Pleasant Ways in Science. 273 



velocity, and out of tliis intense development of motion a 

 sudden neat ensues. 



Heat, magnetism, and electricity are ceaselessly occupied 

 in generating motion, so that no substance we are acquainted 

 with is absolutely still. As a mass it may be at rest ; that is, 

 it may only partake of its necessary share of the common 

 motion of the globe and the system to which it belongs ; but 

 its molecules are never quiet. The least change of tempera- 

 ture moves them more or less, the least change of position 

 places them in a different relation to the magnetic axes of the 

 earth, and then again a change is produced, at any rate, in 

 most bodies. Every house affords an illustration of the way 

 in which internal motious occur in substances that might be 

 thought free from detrimental disturbance. Bell-wires become 

 rotten because the particles of the copper have rearranged 

 themselves in a new form, by which cohesion is lessened ; and 

 iron has a tendency to grow brittle, apparently under the 

 influence of continued concussions, though this is not per- 

 fectly clear. A piece of glass tube might be thought a settled 

 thing, so far as its internal structure is concerned, but thermo- 

 meter makers tell us that if newly-made tubes are exactly 

 graduated, sufficient changes are likely to occur in the course 

 of a few months to affect the accuracy of the instrument. 

 Metallic substances, such as gold and German silver, are 

 employed to make the vacuum chambers used in the construc- 

 tion of aneroid barometers, and these, too, are subject to mole- 

 cular motions, which change the elastic power of their delicate 

 walls, and no one has yet arrived at the art of making these 

 vacuum chambers so as to insure this action being so small as 

 to have no practical effect in lessening their accuracy. Those 

 which stand tests for six or more months are likely to remain 

 good ; but a new instrument, good to-day, may be worth little 

 next year. 



From the internal motions to which all bodies are subject, 

 it is very difficult to make a good standard measure of length, 

 and such a standard can only be perfectly right at the exact 

 temperature to which it was adjusted. Instruments have been 

 contrived by which motions of expansion and contraction can be 

 measured to infinitesimal portions of an inch, and by which 

 the exact length of any object can be taken, or the minutest 

 deviations from a true plane surface detected. As a specimen 

 of this class of instrument we may mention a jolanometer, and 

 our description is taken from one constructed by Mr. Browning. 

 An aluminium circle stands upon three legs, arranged at equi- 

 distant points of its circumference, and of precisely the same 

 length. In the centre of the circle is another leg, which can 

 be elevated or depressed by a delicate screw, and the extent 



VOL. VIII. NO. IV. T 



