52 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[February, 



wnches to pull the cradle forward. The outer part of the cradle, which 

 bore the weight of the raking braces, was, to save expense, supported on 

 sliding balks. On one occasion, when the ropes were tightly stretched, a 

 few blows given to these shding balks caused the cradle, with its enormous 

 ■weight, to start suddenly forward a distance of nearly two feet. In conse- 

 quence, they were always tapped afterwards, whenever the cradle encoun- 

 tered any obstacle to its motion. M. le Bas experienced the same difBculty 

 in moving the Luxor ohelisk, both in Egypt and at Paris, where a great force 

 was employed to drag it forward. The huge rock of the statue of Peter the 

 Great at St. Petersbnrgh was moved on balls of brass, turned very accurately, 

 and running in brass channels ; yet they frequently stuck fast, and required 

 the attention of a man to each, with an iron rod, to keep them in motion 

 and equidistant from each other. The use of railways, with wheels moving 

 in carriages fixed to the cradle, undoubtedly saved trouble and expense, and, 

 to a certain degree, reduced the friction. , 



At the time of building the new pier, preparations were made for the site 

 of a light-house, by piling an area of about 20 feet square in the centre of 

 the head, founding upon the piles, and bringing up along with the other 

 work, a mass of masonry, in large blocks, properly squared, and bedded so- 

 lidly in pozzolana mortar. The foundation was therefore in readiness for 

 the reception of the building. On the 4th of October it was brought to its 

 destination. Timber uprights were immediately wedged up under tlie cradle, 

 which permitted the different sheaves and railway balks to be withdrawn. 

 Upon this being done, the masons commenced operations by building on the 

 foundations above alluded to, pillars of stones, with retreating courses, 

 striking the shores, from time to time, as these pillars took their bearing un- 

 der the original masonry. 



The mortar used was made from blue lias lime, with a mixture of sand and 

 pozzolana, and was laid in very thin joints. The chief difficulty arose in 

 making good the last course, as the joint bad to he made rather thicker than 

 usual, for the admission of the masonry. The stones of the course, before 

 insertion in the building, had their upper surface covered with thin sheet 

 lead, firmly beat down ajul lapped for a breadth of two inches over the back 

 jiart of the stone. This was done to equalize the pressure, and to prevent 

 the external masonry from being flushed by the weight of the building it had 

 to sustain. Tlie joints were run with grout through the funnel of a tube, 

 carried up a few feet in height, to give additional pressure. They were pre- 

 viously closed all round with Roman cement, excepting a few apertures left 

 on purpose for the air to escape, and which allowed tiic grout completely to 

 fill the joint. Great care was taken to make the masonry sound and perfect, 

 Iiy properly bonding the joints, both internally and externally, by which 

 means there is not any indication of the building having ever i)een displaced. 

 The masonry was completed on the 12th of November. 



Before cutting into the light-house for the insertion of the cradle, the dif- 

 ferent corners of the base were accurately levelled with an instrument and 

 trial was made whether the building was exactly perpendicular by a plummet. 

 From time to time, as the building was moved forward, other trials were made 

 for the like purpose and also after it bad been brought to its destination on 

 the new pier head. In all cases it was found to be as at first. No settle- 

 ment was ever perceptible, even where the new masonry was placed, on with- 

 drawing the cradle ; nor has the slightest crack appeared since, in any part 

 of the building. 



The timbers and the chief part of the other materials employed, were used 

 in other works, then carrying on by the Harbour Commissioners, and the 

 men who worked at the winches, when they had accoiuplished their task, 

 were taken off to other work connected with the building of the pier. The 

 cost of carrying the work in' question into execution, amounted to i"827. 

 The building was erected in 1802, at an expense of upwards of £1400. If 

 to £827 be added A'280, the estimated cost of a light-keeper's dwelling, gas- 

 house, and other apparatus, it would have made .f 1107 as the total expendi- 

 ture of this department. The estimated cost of building a new light-house 

 on the high ground near the Fort, with a tide light on the north pier, dwel- 

 lings, and other contingencies, amounted to £2000; consequently, by adopt- 

 ing the removal of the building, as is above mentioned, there was a saving of 

 .£893, and no inconvenience was experienced from the want of a harbour- 

 light. 



Since the completion of this undertaking, the author has had the honour 

 to receive the thanks of the Board of Commissioners for bis exertions ; and 

 a piece of plate of the value of £100 has been unanimously voted to him as 

 a further acknowledgment of his services on that occasion. 



ENOiNnr.KiKG IN Egypt.— Mehemet Ali s great dock at Alexandria lias, 

 after a labour of eight years, and a cost of half a million sterling, been opened ;— to the 

 great satisfaction of the Pasha, who came from Cairo for the occasion, anil caused the in- 

 auguration to be accompanied with great ceremonial. 'Ihe engineer, If. Mongel, a 

 Frenchman, haa, it Is said, received from Mehemet Ali direclions to take all necessary 

 measures for the one other great work which that prince has so much at heart— the con- 

 struction of the liarrage of the Nile. The site now fixed upon is the point of junction of 

 the nosetto and Damletta branches of the river— about ten miles below Cairo. The work 

 will consist of two bridges, one over each liranch, joining each other at the extreme point 

 of the Delta. One arch of each bridge will be muile with a lock, for the purpose of navi- 

 gation. In the centre of the Delta, and on the sides of the bridges, will be opened several 

 caaals, to which the water of the Nile will be allowed ingress, as may be required. It is 

 slated, as an example of the saving to be eftected bv this barrage— that of the 60,000 

 saklahs, or Persian water-wheels employed In Lower Egypt, and worked by not less than 

 l."iO,000 bullocks, not more than 10,000 iilll be required to irrigate the lands situate at a 

 great dlstancefrom the canals or above their level. 



