4^ ftUAtfTITY OF KOB.tZb'HrAL REFR ACTtdN. 



Defcriptlon of a iial notch at the pleafure of the pofTeflbr, and alfo by making 



padlock of com' 

 bination. 



groves or fmall notches on the face of each central piece, 

 which anfvver the purpofe of the teeth recommended at the 

 page laft quoted, by preventing the rings from being turned 

 round while any pull is madeagainft the clofure. 



I find fome obfcurity in his defcription of the manner of 

 connefting the central piece and the external engraved part. 

 From the operation, I apprehend, 1 . that each ring has a 

 number of notches at its inner furface, that an fwer to the letters 

 on its outer face ; 2. that each central round piece fits the 

 cavity of its ring, and is prevented from turning by a tooth 

 which it lodges in one of the notches; 3. that when all the 

 four teeth are ranged in a line between F and F, the lock will 

 open; and therefore, 4. when any particular letter is placed 

 over the tooth, that letter becomes the effedive letter for its 

 own ring;. 



Former paper 

 of tde author, 

 and of Monge 



VIII. 



Obfervations on the Quantity of horizontal Refradion; with a 

 Method of meafuring the Dip at Sea. By William Hyde 



WOLLASTON, M. D. F. R. S* 



JlN a Paper which I fome time fince prefented to this Society, 

 (printed in the Phil. Tranf, for 1800,) I endeavoured to afcer- 

 pon horizontal tain the caufes, and to explain the various cafes, of horizontal 

 ;fradlion. refraction, which I had either obferved myfelf, or had feen 



defer) bed by others. 



At the time of writing that elTay, I had not met with the 

 M6moiresfur I'Egypte, pubiiftied but a fhort time before; and 

 I was not aware that an account had been given by M. Monge, 

 of the phenomenon known to the French by the name of 

 miragCy which their army had daily opportunities of feeing, in 

 iheir march through the deferts of Egypt. 



In the perufal of this memoir, I could not fail to derive 

 inftru6lion from the information it contained ; but, as the fads 

 related by him accord entirely with the theory that I had 

 advanced, I was by no means induced to adopt the explana- 

 tion that he haspropofed, in preference to my own. 



From the Philofo^^iical Tianfaaions for J803. 

 3 



The 



