154 STATE BOAED OF AGEICULTUEE. 



Thus yon see that even simple elementary portions of machines not only are of 

 much importance, but they have been subjected to such close analysis that exact 

 methods for shaping and proportioning them have been discovered. Though 

 time permits no farther notice of shape and form of parts, science, in her appli- 

 cation does not stop here, but extends her analysis and scrutiny to every piece, 

 not overlooking the smallest screw or slighting the largest Avheel. These points, 

 unimportant as they may seem, are of vital interest so far as the working of the 

 machine is concerned ; and farther, these are not the only points apt to be mis- 

 shaped by the workmen, but simple examples presented to show the degree of 

 accuracy that is needed to make a machine work perfectly. 



Another thing : you should always closely examine the material of which a 

 machine is nuide. If of wood, satisfy yourself that it is made of varieties fitted 

 to withstand the strains to which they will be subjected, and be sure that each 

 piece is well seasoned and perfectly sound. Too much can not be said against 

 the use of green or unseasoned timber. It shrinks in drying, utterly destroying 

 all appearance of good workmanship, making the joints loose, and rendering 

 the machine rickety and weak. Covering green timljer with paint, is like filling 

 a pit by covering with rushes and a few inches of dirt : it is a pitfall for the 

 unwary. It conceals, but does not remedy defects. On the contrary, it pre- 

 vents proper seasoning of the wood, and dry rot is the inevitable consequence. 

 Your piece fails when least expected, and when most needed ; perhaps a run- 

 away and a broken neck are the dire consequences ; but whatever they may be, 

 you can not too carefully avoid the parricidal use of green timber for important 

 pieces of machinery. 



Iron of various kinds is now more commonly employed in machinery than 

 wood, and from its greater strength and durability this change is attended with 

 many advantages. Iron, however, is not free from defects, and each piece 

 should be carefully examined for flaws or other failings. In general, the use of 

 cast iron in machinery should be condemned. As it usually comes from our 

 foundries it is very hard and brittle, and is entirely useless for any positions 

 requiring much strength either to withstand vibrations or tensions. 



Since cast iron is used in a great many cases, it is well to have some means of 

 testing it. Engineers on public works use the following tests : When broken, 

 the surface of the fracture should be of a light bluish-grey color, and close- 

 grained texture, with considerable metallic luster. Both color and texture 

 should be uniform, except near the skin, or surface, the color may be somewhat 

 lighter, and the grain closer. If the fractured surface is mottled, either -with 

 patches of darker or lighter iron, or with crystalline spots, the casting will be 

 imsafe. It will be still more unsafe if it contains air bubbles. As to appearance 

 of good iron, it should have on the outer surface a smooth, clear, and continu- 

 ous skin, with regular faces and sharp angles. The iron should be soft enough 

 to be slightly indented by a blow of a hammer on an edge of the casting. Cast- 

 ings are tested for air bubbles by ringing them with a hammer all over the 

 surface. 



Objects and Designs of Machinery. 



Soliciting agents meet you on every side, and claim the preference foi", and 

 endeavor to show how much benefit you will obtain, by buying machines whose 

 vaunted capabilities cover every known qualification. You are shown single 

 machines that will churn, wash, tend baby, sweej:), pare apples, pare potatoes, 

 mop, hem, tuck, broider, sew, knit, and everything else you can think of, for a 



