essays-1-第78章
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ual quantities or weights of them to any given temperature； or any given number of degrees； it will be necessary to determine how much of each of the ingredients is employed in forming any given quantity of dough。
Now； in the foregoing experiments； as 1102 loaves of bread were formed of 1736 lbs。 of rye meal； it appears； that there must have been 1。47 lb。 of the meal in each loaf； and as these loaves weighed 2 1/2 lbs。 each when they were put into the oven； each of them must； in a state of dough； have been composed of 1。47 lb。 of rye meal； and 1。03 lb。 of water。
Supposing these loaves to have been at the temperature of 55 degrees of Fahrenheit's Thermometer when they were put into the oven； the heat necessary to heat one of them to the temperature of 212 degrees； or the point of boiling water； may be thus computed。
By an experiment； of which I intend hereafter to give an account to the Public； I found； that 20 lbs。 of ice…cold water might be made to boil； with the heat generated in the combustion of 1 lb。 of dry pine…wood； such as was used in baking the bread in the six experiments before mentioned。 Now； if 20 lbs。 of water may be heated 180 degrees； （namely from 32 to 212 degrees；） by the heat generated in the combustion of 1 lb。 of wood； 1。03 lb。 of water may be heated 157 degrees； （from 55 degrees； or temperate； to 212 degrees；） with 0。4436 of a pound of the wood。
Suppose now that rye meal contained the same quantity of absolute heat as water；as the quantity of meal in each loaf； was 1。47 lb。； it appears； that this quantity would have required； （upon the above supposition；） to heat it from the temperature of 55 degrees； to that of 212 degrees； a quantity of heat equal to that which would be generated in the combustion of 0。06405 of a pound of the wood in question。
But it appears； by the result of experiments published by Dr。 Crawford； that the quantities of heat required to heat any number of degrees； the same given quantity （in weight） of water and of wheat； （and it is presumed； that the specific or absolute heat of rye cannot be very different from that of wheat；） are to each other； as 2。9 to 1；water requiring more heat to it； than the grain in that proportion： Consequently； the quantity of wood required to heat from 55 to 212 degrees； the 1。47 lb。 of rye meal which entered into the composition of each loaf； instead of being 。06405 of a pound； as above determined； upon the false supposition that the specific heat of water and that of rye were the same； would； in fact； amount to no more than 0。02899； for 2。9 （the specific heat of water） is to 1 （the specific heat of rye）； as 0。06405 is to 0。02899。
Hence it appears； that the wood required as fuel to heat （from the temperature of 55 degrees to that of 212 degrees） a loaf of rye bread （in the state of dough）； weighing 2 1/2 lbs。； would be as follows； namely：
Of pine…wood； To heat 1。03 lb。 of water； which enters into the composition of the dough； 。。 0。04436
To heat the rye meal； 1。47 lb in weight； 。。 0。02899 Total； 0。07335 lb。
To complete the computation of the quantity of fuel necessary in the process of baking bread； it remains to determine； how much heat is required； to send off in steam； from one of the loaves in question （after it has been heated to the temperature of 212 degrees）； the 10 1/2 loths； equal to 21/64 of a pound of water； which each loaf is known to lose in being baked。
Now it appears； from the result of Mr。 Watt's ingenious experiments on the quantity of latent heat in steam； that the quantity of heat necessary to change any given quantity of water ALREADY BOILING HOT to steam； is about five times and a half greater than would be sufficient to heat the same quantity of water； from the temperature of freezing； to that of boiling water。
But we have just observed； that 20 lbs。 of ice…cold water may be heated to the boiling point； with the heat generated in the combustion of 1 lb。 of pine…wood； it appears therefore that 20 lbs。 of boiling water would require 5 1/2 times as much； or 5 1/2 lbs。 of wood to reduce it to steam。
And if 20 lbs。 of boiling water require 5 1/2 lbs。 of wood； 21/64 of a pound of water boiling hot will require 0。09023 of a pound of wood to reduce it to steam。
If now； to this quantity of fuel； 0。09023 lb。 we add that necessary for heating the loaf to the temperature of boiling water； as above determined； 0。07335 lb。 this gives the total quantity of fuel necessary for baking one of these loaves of bread； 0。16353 lb。
Now as these loaves； when baked into bread； weighed 2 lbs。 5 1/2 loths = 2 11/64 lbs。 each and required； in being baked； the consumption of 0。16353 of a pound of wood； this gives for the expence of fuel in baking bread 0。07532 of a pound of pine…wood to each pound of rye bread； which is about 13 1/4 lbs。 of bread to each pound of wood。
But we have seen； from the results of the before…mentioned experiments； that when the bread was baked under circumstances the most favourable to the economy of fuel； no less than 80 lbs。 of pine…wood were employed in heating the oven to bake 304 lbs。 of bread； which gives less than 4 lbs。 of bread to each pound of wood； consequently； TWO THIRDS at least of the heat generated in the combustion of the fuel must； in that case； have been lost； and in all the other experiments the loss of heat appears to have been still much greater。
A considerable loss of heat in baking will always be inevitable； but it seems probable； that this loss might； with proper attention to the construction of the oven； and to the management of the fire； be reduced at least to one half the quantity generated from the fuel in its combustion。 In the manner in which the baking business is now generally carried on； much more than three quarters of the heat generated； or which might be generated from the fuel consumed； is lost。
APPENDIX； No VIII。
The following Account of the Persons in the House of Industry in Dublin； the 30th of April 1796； and of the Details of the Manner and Expence of feeding them； was given to the Author； by order of the Governors of that Institution。
Average of the Description of Poor for the Week ending 30th of April 1796。 Males。 Females。 Total。 Employed 74 352 426 Infirm and Incurable 172 585 757 Idiots 16 13 29 Blind 5 10 16 … … 267 960 1227 In the Infirmary。 Sick Patients； Servants； etc。 88 200 ｝ ｝ 343 Lunaticks 15 40 ｝ Total 1570
Employed at actual labour 322 Persons。 Ditto at menial offices 104 ditto … Total 426
Amongst the 1570 Persons above mentioned； are 282 Children and 447 compelled Persons。
Of the Children； 205 are taught to spell； read； and write。
Saturday； April 30； 1796。
1227 Persons fed at Breakfast。
120 Servants in New…House； a 8 oz。 bread 60 ｝ lbs。 loaves lb。 value。 336 Incurables； Children； etc。 ｝ 186 is 41 1 1/2 L。 1 14 a 6 ditto 126 ｝ 771 Workers； etc。 got Stirabout。 … 1227
Weight of meal for Stirabout 4 cwt。 costs L。 3 1 8
120 Servants in New…House ｝ get 1 quart butter…milk Gal。 P。｝ each 30 0 ｝ 167 gallons of 1084 Workers； Incurables； etc。 ｝ butter…milk 1 pint ditto 135 4 ｝ value 1 L。 23 Sucklers get no butter…milk ｝ … Allowed for waste 1 4 ｝ 1227
Brought down； L。 5 15 8 s。 d。 Fuel to cook the Stirabout； 3 bush。 cost 2 3 ｝ ｝ 0 3 0 1/2 Salt for ditto； 1 qr。 3 lb cost 0 9 1/
