Mr. M. Smith on the Construction ofSundials. 259 

 peared with an excess of this acid,-quite similar in this respect 

 to the arsen.te of silven If, however, this experiment be^re 

 peated several times, the yellow deposit does not re-appear- 

 and mstead of it a brownish one is produced after theTpse 

 from Si °f"^ which might be compared to that arilg 

 rrom nitrate of silver and arsenic acid. ^ 



with .I'!.'" ^'^^ fi'-^t-named re-agents have the same relations 

 hnf f '^°"^'^" ?{''T''^ ^' ^^th a solution of arsenious acid: 



XXXVI. On the Construction of Sundials,- isoith a Table for 

 that jnirpose. By Mr. M. Smith. 



To the Editor of the Philosophical Magazine and Journal. 

 Sir, 



'pHE following table for the construction of sundials is care- 

 fully computed m conformity with the principles demon- 

 strated in my letter published in your last Number, page 168. 

 All similar tables which I have hitherto seen are extremely 

 erroneous, being calculated on the supposition that the sha- 

 dow is cast from the centre of the sun ; and therefore any dial 

 constructed from such tables must invariably err one minute 

 of time from the truth. The following table is computed on 

 the principle that the shadow is cast from a point in the sun's 

 disc one minute of a degree within that limb which is nearest 

 to the meridian, and consequently fifteen minutes from the 

 sun s centre. The shadow therefore coincides with the meri- 

 dian at one minute of time from noon. This remark must 

 be particularly attended to, as it follows that the hour-anffle 

 tor noon is negative; consequently the upper line in the table, 

 or that standing opposite to 0" O'" is to be set off from the 

 meridian in the contrary direction from all the others, which 

 will have the effect of contracting the double hour-line for 

 twelve o clock, so as to render it somewhat less in thickness 

 than the gnomon or style of the dial. 

 I remain, sir, 



Your obedient servant, 

 ^^'°'^^^^'^«~^- M. Smith. 



Kk2 The 



