TEL 



167 



TEL 



Brewster has suggested (Treatise on New Phil, lust., 

 p. 400) that it may be possible to remove, or at least very 

 much diminish, the unconnected colour in the image by 

 the use of two lenses of the same kind of glass with the 

 same or different dispersive powers. He proposes that 

 the exterior lens should have the meniscus form, the con- 

 vex side being outwards ; in order, from the obliquity 

 of the incident rays to the surface, that the dispersion pro- 

 duced by that lens may increase in a higher ratio than its 

 refraction, so that the dispersion produced by the other 

 lens maybe corrected; while in each pencil the rays, 

 alter refraction through both, may be convergent. 



It would be improper to omit here to mention that M. 

 Amici at Modena, some years since, invented a species of 

 achromatic telescope by a combination of four prisms, all 

 of the same kind of glass : the refracting edges of one pair 

 of the pri-ms were parallel to one another, and those of 

 the other pair were also parallel to one another, but per- 

 pendicular to the edges of the first pair; and each pair 

 funned an achromatic combination. By the refraction in 

 the first pair the breadth of the object is magnified, and 

 by that in the second pair the length is magnified in the 

 same ratio : thus the result is an image undistorted and 

 magnified. Sir John Hen,cliel states that, in 1K26. lie 

 *aw in the hands of iU inventor one of these telescopes, 

 which magnified about four ti:i. 



TKLESCO'PIU-M .the Tele-rope , a constellation of 

 in the southern hemisphere, surrounded by Ara, 

 Sagittarius, and Oplu'uchus. Its principal stars are 

 as follows: 



TELESCO'PIUM, De Montfort's name for the Cen- 

 t/mini T<;'<'*i-t:/<tuM of authors. [ENTOMOSTOMATA, vol. ix., 

 p. Lll.] 



TELFOKD, THOMAS. In the life of this eminent 

 man, as has been observed in a brief notice of the l; 

 of that science of which he v\a- MI distinguished an orna- 

 ment, in the preface to the ' Transactions' of the Institu- 

 tion of Civil Engineers, ' another striking instance is 

 added to those on record of men who have, by the force 

 of natural talent, unaided save by uprightness and perse- 

 vering industry, laised themselves from the low estate in 

 which they were born, to take their stand among the master- 

 spirits of their age.' Telford's father was a shepherd in the 

 ia! district of Eskdale in Dumfriesshire, where, in the 

 : kirk, his only son was born, on the 9th of 

 1st, \7~>7. Hi father dying while he was yet an infant, 

 i Telford's early years devolved upon his mother, 

 Janet Jackson, for whom he cherished an affectionate 

 (1 until her death in 1794; he having been in the 

 habit, according to Mr. Hi'ckman, of writing letters to her 

 in jn-iiitcil characters, that, she might be able to read them 

 without assistance. He received the rudiments of educa- 

 tion in the parish school of VVc-terkirk ; and, while engaged 

 during the summer ;'herd boy in as.-: 



IIH uncle, he made diligent use of his leisure in studying 

 the hooks furnished by his village friends. At the age of 

 fourteen he was apprenticed to a stone-mason , 



ing town of Langholra ; and for several years he 



wa employed, chiefly in his native district, in the various 



:tioiu usually performed by a country masoti in a dis- 



i! there \ little occasion for the higher depart- 



his art. The construction of plain bridges, of 



i'aim buildings, and of simple village churches and m 



afforded however good opportunities for obtaining prac- 



knowledge. Tellbrd himself has expressed his sense 



o!' the value of tins humble training, observing, that 



e and usefulness only are studied in 



ar advantages are of fried to the 



your In adopt his o\vu words, *({. 



is not sufficient employment to produce a division of labour 



in building, he is under the necessity of making himself 

 acquainted with every detail in procuring, preparing, and 

 employing every kind of material, whether it be the pro- 

 duce of the forest, the quarry, or the forge ; and this 

 necessity, although unfavourable to the dexterity of the 

 individual workman who earns his livelihood by expert- 

 ness in one operation, is of singular advantage to the 

 future architect or engineer, whose professional excellence 

 must rest on the adaptation of materials and a confirmed 

 habit of discrimination and judicious superintendence.' 

