These boilers have many good points: there is great heating surface; and the shell being heated all round does not strain the plates and seams by unequal expansion so much as in the horizontal plain cylindrical boiler heated only at the bottom; and as both ends are spherical there is no alteration of shape under internal pressure. Moreover in consequence of the upright position of the boiler a safe depth of water can easily be maintained, and yet the steam is taken off so high above its surface that there is little priming; and every part can most easily be cleaned and examined, as a man can stand upright both in the boiler and in the flues. But the great drawback to this class of boilers is that they must stand in the midst of the workmen; so that, although they are not more liable to explode than any other form of boiler, yet when they do burst they necessarily endanger more lives than is usually the case with other boilers that can be placed more away from the men employed at the works. Should anything arise with the boiler to make it desirable to withdraw the fire, this cannot be done without much delay, as the furnaces have to be stopped and the iron run out. Also an explosion can hardly happen without some of the melted iron being scattered among the men at work. [Illustration: _Fig. 45._] [Illustration: _Fig. 46._] Some of the most fatal explosions of these boilers have arisen from careless construction. Such was the case in an explosion at Dudley in 1862, shown in Fig. 45, where the crown plate forming the top of the centre tube was attached to the sides of the tube by so slight an angle iron, as shown enlarged in Fig. 46, that the pressure of steam on the flat crown plate fairly sheared the angle iron through, and allowed the plate to be blown down the centre tube into the chimney flue, whereupon the boiler was violently thrown off its seating by the reaction of the issuing steam and water thus liberated. * * * * * The double-tube horizontal boiler is also used in connection with iron-making furnaces in many places, one furnace working into each tube. Although by this arrangement the boiler can be placed a little further from the workmen, some very fatal explosions have happened to such boilers, as at Masborough in 1862. * * * * * [Illustration: _Fig. 47._]