
ON ELECTRICAL OBSERVATIONS AT KEW. 127 
Table XVIII. exhibits the mean monthly electrical tension at each ob- 
servation-hour deduced from the observations of ¢hree months, also the 
mean summer, winter, and yearly tensions deduced from the observations of 
three summers, winters, and years. The last line in the table to which the 
word “ Means” is prefixed, exhibits the mean tension in each month as de- 
duced from all the separate monthly observations ; 7. e. the mean tension of 
_ January, 150°7 div., is the result of all the January observations in the three 
years. ‘The same thing holds good of the seasonal and yearly mean ten- 
sions. 
The curves projected from these numbers will be found on Plate IX. 
The tensions that enter into the preceding discussion range between 2 
div. and 2000 div. in terms of Volta’s standard electrometer No. i. It has 
- however been considered that tensions above 100 div. of this electrometer, 
or those measured by Henley’s instrument, are not susceptible of that accu- 
racy of determination which is requisite in the deduction of results, such as 
characterize those of modern science. In addition to this, it is apprehended 
that the electrical tension known more particularly as the tension of serene 
weather, seldom (if at. all) rises above 100 div., although there may be move- 
ments indicated by Henley’s instrument which partake of the character of 
those of serene weather. ; 
In immediate reference to these points, and considerably elucidating them, 
remarks occur, either in the body of the Journal or in the notes and addenda 
accompanying it. In the description of the instruments at Kew published in 
the volume for 1844 (Reports, 1844, p. 124), the following occurs in refer- 
ence to Henley’s instrument:— . 
*« This electrometer has seldom been observed until the Volta No. 2 had 
risen beyond 90° (in terms of the first, ¢. e. 18 lines x 5); and since the un- 
certainty and difficulty of measuring the higher tensions increase in a rapid 
ratio with the increments of tension owing to unavoidable and sometimes 
almost imperceptible ‘ spirtings,’ and particularly to the falling of rain from 
the dish or funnel N (fig.2), proportionably less confidence must, of course, 
_ be placed in our notations of such tensions by means of this instrument.” 
In the account of the experiments having reference to the employment of 
photographic methods for self-registering the indications of the instruments, 
which is appended to the vqlume of observations 1845 and 1846, we have 
the following remark relative to the objection of the Astronomer Royal as to 
the non-registry of the kind of electricity :— 
“‘T had not of course overlooked the objection as to not registering the 
kind of electricity, but as every former observer of the: periodical electricity 
of serene weather, (7. e.) that alone which is susceptible of exact measure- 
ment, and that which is by far the most important and interesting, had ar- 
_ rived at the same conclusion as myself, (viz.) that it is positive, and that the 
_ exceptions to this law are extremely rare, and always accompanied by an easily 
distinguished feature in meteorology.” 
In the above extracts we have clearly a restriction of the electricity of 
serene weather to a comparatively low tension, and that the higher tensions, 
although more difficult to measure accurately, are not near so important as 
those which characterize serene weather. In immediate connection with this 
comparatively low tension we have the following remark, recorded on June 23, 
_ 1844 :— 
_ “The weather of this day, considered as serene, has been rather remark- 
able. The signs a little after sunrise were the highest for such weather that 
_we have had. The thermometer até nine stood at’75°5, the max. also, and the 
barometer at 29°938. The atmosphere quite clear; the clouds were light, 
