essentially upon the field strength. Motors for low speed or high