348 
LIEUT.-COLONEL SYKES’S DISCUSSION OF METEOROLOGICAL 
mani^re d’operer me parait d’autant plus necessaire, quit reste heaucoup d' incertitude 
sur plusieurs des elements numdriques que entrent dans le calcul, notamment sur la 
chaleiir spicifique de Vair, sur celle de la vapeur, et sur la chaleur absorhde par Veau 
lorsquelle se vaporise dans Vair” — (P. 212.) 
These are the opinions of M. Regnault expressed twenty years after August had 
invented his formula, and after his own elaborate experiments ; and 1 have preferred 
giving them in his own language to free myself from any possible misconstruction in 
translation. M. Regnault thought that August’s formula modified by him, would 
meet some of the difficulties expressed by him in the above quoted opinions, and he 
gives tables of results, which, with a limited range of the thermometer and small de- 
pressions, are sufficiently satisfactory ; but he candidly admits I’accord a ete beau- 
coup moins parfait dans les bas temperatures et dans Fair tres humide in these cases 
M. Regnault says the fractions of saturation calculated are always above the frac- 
tions of saturation obtained by direct means, often to a very notable extent. M. Reg- 
nault got one of his pupils to try the psychrometer, under considerably diminished 
pressure, on the mountains in Switzerland, but the results were so little satisfactory 
that he does not give a detail of them. He induced also his friend M. Izarn, to compare 
the readings of the wet bulb in the Pyrenees, at a pressure of 700 millims. (27‘559 in.), 
with the readings of an hygrometer invented by himself, which he calls ‘‘ hygrometre 
condenseur,” and which he considers to be free from the objections to which Daniell’s 
hygrometer is subject; and in a table given, the fractions of saturation are almost 
always much higher by the wet bulb than by the condenser, the depression in no 
case exceeding 3°'91. In a depression of 1°'97, the differences of the fraction of 
saturation are respectively 0’7542 and 07937. Finally, M. Regnault says that his 
modified formula may give the elastic force of x a. little too high, and that the coef- 
fieient 0‘480 might be used instead when the fraction of saturation exceeds 0'40 Cen- 
tigrade, but that the coefficient 0'429 gives results nearer to the truth where the 
fraction of saturation is below 0‘40. When the wet bulb descends below zero, he 
considers that the value of A should be increased from 610 to 689, but in the present 
imperfect knowledge of the true numerical value of several of the elements he will not 
venture to put forth a new formula for the psychrometer (p. 227). Several other phi- 
losophers have also given formulae : Burg from observation,/"=/'— •0004528(^— ^')jo; 
Bohnenberger also from observation,/"=/'— •0003962(^— ; also Kupffer, Erman 
and Kamtz. 
On the 24th of November 1 834 and 27th of April 1835, Dr. Apjohn of Dublin read 
to the Royal Irish Academy a very elaborate and able paper upon the theory of the 
moist bulb hygrometer. The results of his observations, experiments and theore- 
tical considerations, induced him to adopt the formula 
J —J 87^30’ 
at p. 436 of the volume of the Proceedings of the Royal Irish Academy for 1840, the 
formula is yP_^ . 