 Chambers states that during this period of his life Telford 

 was remarkable for the neatness with which he cut letters 

 upon gravestones. In 1780, being then about twenty- 

 three, and considering himself master of his art, he visited 

 Edinburgh, apparently with a view to obtaining employ- 

 ment. The splendid improvements then in progress in 

 that city enlarged his field of observation, and enabled 

 him to contemplate architecture as applied to the object 

 of magnificence as well as utility ; and he seems at this 

 time to have devoted much attention both to architecture 

 and drawing. After remaining there about two years, he 

 removed to London, where he obtained employment upon 

 the quadrangle of Somerset House, then erecting by Sir 

 William Chambers, an engagement in which, according 

 to his own account, he obtained much practical informa- 

 tion. About 1784 he was engaged to superintend the 

 erection of a house for the resident commissioner'in Ports- 

 mouth dockyard, from the design of Mr. S. Wyat. Tel- 

 lord's good character and promising talent had secured for 

 him the friendship of two families resident in his native 

 district, the Pasleys and the Johnstones, and to their in- 

 fluence his early employment on important woiks is, in 

 some measure, to be attributed. He was engaged upon 

 various buildings at the Portsmouth dockyard for three 

 years, during which time he became well acquainted with 

 the construction of graving-docks, vvhai f-walls, and similar 

 engineering works ; and in 17H7, having completed his 



.ementa there, he was invited by the late Sir William 

 Pulteney (a member of the Johnstone family) to take the 

 superintendence of some alterations at Shrewsbury Castle. 

 He therefore removed to Shrewsbury, where he was also 

 employed to erect a new gaol, which was completed in 

 17'M. and was subsequently appointed county surveyor, in 

 which office (retained by him until death) he had to fur- 

 nish plans for, and oversee the construction of, bridges 

 and similar works. The first bridge which he designed 

 and built was that over the Severn, at Mont ford, about 

 four miles wot of Shrewsbury, consisting of three elliptical 

 stone ail-In-, dm- of fi fly-eight, and the others of fifty-five 



pan. His next was the iron bridge over the Severn, 

 at Buildwas, consisting of a very flat iron arch of a hundred 

 and thirty feet span, constructed upon very superior prin- 

 ciples to that erected a few years previously at Coalbrook 

 Dale : Telford's object was rather to introduce the trussing 

 principle of a timber construction than that of a stone 

 arch. This bridge was built in the years 17'Ji) and 17!Jl>. 

 Forty smaller bridges were erected in Shropshire under 

 Telford's direction. 



The Ellesmere Canal, a series of navigations intended 

 to unite the Severn, the Dee, and the Mersey, and extend- 

 ing altogether to a length of about one hundred and three 



, was the first great work upon which Telford was 

 rd, his satisfactory execution of the county works 

 intrusted to him having led its projectors to select him as 

 their engineer; and from this engagement, which com- 

 menced about 1793, in which year (he. act of parliament 

 was obtained for the scheme, his attention was directed 

 almost solely to civil engineering. The uneven character 

 of the country occasioned many serious difficulties in the 

 HI of this canal, and rendered necessary the 

 execution of some works of astonishing magnitude, i 

 cially in crossing the valleys of the Ceriog, or Chirk, and 

 of the Dee. In the former the canal crosses the river at 

 an elevation of seventy feet, by an aqueduct-bridge often 

 arches, each of which is of forty feet span, in the construc- 

 tion of which some important deviat'ions were made from 

 the previous practice of engineers. It had been usual in 



-tinctures to form the bed for the canal of puddled 



.ontined in masonry, a practice which involved 

 Xpnue, and some danger in time of frost, from the. ex- 

 pMrion of the moist puddle. The great elevation of tne 



i aqueduct would have increased the difficulty, but. 

 Telford abandoned the puddling system, and formed the 



