Mr. Mackain <m the Compression of Water. 249 



moon, or 3 more than the average. But it also appears that on the 

 dajs of full moon and of the first quarter, the changes are still more ; 

 and that all of them are exceeded bj several of the intermediate dajs on 

 wliich no lunar influence is supposed to be exerted. It will farther be 

 remarked, that on the days of the new moon, the changes vary from 

 from 8 in 1842, to 20 in 1841, and that in three of the years they are 

 below the average of 13i. To account for the omission of certain days 

 in the table, it is necessary to explain that as each quadrature had to 

 be placed in the same line, and the number of days between the qua- 

 dratures being unequal, blank days frequently occurred in the columns 

 which had been marked the 5th and 6th, 13th and 14th, 21st and 

 22d, 29th and 30th days of the moon, and that these have been 

 omitted for the purpose of shortening and simplifying the table. 



4th. Have tides been observed in the atmosphere ? 



They must be assumed to exist by all who acknowledge the moon's 

 influence upon the atmosphere, for scarcely in any other way can such 

 influence be supposed to be exerted. An atmospheric tide, however, 

 like that of the ocean, must twice every day have its ebb and 

 flow — changes quite as great as those to which the effects in ques- 

 tion are attributed ; and yet we never hear of changes of weather 

 being expected in correspondence with each of these atmospheric 

 waves. But even the existence of any double diurnal oscillation pro- 

 duced in the atmosphere by the attraction of the sun and moon, has 

 never yet been detected. It is altogether insensible to an ordinary 

 barometer. Laplace reduced a great number of exact observations, 

 and found the differences due to those attractions so minute and so 

 variable as to leave him in doubt of their sensible existence. And 

 Arago, to wliom we are indebted for much information on this sub- 

 ject, after a minute detail of the observations of Flaugergues and 

 Bouvard, arrives at essentially the same conclusion. 



We have no ground, therefore, either from theory, or direct obser- 

 vation, for believing that the moon produces any change upon the 

 weather; or, at least, it must be allowed, (to use the words of Dr. 

 Olbers,) that its influence is so slight as to be lost among the infinite 

 number of other causes and forces which destroy the equilibrium of 

 our easily disturbed atmosphere, and therefore altogether insensible 

 to ordinary observation. 



The following communications were then made : — 



LIV. — On a Theoretical Rule for the Compression of Water. By 

 Daniel MacKain, M. Inst. C.E. 



The extreme elasticity of air, when considered with reference both 

 to the amount of force which we can apply to it, capable of producing 

 important changes in its volume, without any great effort, and to the 

 strength of the materials of which the instruments used for ascertain- 



