32 report — 1842. 



10 a.m. and 10 p.m., being, with only one exception, a uniform result for 

 the whole series. 



The hours of minima are 4 a.m. and 4 p.m., being also a uniform result of 

 the five years without any exception. 



When we consider that the few deviations from a fairly continuous curva- 

 ture, as laid down in the graphic delineation of the mean hourly march of the 

 atmospheric pressure (pi. iv.), apply to the fourth place of decimals, we have 

 fair ground for believing that by these observations we have really arrived at 

 the general laws of the horary oscillation at Plymouth. 



My Report for the year 1839 contains many general deductions from these 

 observations ; and Professor Airy, to whom they were submitted, seemed to 

 think that at present little more could be effected by them. I cannot, how- 

 ever, but again remind the Association that 48,000 hourly observations on 

 pressure and 87,600 hourly observations on temperature, of a very fair and 

 perfect kind, made and discussed with great labour and at great cost to the 

 Association, should not be lost sight of, and be allowed to exist only in the 

 fragile form of a manuscript, but should be permanently secured and placed 

 at the disposal of the scientific world generally. There are members of this 

 Association so highly gifted with powers of physical research, that it would 

 be by no means unreasonable to hope that, should their attention become di- 

 rected to particular views of this branch of science, they would with such a 

 mass of accredited observation at their command, find themselves in a posi- 

 tion to contribute essentially to the future advance of meteorology. 



My last Report on Whewell's anemometer contained an account of the 

 general indications of the instrument for one year, and of the means I pro- 

 posed to pursue with a view of giving its indications not only a relative but 

 an absolute value. Although the tempestuous and unsettled weather which 

 marked the close of the last meeting of the Association greatly impeded the 

 experiments which I proposed to make, and the early period of our meeting 

 this year has somewhat abridged the intermediate time, I have been to a 

 great degree enabled to realize the view I entertained of the possibility of 

 deducing, by actual experiment, the absolute velocity of the aerial current 

 corresponding to certain indications of the instrument, so as not only to de- 

 termine the mean direction of the wind for a given time, but the absolute 

 mean rate at which it has moved. 



It does not appear requisite at present to enter into a minute detail of the 

 various experiments ; it may perhaps be sufficient to state that the pressure 

 and velocity of the wind were observed simultaneously with the anemometer, 

 and the results tabulated and discussed. From these results the following 

 deductions were arrived at. 



1. When the pencil tracing the integral effect of the wind moved by the 

 revolutions of the fly at the rate of 1 division of the scale of measure, or *1 

 of au inch per hour ; the current of air for the same time moved at a mean 

 rate of 11 feet per second. 



2. The space described by the pencil appeared to be proportional to the 

 square of the velocity of the aerial current acting on the fly. Thus when 

 the pencil described 4 divisions of the scale in an hour, the velocity, by a 

 mean of many observations, amounted to 22 feet in a second. 



When the velocity was 15 feet in a second, the pencil had described about 

 two divisions of the scale in an hour, and so on. 



Having then the velocity due to a given rate of indication per hour taken 

 as unity, it is easy to find the velocity of the wind due to any other rate of 

 indication, since we have only to multiply the square root of the given rate 

 by the constant 1 1 , the velocity per second corresponding to a space of 1 divi- 



