140 Remarks on Dr. Enfield’s Institutes 
true amplitude is 8° towards the same — then the varia~. 
tion of the needle is 2° east. 
Prop. 10. Sehol. 2. The late and accurate obtervati tions 
of Mr.. ‘Gilpin*® and Col, Beaufoy on the diurnal variation 
of the needle in different months of the year, present wide 
deviations from the results here given from Mr. Canton. a 
the observers just mentioned, make the extremes of the 
mean diurnal. variation in different months, about 11’ and 
4/; and both place the time of the maximum earlier in the 
year than was done by Mr. anton. Col, Beaufoy otanind 
son’s Annals, 1819,) places it in April. 
Prop. 11. Schol. 1... The dip of the needle is here rep- 
resented as probably “unalterable at the same place.”— 
Whatever be the cause of the dip, this supposition is ex- 
tremely improbable, while the declination is known to be ~ 
variable, and to be, in common with the dip, the result of 
the Psat et _ whiehy the. needle — itself 1 in) the —_ 
netical line 
_ 
made i In t n this 
She saeacuted: Pig: Whiston in n 1724,- it was 75° ° 
t with this result is that of Gra- 
, obtained in the following g year. Cavendish, in 1775, 
found it to be 72° 30’, and Gilpin, in 1805, 70° 21. These 
observations, after every allowance. is made for the imper- 
fection of the instruments employed, leave no doubt that . 
the inclination of the needle has undergone a gradual dimi-~ 
nution in London, = ae the last century. According to ~ 
M. Humboldt, (see Biot—Traité de Physique, IIL. 136,) a 
similar diminution has. taken place, urine the same gaan 
in France. pete 
~ Book VI. Prepe 13. Sehiol: 1... The statements pence 
ing the ratio of the sine of the angle of incidence *to that 
of deviation, in passing. to and from water and» glass, are 
true only under a limitation which is not distinctly pointed 
oie seals that. the angle of incidence is indefinitely 
Schol. 2. The partial reflection of light by the second: 
surface of transparent media, when the angle is within the 
limit for light to _be refracted, is erroneously ascribed. to 
*imaguliis” of the etion: | if this were the trae causes 
* Philos. ates isle. 
