366 



On Heating by Hot Water, 



that the motion of water within them being retarded by friction in 

 a much greater degree than in large pipes, they can never be 

 brought to so high a mean temperature. So that, under similar 

 circumstances of pressure, &c, 200 feet of one inch pipe could 

 never be made to produce the same effect as 50 feet of four inch, 

 though their surfaces would be nearly equal; besides which the 

 original expense of the one inch pipe would be nearly three times 

 that of the four-inch. 



A little consideration will enable us to determine whether such 

 rapid communication of heat be really essential to the efficiency of a 

 heating apparatus. In hot-houses, where permanent heat is required, 

 it is evidently unnecessary. The only place where it may be desirable 

 is in buildings where occasional heat only is employed. Now if any 

 one will take the trouble to note hourly the variations of the ther- 

 mometer by night, in weather in which frost is so severe as to be 

 dangerous, they will find that, instead of a sudden jump of 10° or 

 20 , the th ermometer begins to fall slowly an hour beforesunset, 

 somewhat more rapidly afterwards, and continues falling steadily 

 till about 11 p.m. After that time it falls still more slowly till 3 or 

 4 a.m., by which time it will have almost reached its minimum. Its 

 variation will be something like three or four degrees per hour for 

 the first four hours, after that about one degree per hour for the next 

 two or three, and then from 1 to ^ of a degree till it has reached its 

 minimum. Now it is evident that to meet this variation, supposing 

 the temperature of the house to range exactly with outer air, an 

 apparatus which occupies three or four hours in reaching its 

 maximum, would be much more accurately adapted to the 

 emergency than one which could be heated in an hour. But we 

 may observe that, except in iron-roofed houses, that the temperature 

 within the house does not keep pace with that of outer air, but 

 falls much more slowly, owing to the specific heat contained in the 

 objects within the building, which is gradually transmitted by the 

 roof, so that in fact the necessity for rapid heating, even in green- 

 houses, is really less than at first sight appears. 



