and on the Encke Planet. 1 79 



According to this Rule, a temperature of 118°-6 would be 

 necessary to produce a velocity of 1142 feet per second, as 

 given by Dr. Derham's Experiments, and adopted by New- 

 ton, but certainly his Experiments were not made at so high 

 a temperature as this. 



I have in column 2 added the mean pressures observed in 

 Dr. Gregory's Experiments, in order to examine, whether they 

 would appear to bear (as it is evident they do not) any such 

 relation to the velocities, as seems indicated by Mr. Ivory in 

 page 426 of the same volume, by the expression V32%xlx%, 

 or its equivalent 6-549 ^~l, for the velocity, in feet, of Sound 

 per second ; I being the height of a supposed homogeneous 

 atmosphere, in feet. 



Now since mercury is about 11,262 times the weight of 

 air, in a mean state, at the earth's surface ; we have I = y 1 ^ x 

 11262X7«; in being the height in inches of the barometric 

 column of mercury, balancing such an atmosphere; and the 

 above expression becomes, 200-63 V m, = the velocity of 

 Sound per second, in feet. 



The pressures, or mercurial columns m, being the same, viz. 

 29-82 inches, when the greatest and when the least velocities 

 were observed by Dr. Gregory (the difference being 21-9 feet), 

 it seemed useless for me to calculate, in a fifth column, the 

 velocity of each of his observations, by the formula last men- 

 tioned : I will do so, however, for the mean of all the Doctor's 

 observations, as to pressure and velocity ; and these I find to 

 be 29-79 inches, and 1107 feet. Accordingly 200-63 x \/ 29*79 

 = 1095*3; which is 11-7 feet less than the mean of the ob- 

 served velocities. 



If by this same formula we calculate, what must be the 

 pressure, to occasion a velocity of 1142 feet per second, we 



■j = 32-40 inches ; which is a barometric pressure, 



200-63 



not less excessive, than we have just found the temperature to 

 be (according to the first of these Rules, which are but em- 

 pirical ones) for producing this velocity, which so long has 

 been received as the mean velocity of Sound. 



If we compare in p. 414 Mr. Goldingham's Temperatures 

 with his velocities, we shall find his minimum temperature in 

 February, but his least velocity in December; and again, his 

 maximum temperature in May, but the greatest velocity in 

 July; whence it is manifest, that no Rule on the same prin- 

 ciple as Dr. Gregory's wherein the velocities are directly as 

 the temperatures, can be applied to represent the velocities of 

 Sound at different seasons, at Madras. 



Z2 On 



