292 Remarks on the Mode of taking Lunar Ohservatipns. 



straight line MH be divided in G, so that HM may be to HG in 

 a constant ratio, and revolve so as to keep M and G respectively 

 in DE and AB, two fixed straight lines crossing at right angles, 

 and also that MH be always perpendicular to the path H de- 

 scribes : then shall the Iccus of H be an ellipsis, whose axes are 

 in the ratio of \/HGto VMH. 



The principle of the elliptical compasses is much simpler than 

 this; and differs from it, in that the bar to which the tracing 

 point is attached is never perpendicular to the curve, but at the 

 vertices of the axes ; and also that the distances of the sliding 

 dovetails from the tracer are equal the semiaxes. 



A correspondent to the Annals of Philosophy lately objected to 

 periodical mathematical works, for not adn)ittingany thing unac- 

 companied by demonstration ; and in effect proposed that de- 

 monstration be done away with in future. This, no doubt, he 

 did for the same reason that smugglers wish to be disencumbered 

 of *' those oppressive laws," hoping thereby to get off his con- 

 traband doctrines with more facility. I cannot, however, second 

 his motion, on the ground that no new doctrine on which any 

 stress is laid can ever be safely let loose into the world without 

 some evidence of its truth. It is this which makes the grand di- 

 stinction between true science and unmeaning jargon. Things, it 

 is true, which have often been demonstrated, may be quoted or 

 given without demonstration. But many erroneous propositions 

 are copied from book to book, without the least suspicion of their 

 accuracy. A remarkable instance of this is the " Construction 

 of the dialling lines," which, though admitted into our first-rate 

 works of science, is nevertheless grossly erroneous. It does not 

 appear that an absurdity becomes more correct by frequent copy- 

 ing, but it certainly becomes more dangerous. 



Periodical mathematical works, however, are liable to a more 

 serious charge than the one above mentioned; in that they are 

 mostly made up of mere puzzling questions, totally useless and 

 unconnected with science. Persons who torment themselves with 

 such nonsense, might as well be assisting Sisyphus in rolling his 

 stone, for any good they are doing either themselves or others, 

 except it be to keep them out of mischief. But even this lauda- 

 ble end might be attained in solving questions serving some more 

 useful purpose. I am, sir, your most obedient servant, 



Berners Street, Oct. 1, 1819. Henry MeikLB. 



P.S. — In concluding the last paper on friction, I made a re- 

 mark on a position which I then thought inconsistent with the 

 case under consideration. The truth however is, that it is not 

 simply so, but impossible that bodies of different weights with 

 equal bases can make impressions proportional to their weights 



