THE PRESIDENT'S ADDRESS. 309 



work will contain " The Journal of the Mayor of Penzance," 

 1816-17, a very interesting municipal document, hitherto quite 

 unknown to the public. I am sure that you will join with me in 

 congratulating Mr. Boase on the prospect of an early completion 

 of this comprehensive and valuable contribution to Cornish books 

 of reference. 



I am not aware whether attention has ever been directed to 

 an important error in the longitudes of the principal stations of 

 the Ordnance Survey, given in most of the parochial and des- 

 criptive works on Cornwall, even in those of recent date, and 

 taken originally from a preliminary report on the Survey pub- 

 lished in the "Philosophical Transactions" for 1800. This 

 error was first pointed out by Dr. Tiarks, who in 1822 deter- 

 mined the longitude of Funchal, by means of fifteen chronometers 

 conveyed to and from the island of Madeira, making Pendennis 

 Castle one of the intermediate stations. By this operation, the 

 new determination of longitude placed Pendennis Castle con- 

 siderably to the westward of that assigned to it by the Ordnance 

 Survey. In the following year, Dr. Tiarks repeated the operation 

 by the transmission of twenty-six chronometers between Dover, 

 Portsmouth, and Falmouth, the results of which fully confirmed 

 his previous observations, showing an error of more than four 

 seconds in time in the longitude of Pendennis Castle and other 

 Cornish stations, as given in the preliminary report.* 



The Director- General of the Ordnance Survey has recently 

 informed me, that the longitudes originally given in the table in 

 the " Philosophical Transactions " are largely in error, owing 

 to the neglect of the application of a correction depending on 



* Davies Gilbert states in his " Parochial History, Vol. II, p. 23, that " the 

 longitude of Falmouth has been ascertained by Dr. Tiarks with the greatest care 

 (see Phil. Trans, for 1824) : the flag-staff at Pendennis Castle, 20m. H'53 west." 

 — The longitude printed in the preliminary report of the Ordnance Survey is 20m, 

 6"9s west, or more than four seconds of time too small. 



The Ordnance Survey authorities at Southampton have lately, at my request, 

 accurately determined the geographical position of the Falmouth Observatory, 

 using the best modern data, to be as follows : — 



North Latitude ... 50° 8' 59-0" 



West Longitude ... 5° 4' 34-5" 



West Longitude in time ... 20m. 18"3s 



(Report, R.C.P.S., 1888, pp. 108-110). 



