to Health and Comfort . 
225 
1878.] 
posing iced water to be the cooling medium, and steam of 
low pressure to be the heating one, the relation of difference 
of temperature of the air to be cooled or heated to that of 
the iced water or steam is such that about the same extent 
of surface would be required in either case to transfer the 
heat. But a pound of coal produces, in ordinary steam- 
boilers, quite 9000 units of heat on the average, while a 
pound of ice (in cooling to 6o°, let us say) will produce only 
170°, so that about 54 lbs. of ice would be demanded to 
effect the transfer of an equal quantity of heat, to what 
would be effedted by 1 lb. of coal ; or, accepting the one- 
fourth cooling effedt, then 13^ lbs. of ice would be demanded 
for the cooling of air in summer against each pound of coal 
required for warming in winter. Unfortunately for the pro- 
position for cooling air in summer, even this statement is 
too favourable, for the requirement will be found that the 
air must not only be cooled, but must be divested of a por- 
tion of its moisture, unless the cooled air is deemed satis- 
factory in the form of a cloud of vapour. Air at 85° and 
70 per cent of humidity must be taken to be cooled to 70° 
and 70 per cent of humidity, and one and one-fourth times 
as much cold will be demanded to condense the vapour 
2*3 grains per cubic foot as that which is requisite to cool 
the air the 15 0 supposed. This leaves the ratio of ice 
needed to obtain a spring condition for the air on a hot day 
in summer to be that of 30 to 1 of coal usually demanded 
to heat air on a cold day in winter, or assuming that the air 
on such a day is so dry that no moisture should be removed, 
about 15 to 1. Our ideas of the necessity of civilised wants, 
as compared with civilised luxuries, scarcely yet reach high 
enough to warrant the expenditure of money to cool air 
under the circumstances stated. 
In view of the great cost of cooling air by ice, it has been 
proposed to cool it by mechanical means on a large scale. 
Air, if compressed, becomes sensibly hotter. In fadt the 
compression can be carried to the extent that the heat will 
ignite tinder, as the cigar-lighters of twenty or thirty years 
since bear witness. And it has been proposed to use large 
air-pumps which shall compress the air until its temperature 
is elevated sufficiently for it to give off heat to surfaces 
cooled by currents of water, at such temperature as water 
is to be had from streams or aquedudts in summer. This 
compressed air, when deprived of a portion of its heat, is 
then allowed to expand, and the result is a cooler air. This 
process has, in reality, much merit, and it is probable that 
the cost of producing cold in this way compares favourably 
VOL. viii. (n.s.) q 
