34 RECORD AND RESULTS OF 



Let (3 = reading -of barometer in inches, and reduced to 32° Fah. 



t = temperature ^f the air in degrees of Fah. ; then the correction to the 

 number of vibrations made in a mean solar day by a brass pendulum 



" ^-^^^^ 1 + 0.0023 {t — 32) 



The average reading of the barometer (reduced to 32°) at Cambridge is 29'°. 72, 

 and at Port Foulke 29'°. 82, the observations have therefore been referred to the 

 convenient average reading 29'°. 8 by the formula 



0.3541 i(3 — l3,) 

 ' 1 + 0.0023(^ — 32) 



The average t at Cambridge is 70°. 9, and at Port Foulke + 22°. 8 hence the cor- 

 rection for Cambridge 0.325 (^ — 29.8), and for Port Foulke 0.362 (/3 — 29.8). 

 The reduction to vacuum is always additive. The variations from the average t at 

 either place are small. 



Reduction to the Level of the Sea. 



Let N = number of vibrations at the elevated station 

 JVTj :;^ corresponding number at the sea level 



H = the elevation and E = the earth's radius, then the reduction to the 

 number of vibrations in a day (see Vol. VII, p. 28, Mem. Roy. Ast. Soc.) 



=0.666 N— a correction vi^hich is always additive. For Cambridge 



we have 0.00276 H, and for Port Foulke 0.00277 if, the elevation, 

 above half tide being expressed in feet. 



From the preceding record the following abstract of observed times, arcs, temper- 

 atures and atmospheric pressure has been formed. 



The first column contains the number of observed times united into a mean ; the 

 second column the average clock times of vibrations from right to left ; for an odd 

 number of times the mean corresponding to the middle one is set down; for an 

 even number either the first or last observation was omitted ; the middle times, in 

 aU cases are marked thus x in the preceding record ; the third column contains 

 the arcs of vibration; when not directly observed they were interpolated by a 

 graphical process, the arcs are inversely as the squares of the times, and the curves 

 constructed on a sufficiently large scale proved them to be quite smooth and regular ; 

 the fourth column contains the average temperatures observed or interpolated. The 

 next column contains similar information for vibrations from left to right, and the 

 last column gives the observed height of the barometer when referred to tem- 

 perature 32° Fah. 



The first means for face 3 have been corrected by subtracting one second to 

 refer to "right" and "left" respectively. 