IiMPROVED LEVEL, 



Some Account of Levelling Instruments, with Descriptk 

 Improved Form, 



i of one of a 



By Thomas Stevenson, Civil Engineer, Edinburgh. 



The improved spirit-level about to he described, was made for me in 1840 

 by Mr. Adie, and afterwards described in a letter to the Institution of Civil 

 Engineers (see Journal, Vol. IV., pp. 357 and 373.) 



Since then I have bad two levels of the common construction altered to 

 the new form. On a late occasion these instruments were e-f hibited to the 

 Royal Scottish Society of Arts; and as no drawings had been laid before Ihe 

 public, that Society requested me to prepare a description with diagrams, 

 that these might be pnblished in their Transactions. In accordance with 

 their request, therefore, 1 have drawn up the present description ; and have 

 also consulted many early writers, to ascertain, as well as I could, the history 

 of one of the most important of geodetical and engineering instruments. 



There unfortunately e.xists a great deal of conUicting evidence regarding 

 the true inventors of the dillerent parts of the spirit-level. Indeed, there are 

 hardly two authorities who agree upon the suliject. Tlie original form seems 

 to have been' that of a plummet, and is described as " inslrumentum quo 

 pliimbo rt filo et giiomone pendente, reclio, sive oblirjuitas operis perpendi- 

 tur. '' 



The great lliiygens appears to have been the first to apply the telescope to a 

 level of liis, which was constructed on the principle of the plummet. This is 

 noticed, in the life prefixed to his works, s in the following terms : — "Ibi" 

 (Lutetoe) " vixit ab anno 1GG6 ad' annum IG81. Durante hoc tempore pul- 

 clierrima subtilissimaque multa in mathcmaticis delexit. veriaque ex lis 

 operibus conscripsit, quce nunc in unum corpus coUecta, quid in variis Ma- 

 thcseos partibus pia-.stiterit, sub oculis ponunt. Praeter ipsius jam memorala 

 inventa praiclara, inter alia duo insigni usu eminent. Libellam telescopio 

 munitam ita construxit ut ipsi prae ceteris, fides liaberi possit," Ste. 



The honour of having first applied the air-bubble to the determination of 

 horizontalily seems to be due to that universal genius Dr. Ilooke. From all 

 that I can gather, it appears that his invention must have been made subse- 

 quent 10 25ih March, 1674, and prior to the year 1675, as, in his " Attempt 

 tn prove the Motion of the Earth by Observations," by date 25ih March, 

 1C74. he describes a new method of slillhi^ the plummet by immersion in water. 

 While in his " Animadversions,' " published also in 1674, after fully describ- 

 ing bis invention of the air-bubble confined in a lube, he speaks of its pecu- 

 liar advantages, and great delicacy of movement, and remarks,— " This can 

 hardly be performed by the ordinary way of plummets, without hanging 

 from a vast height, which is not practically to be performed without almost 

 infinite trouble, expense, and difficulty," &e. 



Hutton, in his Mathematical Dictionary, remarks, that the applications of 

 the air-bubble to the level ''is said to be due to M. Thevenot ;" but with 

 what justice I cannot say, having been unable to meet with any reference to 

 this instrument in the writings of that author. Thevenot was born in 1G2I, 

 and he died in IG;)2. 



I have been unable to discover who was the inventor of the circular level, 

 which I imagined had been of recent date ; but Suitzer, at page 91 of his 

 Treatise on Water works, which was published in 1734, remarks, Ihat the 

 circular level was then employed in the construction of the surveying instru- 

 ment called a Plane-table. 



According to Sir John Herschel, tlie cross-hair, which gives so much accu- 

 racy to all astronomical as well as levelling instruments, was the invention 

 of Gascoigne, a young Englishman, who used it in 1640. He was killed at 

 the age of 23. at the battle of Marston Moor. 



M. Le Bion° a))pe3is to have been the first to conjoin the telescope of Iliiy- 

 gtvs with the air-bubble of Dr. Hooke ; and this must have been subsequent 

 to the year 1684, as such an instrument is not shown in De La Hire's edition 

 of Picard's Treatise on Levelling." 



But it was not tillSisson's improvements that the level could be considered 

 as in any way an accurate or philosophic instrument. All that were made 

 previously to his time were coarse instruments, adjusted by a ball and socket, 

 and in other respects rese:nbling the common perambulatory survey-level, 

 which, from the nature of the construction, can be levelled in only one direc- 

 tion, and cannot be reversed, or moved even in the slightest degree, without 

 requiring readjustment. Sisson may, therefore, be considered as the inventor 

 of the instrument in common use. The miin feature in his improvements 



1 Plin. Lib. 7. 



2 Polydori Virgilii de Tlerum Inventoribus Libri Octo. 12mo, p. 258. Urbini, IMO. 

 a Christ. Hugenii Op." Var. Lugd. Bntav. 1724. 



4 Animadversions on the first part of the Machina cicleslis of the Hon., learned, and 

 deservedly famous Astron. Johannes Hevelius, Consul of Dantzick, together with an ex. 

 plication of some instruments made by Hob. Hooke, Prof. Geora. in Uiesh. Coll., aud 

 I'.R.S. Lond. 1674, p. HI, -et seq.' 



3 Traiti- de laConslructlnn et desPrincipanx usages dea Instrumens de Sfathematique. 

 Par N. Le lilon, Ing^uieur du Koi i)Our Ics Instrumens de Math. Nouv. Edit. A La 

 Haye, 1723. 



rrMi du Nivellement Par M. Pical'd, mis en lumlerc paries solus deSI.DeLa 

 Hire, I2mo. A Paris, 1(;84. 



