15 



therefore the roof ought to slope at an angle of 34 

 (legs. 55 min. 



In latitude 52 degs., Mr. Knight found from length- 

 ened experiments, that the hest angle is about 34 

 degs., considering the services of a hothouse through 

 the year ; and to illustrate this, he gave the preceding 

 diagram. About the middle of May, the elevation 

 of the sun at noon corresponds nearly with the 

 asterisk, a ; in the begining of June, and early in 

 July, it will be vertical at b, and at midsummer at c, 

 only six degrees from being vertical. The asterisk, 

 d, points out its position at the equinoxes, and e its 

 position at midwinter. If the best glass be em- 

 ployed, it is an excellent plan to have it put double 

 in each sash, an interval of half an inch being left 

 between the two panes, and a small hole at the corner 

 of the inner one to prevent the glass being broken 



