Again at other establishments semi-cylindrical retorts, placed horizontally upon their flat surfaces were employed; fig. 18. pl. V. The length of these retorts was from five to six feet, and their vertical and horizontal diameters were to each other as 6 inches, to 18 inches. And at a few establishments, ellipsoidal retorts, fig. 17, plate V. were used; these measured from five feet and a half, to six feet in length, their major and minor axes bore different proportions to each other at different establishments. At the first adoption of these retorts, the proportions varied but little from the cylinder, but subsequently the difference between the major and minor axes became gradually increased till at last the major axis has become to the minor axis, as 20 to 10 inches, and at some gas works the proportions are as 25 to 10 inches. With vessels of these forms the distillatory process was carried on for some years, and the quantity of fuel employed to decompose a given quantity of coal by means of them, amounted to from thirty to thirty-six per cent. When the dimensions of the retorts were increased, both the quantity of fuel and time required for the decomposition of a given quantity of coal was in a far greater ratio; and the operations of charging and discharging the retorts, very troublesome. Retorts of smaller dimensions have likewise been tried, but the more frequent charging and discharging, which they require, occasioned such a waste of time and labour, and such intermissions, in the temperature necessary for the process of distillation, (besides being attended with other disadvantages which will be afterwards explained), that they were speedily discontinued at the gas works where they had been adopted. The use of conical retorts, as well as of those of a semi-cylindrical and parallelopipedal form, has of late been discontinued in most establishments. The conical shape not only diminishes the capacity of the vessel, but also renders it incapable of being heated economically. From two comparative series of operations made on a large scale, and continued for upwards of six months with conical and cylindrical retorts, with a view to determine the comparative power of these vessels, it has been proved that the same quantity of gas which can be obtained by means of forty conical retorts, may be procured in the same time and with the same quantity of coal and fuel, by means of thirty-four cylindrical retorts.[10] [10] These Experiments were made at the commencement of the new art of lighting with gas, at the Westminster Chartered Gas Works, by Messrs. Grant and Hargraves.