necessary to expel one-half the water that entered the boiler,—say at a temperature of 250-the remaining half of the water being converted into steam and utilised. Now, because the sensible temperature of the steam is the same as that of the water, it must not be supposed that one-half the value of the fuel has been lost by this process. It must be taken into account that the steam has absorbed as much heat as could, if a change of state had not taken place, have raised its temperature to 1212°; therefore, the loss incurred from priming the boilers and the prevention of deposit would be found only to be about 12 per cent. of the fuel used. Centrifugal pumps, driven by engines of about 40 horse-power, will throw 60 tons of water per minute. From Mr. Froude's Trials in "Greyhound:" "The power actually indicated throughout the towing trials on an average realized from 33 to 42 per cent. of the power expressed in indicated horse-power, at trial trip of the engines for the same speed of ship." "A considerable portion of this loss of power is said to be due to the action of the propeller diminishing the water pressure against the after part of the vessel." N.B. This action causes an increase of resistance when the propeller is allowed to revolve freely, as compared with fixing it up and down for sailing purposes. USEFUL NOTES ON COALS. Vide Main and Brown. As much as possible avoid breaking the coal in its transit to the coal bunkers; weigh a basket or bag frequently to estimate total weight received. Coal that has been Exposed to the Atmosphere. The power for raising steam is reduced one half, after lying in the open air for six months, by disintegration and loss of heating properties. The destruction of coal by exposure depends on its ability to absorb oxygen, converting the hydro-carbons into water and carbonic acid. Combustion of Coal The gas which escapes from the coal during the first process of combustion, will, if it be supplied freely with oxygen, and its temperature be sufficiently high, guite in our tire places and furnaces. Coal containing a great quantity of hydrogen is called bituminous. NEWCASTLE COAL is of this character. CANNEL COAL is more particularly adapted for gas manufactories on this account. WELSH COAL, or Anthracite, is coal containing but little hydrogen; its heating power is considerably greater than bituminous coal, cause a great quantity of the heat in the former case is employed verting a portion of the solid matter into a gaseous form, and ently heat is absorbed by this process. For the same reason, om which the bituminous matter has been previously expelled, able for furnaces where great local heat is required, besides is smokeless. Ish coal, when good, partaking of this character, is preferred in war ships and deets, the localized heat derived being more 1 beneficial in our present form of boiler with its necessarily contracted combustion chamber, within which the gasses mix, but Welsh coal long in store or on board ship loses its cohesion, and becomes small, the waste is then very great, unless a more lively bituminous coal can be mixed with it; from this cause a mixed coal is better adapted as a general rule, and an attempt is made to properly supply the furnaces with oxygen to prevent the smoke from the bituminous portion of the coal. Usual mixture,-one-third North Country, two thirds Welsh. To find the Co-efficient of Speed. Suppose Agincourt and Northumberland running equal distances, or equal time,—say 4 hours,-the Agincourt makes 5821 revolutions, and the Northumberland 5962; then, taking Agincourt's co-efficient as 1, the Northumberland's would be 1.024. Therefore, for practice, if Northumberland is required to go the same speed as Agincourt-say, at 40 revolutions,-she would have to go 40 x 1024-41 (nearly) to keep station. RULE. Take the number of revolutions in a given time as a divisor into the number of revolutions for the same time in any other vessel-the quotient is the co-efficient. Another approximate method is obtained by dividing the pitch of screw of any vessel by the pitch of the screw of flag-ship or leading vessel, the result being the co-efficient, which is used as before. In paddle-wheel vessels the circumference of the wheel divided by 101 will give a constant, which, if multiplied by the number of revolutions per minute, will give the speed of wheel in knots per hour. This 101 is obtained by dividing the feet per knot, 6080, by 60=101.33. |