ROO 



times attached, for the purpose of 

 watering or sprinkhng fluid manures 

 over the young plants. As tiiey are 

 rolled, the man who leads the horse 

 manages the watering by a string at- 

 tached to the end of the lever (A), 



ROO 



which, being raised, lets the fluid out 

 through the small holes (c). The 

 cylinder, or roller, is also set with 

 scarifiers and other contrivances for 

 cutting or crushing the soil, as in the 

 clod crusher {Fig. 3) In some imple- 



ments the roller is of a conical form, 

 or of the figure of two cones, united 

 at their bases for the purpose of run- 

 ning between furrows or ridges ; they 

 are also made of a series of separate 

 rings or wheels which run between 

 drills (drill rollers), and may be used 

 in marking the ground before sowing 

 seeds 



ROMAN VITRIOL. Blue vitriol, 

 sulphate of copper. 



ROOD. A square measure, equal to 

 40 perches or square poles. The 

 fourth part of an acre. 



ROOF. The covering of a build- 

 ing. The pitch, or inclination, should 

 be much greater in northern posi- 

 tions than at the south, since snow 

 and rains tend to rot the timbers 

 more rapidly. Those which are cov- 

 ered with shingles must also he 

 more pitched than those of slate or 

 metal. 



" A roof, as respects its construc- 

 tion, involves some knowledge of 

 mathematics Of the general princi- 

 ples on which its proper construction 

 666 



depends, we shall here subjoin some 

 account. The obvious mode of cov- 

 ering a building, where a greater or 

 lesser inclination of the sides of the 

 roof is required by the climate, is to 

 place two sloping rafters, C C, upon 

 the walls, B B, as in 

 the subjoined diagram 

 (Fig-. 1), meeting at 

 the apex, A , where we 

 will suppose them so 

 connected with a 

 hinge as to be insep- 

 arable, but capable of r)l 

 descending by their 

 gravity, as shown in 

 No. 2. The walls are 

 considered as solid ''■'' 



masses, moveable on points, P. If 

 the walls be not of sufficient weight, 

 the thrust that will be thus exerted 

 on them by the tendency of the raf- 

 ters to spread at their feet will throw 

 the walls out of an upright, as in No. 

 2, and the whole assemblage will be 

 destroyed. By the laws of mechan- 

 ics, it is known that the horizontal 



