OF CERTAIN HOT-SPRINGS IN THE PYRENEES. 455 



manently changed its temperature, but merely having oscillated l)etween these 

 extremes. One advantage derived from a comparison of my observations with 

 those of Principal Forbes is that we both observed at the same time of year— 

 namely, in or near the month of August. 



There are certain circumstances calculated, if not satisfactorily explained, to 

 cast discredit upon the results of an investigation of this kind : they respect the 

 character of the instruments employed ; the scrupulosity of the observer ; and 

 certain local conditions. 



1. Instruments. — One of the difficulties that we have to contend with in com- 

 paring observations made at long intervals, is that of ascertaining the nature of 

 the instruments employed by former observers. Unless the errors of the instru- 

 ments are recognised and corrected, it is quite possible that what may appear 

 to be a change of temperature may be nothing more than the fault of a ther- 

 mometer. Principal Forbes has been most minute in describing the means adopted 

 for the verification of the instruments which he employed ; and M. Filhol has 

 also been at great pains to check the errors of his instruments. 



Knowing that accuracy of result depended greatly upon the character of the 

 instruments employed, I applied to one of the first houses in Edinburgh, request- 

 ing them to make for me four thermometers of the best description, two going to 

 140° Fahr., and two to 212° Fahr. These were marked respectively, A, B, C, 

 and D. They were tested by Mr Buchan, the acting Secretary of the Scottish 

 Meteorological Society, who having examined them and compared them with the 

 Society's standard thermometers, pronounced three of them perfectly accurate, 

 and the fourth very slightly faulty. The latter was rejected. All the instru- 

 ments are very sensitive, the mercury rising in the tubes very rapidly when heat 

 is applied. As thermometer D was used at every spring, returned uninjured, 

 and was again tested by Mr Buchan and found perfect, I need only describe it. 

 For convenience of carriage, its length is only '^from the extreme ends) thirteen 

 inches. It is a simple glass tube without mounting, the scale being marked on 

 the tube, and containing eighteen degrees Fahr. to an inch. The degrees are not 

 subdivided ; but I had no difficulty in estimating the parts of a degree. 



2. The Observer. — Having obtained trustworthy instruments, the next fear of 

 error was from carelessness in using them. Any error from this source I was 

 particularly anxious to avoid, and fearing that — from the awkward positions in 

 which many of the observations were made — I might, in the reading of a figure or 

 otherwise, make a mistake, I always endeavoured to associate myself with some 

 person of local reputation who would accompany me to the springs, and make an 

 observation with his own thermometer at the same time. After the observa- 

 tion at each of the springs was made and entered in my book, the Centigrade 



' degrees of my associate's thermometer were reduced to those of Fahrenheit ; if 

 the observations accorded, or were within one or two-tenths of a degree, we 



VOL. XXIII. PART III. 6 H 



