May 29, 1685.] 



♦ KNOWLEDGE • 



455 



is to say they must both touch the work, and yet pass 

 \\-itbout being foiced. The distauco between the points, 

 measured on a rule, gives, of course, the size of tlie work. 

 The tool is manifestly useful for ascertiiining the thickness 

 of a plate, block, i\:c. 



E.\. LXIV. — for taking inside measurements, the form 

 of calliper employed is that shown in Fig. 31, which con- 



Fig. 3-i. 



sists of two legs of flat steel cut to the shape indicated and 

 hinged at their thicker ends. ^Measurements are made by 

 causing the points which protrude outwards to pass into 

 the hole, ic, just, but quite, touching the opposite sides. 

 The disttmce between the points indicate, as before, the 

 dimensions sought. 



Fig. 35. 



Ex. LXV. — A form of calliper sometimes, but not 

 largely, used is the compound type illustrated in Fig. 1)5. 

 It embodies the features of the tools for external and in- 

 ternal measurement. The drawing explains itself. One 

 advantage claimed for this form is that both ends indicate, 

 at the same moment, the same measurement. But this 

 advantage is a very questionable one. It is apparent that 

 a tool which is frequently used is more likely to wear than 

 one which is not used so much. Now, external callipers 

 are much more frequently requisitioned than internal ones, 

 whence our compound callipers stand a good chance of being 

 worn much more rapidly at the end A than at the end B. 

 When that takes place the indications of the two ends are 

 no longer identical. 



Ex. LXYI. — There are a few other tools which will be 

 required occasionally, such as a square, T'^ci^'*''^) plane, 

 saw, ic, but they are so well known and so often u.sed that 

 nothing further need be said about them. 



I think I may now fairly suppose that our young 

 electrician is sufficiently well equipped with tools to enable 

 him to make a good start with the study of experimental 

 electricity, and I shall therefore turn his attention next to 

 the experiments themselves, and I trust that he will find 

 in them a source of knowledge of a character which will 

 set him thinking and interest him in the deductive 

 branches of the science. I do not, as I said at the 

 commencement of this series, purpose dilating upon 

 theories. I intend rather to simply enunciate such 

 laws and principles as are more generally accepted 

 as being correct, and then to set before the young elec- 

 trician such experimental demonstrations as he may be able 

 to repeat, and so make himself master of the subject. I 

 shall be greatly disappointed if it turns out that he does 



not learn to regard electricity as a subject full of interest, 

 full of attraction, and yet as a science whose possible apipli- 

 cations wo so fur know little about. If, in the course of 

 half a century it is possible to initiate and bring to its 

 present state of (relative) perfection the electric telegraph, 

 to startle the world with tlio teleplioue, and to illuminate 

 its dark places with the electric light, what, in fair reason, 

 ought the immensity of the future to bestow upon 

 posterity 1 



OTHER WORLDS THAN OURS. 



A WKEK'S CONVEKSATION ON TITF PLURALITY OF 

 WORLDS. 



By Mons. de Fontenelle. 



with notes uy richard a. proctor. 



THE SIXTH EVENING {continued from p. Ml). 



" Ij^OR my part," says I, " I am such a foe to a perfect 



JL equality, that I cannot even allow all the turns 

 which the earth every day makes on herself should bo pre- 

 cisely in twouty-four hour.s, and always equal one to another; 

 I should be very much inclined to think that there are 

 variations." 



" Variations!" cried the lady; "why, do not our pen- 

 dulums mark an entire equality 1 " 



" O," says I, " to your pendulums I must object, for they 

 cannot be altogether just; and sometimes when tlioy are, 

 ill showing us that one circuit of twenty-four hours is 

 longer or shorter than another, we should rather be inclined 

 to believe them irregular than to suspect the earth of any 

 irregularity in her revolutions. What a complaisant 

 respect is this we have for her ! I would no more depend 

 on the earth than on a pendulum ; and the very same casual- 

 ties almost which will disorder the one, will make the other 

 irregular ! Only, I believe there must be more time allowed 

 for the earth, than a pendulum, to be visibly put out of 

 order ; and that is all the advantage we can give on her 

 side. But might she not by degrees draw nearer to the 

 sun 1 And there finding herself in a situation, where the 

 matter is more agitated, and the motion more rapid, she 

 will in less time make her double revolution both about 

 the sun and herself; so consequently her years and days 

 will be much shortened, but not to be perceived, because we 

 must still go on to divide the year into three hundred and 

 sixty-five days, and the day into twenty-four hours : so 

 that without living longer tlian we now do, we shall live 

 more years ; and on the other hand, as the earth withdraws 

 from the sun, we shall live fewer years than we do now, and 

 yet have our lives of the same extent." 



" There is a great deal of probability," says she, "that 

 whenever it falls out thus, long successions of ages will 

 make but very little variation." 



" I agree with you, madam," replied I, " the conduct of 

 nature is very nice, and she has a method of bringing 

 about all things by degrees, which are not sentible, but in 

 very obvious and easy changes. We are scarce able to 

 perceive the change of the seasons, and for some others 

 which are made with a certain deliberation, they do not 

 fail to escape our observance. However, all is in a per- 

 petual rotation, and not so much as the lady's face in the 

 moon, which was discovered with telescopes, within these 

 forty years, but wliatis grown considerably old. She had a 

 good tolerable countenance, but now her cheeks are sunk, 

 her nose grown long, and her chin and forehead meet, so 

 that all graces are vanished, and age has made her a 

 terrible spectacle." 



