23 
such a manner as to remedy this defect. Without altering 
the size of the bulb, I should propose for a permanent 
instrument a stem say 18 inches long with a bore of such 
diameter that the stem should embrace a range of tempera- 
ture betwen 20° Fahr. and 92° Fahr. Thus somewhat less 
than five degrees will go to the inch. The stem might be 
protected from the risk of accident by an appropriate shield. 
Let such a thermometer be heated for two minutes and the 
size of the lens be somewhat increased. In this case a rise 
of something like 5° Fahr. will be obtained, and this heating 
effect might very easily be estimated to one hundredth of 
the whole, while the same thermometei would serve for all 
the temperatures likely to occur in these islands during the 
course of the year. 
I ought to add that a pasteboard cover, gilded on the out- 
side is made to surround the chamber, and also that between 
the lens and the chamber there is a pasteboard shield with 
a hole in it to permit the full rays from the lens to pass-— the 
object of this shield being to prevent rays from the sun or 
sky from reaching the instrument. 
In such an instrument r or the change taking place in the 
thermometer before exposure to the sun will in all pro- 
bability completely disappear, while r will be extremely 
small. 
At any rate we may be quite certain that R + 
r + r' 
'~T~ 
will accurately represent the heating effect of the sun. 
We may probably suppose that in the same instrument 
the lens (which must always be kept clean) will always stop 
the same or nearly the same proportion of the solar rays. 
But the lens of one instrument may not stop the same pro- 
portion as that of another instrument. This, however, is no 
objection if it be borne in mind that the instrument is a 
differential one. In practice there would be some standard 
instrument which would be retained at a central observatory, 
and all other instruments would, before being issued, be 
