506 SCIENCE OF AGRICULTURE. Part II. 



of large streams, arches of masonry. In the case of small drains, wooden pipes or 

 boarded tubes are sometimes resorted to, and even earthen pipes have been used ; but 

 masonry should generally have the preference. 



BOOK V. 



OF THE OPERATIONS OF AGRICULTURE. 



3111. The operations of agriculture are effected under the direction of man, and by 

 means of the mechanical agents, or implements and buildings, which have passed in 

 review in the preceding book. They are either performed directly on plants or animals, 

 which may be considered the objects of agriculture ; or on the soil and climate, which 

 are the natural agents of growth and culture. They may be arranged as manual labours 

 and operations, operations with beasts of labour, and mixed operations. 



Chap. I. 

 Manual Labours and Operations. 



3112. The labours and operations of any art can seldom be described with great 

 advantage. Whoever wishes to acquire them should resort at once to the scene of 

 practice : no description, however minute, will teach a man to dig, plough, or mow, 

 equal to a few hours' trial in the field, though a knowledge of the mechanical principles 

 on which the implements and the human machine act in sucl^ operations, will aftbrd 

 some assistance in acquiring them, and in performing them with ease. Our observations 

 shall chiefly be directed to these parts of the subject, and to the most suitable weather 

 and other circumstances for the performance of the different field labours of the manual 

 kind. We shall arrange these as manual labours common to all arts ; manual operations 

 on the soil ; and mixed manual operations, or such as are performed on the soil, plants, 

 and animals, together or connectedly. 



Sect. I. Mechanical Operations common to all Arts of Manual Labour. 



3113. All the operations which man performs ivith imjdements or machines are, as far as 

 his own person is concerned, reducible to lifting, carrying, drawing, and thrusting. 

 Man himself, considered as an engine, derives his power from alteration in the position 

 of the centre of gravity, and he applies it chiefly by his hands, arms, and legs acting as 

 levers of the third kind. 



3114. Lifting is performed by first stooping, or lowering the centre of gravity, and at 

 the same time throwing it to one side. The object being then laid hold of by the hands, 

 the body is raised, and the centre of gravity, in being restored to its true position, acts as 

 a counterbalancing weight to the weight to be raised. The weight retained by the hand 

 is now raised a certain height, never exceeding half that of the man ; if to be raised 

 higher, recourse is had to muscular strength, or the power of the arms to act as levers. 



3115. Carrying. To carry a thing is merely to walk with q^^reater weight than 

 before, and walking is performed by a series of alternate derangements and adjustments 

 of the centre of gravity, slow or rapid, as the person may walk or run. According to 

 Delolme, the most advantageous weight for a man of common size to carry horizontally is 

 112 lbs. ; or, if he returns unladen, 135 lbs. 



3116. Drawing. In this operation, the upper part of the body is thrown forward, so 

 as to act as a power to counterbalance or lift up the body or weight to be moved ; and 

 by joining to this lifting motion the operation of walking, the weight is at once lifted up 

 and drawn along. This compound operation is exemplified in a horse, when straining 

 at a draught in a plough or cart: he first lowers his chest, than raises it, and lastly 

 steps forward. When drawing at ease, the lifting motion is scarcely distinguishable 

 from the progressive one. 



3117. Pushing, or thrusting, is performed exactly on the same principles as drawing, 

 and differs from it chiefly in the kind of implement or machine which requires to be 

 employed : all machines which are to be pushed requiring to be attached to the animal 

 machine by parts acting by their rigidity ; whereas those to be drawn may be attached by 

 parts acting by their tenacity merely. 



3118. Wheeling is a mode of carrying materials in which the weight is divided between 

 the axle of the wheel and the arms of the operator. The arms or shafts of the barrow 

 thus become levers of the second kind, in which the power is at one end, the fulcrum 



