332 



pressed tightly together. The air is removed by means of the 

 common exhausting syringe, and it is found that the two 

 adhere together with such force that two strong men cannot 

 pull them asunder. But, if the tap be turned, and air 

 admitted, they come apart without the least difficulty. 



Similarly, if two plates of glass or metal be ground to 

 exactly plane surfaces, and pressed together, they adhere 

 nearly as strongly as if they were one solid piece. 



WE will now turn from Art to Nature, and examine some 

 natural producers of vacuum. 



One of the most celebrated is that series of suckers which 

 may be found upon the arms of the various Cuttles. At the 

 upper part of the illustration a figure is given of part of an 

 arm, on which are four suckers. When the animal wishes to 

 attach itself to any object, it presses the disc of the sucker 

 against it, and simultaneously withdraws the centre, exactly as 

 the boy does with his toy sucker. And, as each arm contains 

 a great number of suckers, it is evident that the holding power 

 must be very great. Indeed, on one occasion when a com- 

 paratively small specimen had fastened on a man's arm, he 

 could not remove it, but was obliged to have it cut away piece- 

 meal by an assistant. 



The common Water-beetle has similar suckers upon its first 

 pair of feet, and can adhere to smooth surfaces with great 

 tenacity. 



ON the left of the cuttle-arm is the common Limpet, shown 

 as it appears when adhering to the rocks. Every visitor to 

 the seaside who has attempted to remove the Limpets may 

 remember how difficult it is to stir them when they have once 

 taken their hold. If they can be taken by surprise, they come 

 away with a touch ; but if they become alarmed, they press the 

 edges of the foot firmly against the rock, withdraw the centre, 

 and thus create the necessary vacuum. 



NEXT follows a Star-fish, shown as it appears when in the 

 act of walking, or rather, gliding along. 



This movement is obtained by the use of a vast number ol 

 long puckers, exactly resembling the pneumatic peg, except 



