30 OBJECTS, ADVANTAGES, AND 



the thing and the place ; but if you wish to draw it as quickly as pos- 

 sible, and do not regard the loss of force, you must pull it obliquely, 

 by drawing- it in two directions at once. Tie a string to a stone, and 

 draw it straight towards you with one hand ; then, make a loop on 

 another string, and running the first through it, draw one string in each 

 hand, not towards you, but side-ways, till both strings are stretched in 

 a straight line : you will see how much swifter the stone moves than it 

 did before when pulled straight forward. Now this is proved, by ma- 

 thematical reasoning, to be the necessary consequence of forces ap- 

 plied obliquely: there is a loss of power, but a great increase of 

 Velocity. The velocity is the thing required to be gained in the third 

 eyelid, and the contrivance is exactly that of a string and a loop, moved 

 each by a muscle, as the two strings are by the hands in the case we 

 have been supposing. 



A third eyelid of the same kind is found in the horse, and called the 

 haw ; it is moistened with a pulpy substance (or mucilage) to take hold 

 of the dust on the eyeball, and wipe it clean off, so that the eye is 

 hardly ever seen with any thing upon it, though greatly exposed from its 

 size and posture. The swift motion of the haw is given to it by a gristly, 

 elastic substance, placed between the eyeball and the socket, and striking 

 obliquely, so as to drive out the haw with great velocity over the eye, 

 and then let it come back as quickly. Ignorant persons when this haw 

 is inflamed from cold and swells so as to appear, which it never does 

 in a healthy state, often mistake it for an imperfection, and cut it off: 

 So nearly does ignorance produce the same mischief as cruelty ! 

 They might as well cut off the pupil of the eye, taking it for a black 

 spot. 



If any quantity of matter, as a pound of wood or iron, is fashioned 

 into a rod of a certain length, say one foot, the rod will be strong in 

 proportion to its thickness ; and, if the figure is the same, that thickness 

 can only be increased by making it hollow. Therefore, hollow rods or 

 tubes, of the same length and quantity of matter, have more strength 

 than solid ones. This is a principle so well understood now, that 

 engineers make their axles and other parts of machinery hollow, and 

 therefore stronger with the same weight, than they would be if thinner 

 and solid. Now the bones of animals are all more or less hollow ; and 

 are therefore stronger with the same weight and quantity of matter than 

 they otherwise would be. But birds have the largest bones in propor- 

 tion to their weight ; their bones are more hollow than those of animals 

 which do not fly ; and therefore they have strength without having to 

 carry more weight than is absolutely necessary. Their quills derive 

 strength from the same construction. They have another peculiarity 

 to help their flight. No other animals have any communication between 

 the air-vessels of their lungs and the hollow parts of their bodies ; but 

 birds have ; and by this means they can blow out their bodies as we do 

 a bladder, and thus make themselves lighter when they would either 

 make their flight towards the ground slower, or rise more swiftly, or 

 float more easily in the air. Fishes possess a power of the same kind, 

 though not by the same means. They have air-bladders in their bodies, 

 and can puff them out, or press them closer, at pleasure : when they 

 want to rise in the water, they fill out the bladder, and this lightens 



