54 



FORM OF GREENHOUSES. 



and indeed we find that tliis subject had attracted the attention of the 

 celebrated Boerhaave. Phihp Miller applied it to plant stoves, and 

 Williamson, Knight, the late Sir George M'Kenzie, and others, have also 

 directed their attention to the subject. The " Horticultural Transactions," 

 and the Encyclopsedia of Gardening," contain a variety of opinions upon 

 this subject ; but so far as we know, few hot houses have been erected 

 with much attention to the nicety of these theories. 



Most hot houses are built to about an angle of forty five degrees or 

 thereby, and we hear of few complaints against such structures. Indeed, 

 this angle appears to be the least that will effectually drain off the 

 water, and appears to us to be exceedingly well calculated for general 

 purposes. 



The follovdng has been laid down as data to determine the angles of 

 the roofs of hot houses by Wilkinson, in the Horticultural Transactions, 

 vol. 11. p. 237: "The angle contained between the back wall of the 

 forcing house and the inchned plane of the glass roof, always equals the 

 sun^s altitude when his rays fall perpendicularly on that plane, provided 

 that the inclination of the plane to the horizon be at an angle not less 

 than twenty eight degrees, two minutes, nor greater than seventy five 

 degrees. Within the above hmits, the sun's rays are perpendicular 

 twice in the year, once in going to, and once in returning from, the 

 tropics." Hence, then, having determined in what season we wish to 

 have the most powerful eflfects from the sun, we may construct our 

 houses according to the following rule : Make the angle contained 

 between the back wall of the house and its roof, equal to the com- 

 pliment of latitude of the place, less or more the sun's declination 

 for that day on which we wish his rays to fall perpendicularly. From 

 the vernal to the autumnal equinox, the dechnation is to be added, 

 and the contrary. Thus, to apply those principles to the slope of 

 roof recommended by Knight, for ripening grapes in July, we have, 

 say at London, 



Latitude of London . . . 51° 29' 



Sun's declination on the 21st of July . 17° 31' 



33° 58' or 34° nearly. 



As we want the genial warmth of the sun most in spring, therefore, 

 for general purposes, that construction would perhaps be best which 



