260 SCIENTIFIC ILLUSTRATIONS [Par 



qualities, may be taken by itself (as . theine, not as tea) without 

 any appreciable effect. The water which will allay our burning 

 thirst augments it when congealed into snow ; so that Captain 

 Ross declares that the natives of the Arctic regions prefer 

 enduring the utmost extremity of thirst rather than attempt to 

 remove it by eating snow. Yet if the snow be melted, it be- 

 comes drinkable water ; and it is melted in the mouth ! 

 Nevertheless, although, if melted before entering in the mouth 

 it assuages thirst like other water, when melted in the mouth, 

 it has the opposite effect. To render this paradox more striking, 

 we have only to remember that ice, which melts more slowly 

 in the mouth, is very efficient in allaying thirst. L. P. 



The Parallax Movement. 



Parallax movement is that apparent shifting of bodies which 

 arises from changing our own position. We cannot stir a step 

 without producing examples of it. If we pace up and down 

 the street opposite to any object on the other side, as a door 

 or a lamp-post, the angular direction or parallax of the, object 

 changes at every moment. If we sail down a river, and fix 

 our eyes on some church spire at a distance from its bank, we 

 find that the direction in which we see it is always altering. 

 At first the spire appears in advance of us, then to our side, 

 and lastly it lies behind. If, instead of limiting our attention 

 to one object, we look at several that can be easily observed 

 together, we find that as we move they move, or rather seem to 

 move, and the angles formed by their lines of direction are 

 changed relatively to each other and to us. In these instances 

 of this parallax shifting it must have been remarked that the 

 effect of a change of our position in altering the direction of 

 objects is greater when they are near than when they are dis- 

 tant. A few paces will sensibly alter the angular position or 

 direction of the door or lamp-post, or the opposite side of the 

 street. But if we look at a church some miles off, or at ships 

 anchored in the offing, we find that we require to move much 

 more than a few paces in other words, the length of the bases 

 needs to be considerably increased before we can make any 

 sensible change in the angle or direction in which we see them. 



