strength to resist as much strain as the bar itself. Although these precautions may appear unimportant, and unfortunately are often so regarded, yet it is upon just such details as these that the lives and the safety of every locomotive runner, fireman and others near them are constantly dependent. QUESTION 113. _How much water is usually carried in a locomotive boiler?_ _Answer._ There must always be enough water in the boiler to cover completely all the parts which are exposed to the fire, otherwise they will be heated to so high a temperature as to be very much weakened or permanently injured. In order to be sure that all the heating-surface will at all times be covered with water, it is usually carried so that its surface will be from 4 to 8 inches above the crown-sheet. QUESTION 114. _How much space should there be over the water for steam?_ _Answer._ No exact rule can be given to determine this. It may, however, generally be assumed that the more steam space the better. In order to increase the steam room, locomotive boilers are very generally made in this country with what is called a _wagon-top, C_, fig. 41, that is, the outside shell of the boiler over the fire-box is elevated from 4 to 12 or even 18 inches above the cylindrical part. QUESTION 115. _What is a steam-dome and for what purpose is it intended?_ _Answer._ A _steam-dome_, _X_, fig. 41, is a cylindrical chamber made of boiler-plate and attached to the top of the boiler. Its object is to increase the steam room and to furnish a reservoir which is elevated considerably above the surface of the water, from which the supply of steam to be used in the cylinders can be drawn. The reason for drawing the steam from a point considerably above the water is that during ebullition more or less spray or particles of water are thrown up and mixed with the steam. When this is the case, steam is said to be _wet_, and when there is little or no unevaporated water mixed with it it is said to be _dry_. It is found by experience that wet steam is much less efficient than that which is dry. There is also danger that the cylinders, pistons or other parts of the machinery may be injured if much water is carried over from the boiler with the steam, because water will be discharged so slowly from the cylinders that there is not time for it to escape before the piston must complete its stroke, so that the cylinder-heads will be “_knocked out_,” or the cylinder itself or the piston will be broken. The reason for drawing or “_taking_” steam from a point considerably above the water is because there is less spray there than there is near the surface, and the hottest steam, which is also the dryest, ascends to the highest part of the steam space. QUESTION 116. _Where is the dome usually placed?_