DISTRIBUTION OF GAS THROUGH MAINS. 185 



feature in this branch of gas-lighting, and a good plan drawn to a large scale is the only 

 available method that can be adopted ; no written description will answer. 



" In commencing operations for the survey, take several running levels from the works 

 with your first ' back sight' on the curb of any of the gasometer tanks, and fix upon some 

 spot commanding the entrance into several streets, for the position of the equilibrium 

 cylinder or chamber into which the leading main empties itself: this point should be as 

 nearly as possible on the same level as the works : if it should be a little lower no 

 matter, but carefully avoid having its altitude much greater, as its discharge will not be 

 so readily appreciated. Still there are circumstances which will determine the position 

 of this ' starting-point' even of greater consequence than the level, and those are the 

 localities of the principal streets ; and that point where several meet must be fixed upon 

 as the place for the equilibrium cylinder. In the margin of the map the level and 

 distance of this point from the works must be marked, the level in red, either above or 

 below a line, as 5'75 would signify a fall, 5'75 a rise of 5'75 above the datum ; the length 

 should be marked opposite in black ink. If there is room upon the map, the level may 

 be marked as above, within a circle on the spot. 



" As the mains are laid from this point, let them be carefully plotted from the measure- 

 ment-book, so that their exact position will be apparent at one view ; the valves, re- 

 ceivers, and branches distinctly marked, and the relative positions of water-pipes, eewers, 

 &c. occurring in the same street, that may be met with on opening the ground, or other- 

 wise known. 



'' Draw the line indicating the mains with a permanent colour ; ultra-marine is the 

 best, as it never fades, and is capable of being washed out in case of alterations ; the 

 plan is thus preserved distinct by avoiding erasures, which must be made if the line is 

 marked with any but a body-colour. Show the valves by a blue cross, and receivers by 

 a small circle at every point at which they may occur, and let the directions of their 

 drainage be marked by arrows. If the map be to a small scale, write the name of each 

 street within the line of buildings, and suffer no interference with the spaces, by any 

 description but what distinctly belongs to the mains and apparatus. If any remarks are 

 necessary, they must be written in the margin of the map, and referred to by a corre- 

 sponding symbol. Show the connections ; and where mains cross one another, note if 

 under or over. It was a custom to indicate the different sizes of mains by different 

 colours : this is a very bad method, for reasons which must be obvious ; for the positive 

 colours are few in number and are liable to fade, and even under some influences to 

 change their colour completely. Draw the lines with ultra-marine of comparative thick- 

 ness, and mark the diameter with a red figure, dotting to crosses at the termination of the 

 lengths of the respective sizes. It is also advisable that the position of the public lights 

 should be shown, as also the district and parish boundaries. A small book should accom- 

 pany each map, in which the names of the streets should be set down alphabetically, and 



2 B 



