14 



this leads me to observe, that no one having green- 

 houses or stoves should fail to have them insured by 

 the " Hailstorm Insurance Company." Good glass 

 is of little value unless kept clean, and for this pur- 

 pose it should be cleansed on both sides twice anuu- 

 ally, early in February and October, and on the out- 

 side only in June. 



The angle formed by the glass roof of the hothouse 

 is of very considerable importance, because rays of 

 light are reflected in proportion to the obliquity with 

 which they fall upon any given surface ; those which 

 fall upon it perpendicularly from the source of light 

 pass through with very slight diminution, but those 

 falling upon it in a slanting or oblique direction are 

 reduced in number in proportion to the obliquity of 

 that direction. To ascertain how a glass roof may 

 be constructed, so as to receive the greatest number 

 of rays of light from the sun perpendicularly, or near 

 to perpendicularity at any given time of the year, it 

 is necessary to know the latitude of the place where 

 the hothouse is erected, and the sun's declination at 

 the period when most light is required. The latter 

 information may be obtained from most almanacks, 

 and if it be subtracted from the latitude, the remain- 

 der will be the angle desired. If London be the 

 place, and May the 6th the time about when the most 

 light is desired, the latitude being 51 degs. 31 min., 

 and the suu's declension then 16 degs. 36 min. north, 



