These are for tho purpose of preventing the siphon from running dry when the acid gets so low in the cistern as to bo below the siplieu mouth. To effect this, however, the ends most be kept at exactly the same level. This is easily managed, as the siphon depends from two points, as shown at g. In order that the cistern luny be completely emptied, it is given a little well at h, into which tho vessel e can completely enter. To avoid the splashing of the escaping acid, it is not run into the open air but into the lead-lined vessel N, of the same height as the cistern M.
This box has a tap at the bottom which regulates tho outflow of the acid. During the feediug of the cistern M, the siphon is drawu up above the highest level of the acid. The bottoms of the chambers and of the reservoirs M Inc) be of 7 lb. sheet lead, while, for the sidewnlls and the tops of the chambers and for the connection pipes and the smaller parts of the apparatus, 5 or 6 lb. lead will suffice.
The overlapping sheets forming the chamber linings were formerly beaten together and joined with a oetuent, or fastened by tin solder. Now, however, they are melted together by means of a hydrogeu gas flame, without bending the lead edges together. In this way, not only a great saving of time and cost is secured, but it is especially valuable as giving a much more durable seam, for even with tin solder the acid soon found its way through.
The support of the sides and roofs of the chambers is aided by a stroug timber framework, and they are erected in the following manner:—On the strong and level plank flooring of the chamber are spread the leaden sheets destined to cover the floor. They are carefully beaten flat with wooden mallets, in such wise that each sheet shall overlap the next about an inch. This is then "burnt" on to the underlying sheet with the hydrogen flame. In order to give the bottom of the chamber the form of a basin in which to hold the acid, each sheet is turned up vertically about nine inches high all round the chamber.
Before proceeding to erect the side lead, the floor is covered with straw overlaid with planks, so that it shall not receive any damage ; often, also, the bottom lead is the lest to be put in. The timber work is then proceeded With. Four beams, marked o, are laid round the upturned edge of each chamber, and serve to keep it iu place. The uprights p are then morticed into the beams and strutted by the braces q. Then the four erowntrecs r are morticed on to the uprights and struts. Now follows the erection of the sheet lead.
As the burning together of the leaden sheets is much more quickly and easily effected in a horizontal position than in a vertical one, the small sides of the large chamber and all the sides of the small ones are burnt first and afterwards raised in one piece. The long sides of the lerge
chamber must be divided into two or three portions for raising. They are fmally connected by vertical seams, and then form one complete sheet. For this purpose a strong wooden platform is made, on which the lead may lie. This is so placed that the edg,e which corresponds to the lower edge of the side sheet shall be close against the upturned edge of the basin, so that the sheet can be very easily raised into place by pulley blocks from the upper edge. On this platform the sheets mny be joined, as on the floor of the chamber. In estimating the height of the sheet, about nine inches in excess is allowed, as it is necessary that a portion be turned over the crown tree r, and firmly nailed to it.
As soon as the she ts are joined ready for raising, chalk lines are drawn on them indicating the position of the uprights p. On these lines, and about 1 ft. 6 in. apart, are burnt leaden straps about four inches square. Then the top edge of the sheet is bent over the edge of the platform and there fastened, and the whole is then raised into position tight against the framework of the chamber. The straps correspond with the uprights, and are nailed to them. The platform is then removed for further use, as before. When all the side sheets are thus erected, the remaining joints must he "burnt" up. A good upright seam can be made by a workman of sufficient prac tice, in whose hands the lead will not drop down on application of the flame, but rather adhere to the already-formed portion of the seam.
When the sides are completer], the roof is commenced. For this purpose trestles are brought into the chamber, and on theta a platform is constructed. On this platform the leaden sheets forming the top are spread out and lapped two or three inches over the erowntree. They are then buint into one complete sheet, thus closing the chamber. On this ceiling are burnt a series of straps, running parallel to the short sides of the chambers. Between each double row a beam or joist s is laid, whose ends reach the crowntrees r, and to which the straps are securely nailed. Thus, these joists support the ceiling. To prevent the joists from sinking, a strong beam t is laid athwait their middle, and supported by iron clamps depending from the framework of the timber roof of the works, where such roof is required ; hut in this country they are seldom or never used, rind the joists are sufficiently stout to need no support. The wooden platform, &c., is then removed from the finished chamber.