170 GREENHOUSE CONSTRUCTION AND HEATING. 
water boils—-and any farther access of heat is carried 
off in steam, but even this point ought seldom or 
never to be reached, a temperature of 160° to 180° 
being safer and better than a higher range, especially 
where delicate ferns, etc., are cultivated, very hot 
pipes having the effect of unduly dessicating and 
scorching the air. 
It must be borne in mind that the aspect and position 
of a greenhouse of any kind, as well as its construction, 
and the local climate, all have their effect on the average 
temperature, and ought to be taken into consideration 
when calculating the amount of piping required. A house, 
or houses, of whatever class, built on a warm southern 
slope fully exposed to the sun, and sheltered on the north 
and east, will not require so much artificial heat as another 
standing on very bleak or open ground, or in a comparatively 
low, damp, or sunless spot, in order to secure the same 
temperature. Climate also varies considerably, that of 
most places as far north as Yorkshire or Lancashire being 
naturally much colder than the average of Devonshire or 
Cornwall, especially near the coast, where frost never 
appears to penetrate or possess so much power as in 
inland localities. But in all cases it is wise to be on the 
safe side, and provide rather more piping and heating 
power generally than is actually necessary rather than 
too little. 
Lastly, unless the air can circulate freely round the 
heating pipes in all directions they cannot radiate their 
warmth as freely as they would otherwise do. Thus 
pipes confined in a trench or pit cannot be reckoned as 
equally effective with those standing free all round, 
