54 PHILOSOPHICAL TRANSACTIONS. [aNNO 1776. 



hygrometer was last adjusted in Dec. 1775> and as the string has now been in 

 use upwards of 5 years, it is not likely to want re-adjusting soon. 



Of the Varialkm C'impaxx. — In this instrument, tlie box which holds the 

 needle is not fixed, but turns horizontally on a centre^ and has an index fastened 

 to it, pointing to a divided arch on the brass frame on which it turns; and the 

 method of observing is, to move the box till a line drawn on it points exactly to 

 the end of the needle ; which being done, the angle that the needle makes with 

 the side of the frame is shown by the index. It was by tliis method that the 

 error of the instrument, at the time of the observations in 177-1, was found to 

 be lO'. For example, by a mean of the observations made on St-pt. 5, the 

 variation with the needle, in its upright position, was 21.36 by the south end, 

 and '21.27 by the north ; with the needle inverted it was 21.ig by the south end 

 and 21.29 by the north. The mean of all four is 21.28, which is the true varia- 

 tion at that time and place, and is 8' less than that shown in the upright posi- 

 tion of the needle by the south end, which is the end always used in observing; 

 so that by this day's experiment the error of the instrument appeared to be S'; 

 but by a mean of the observations of this and two other days it came out lO'. 

 Since that time the needle has been altered ; and, at the time of the observations 

 in 1775, the error was so small as to be scarcely sensible. 



Great care was taken that the metal, of which this variation compass is com- 

 posed, should be perfectly free from magnetism. There is a contrivance in it for 

 lifting the needle from off the point, and letting it down gently, to prevent 

 injury in carrying from one room to another. The instrument is constructed 

 nearly on the same plan as some made by the late Dr. Knight. The principal 

 difference is, that in his the pin which carried the needle was not fixed to the 

 lower frame as in this, but to the box ; the consequence of which was, that 

 when the needle had settled, and the box was moved to make the index on the 

 needle point to the proper mark, it was again put into vibration, which caused 

 great trouble to the observer. This inconvenience is entirely removefl by the 

 present construction. There is no other material difference, except that of the 

 needle being made to invert, and the addition of the telescope. The contrivance 

 of fixing the pin which carries the needle to the lower frame, is taken from an 

 instrument of Lord Charles Cavendish ; that of making the needle invert I have 

 seen in some compasses made by Sisson. 



- There is a very common fault in the agate-caps usually made for needles, 

 which is, that they are not hollowed to a regular concave, but have a little pro- 

 jecting part in the centre of the hollow ; the consequence of which is, that the 

 point of the pin will not always bear against the same part of the agate, and 

 consequently the needle will not always stand horizontal ; but sometimes one end 



