469 
the Fruit of &icurbita. 
diameter, is filled with oil and placed on the surface of the 
gourd, vertically beneath the needle-point. The micrometer 
is now screwed down until the point comes in contact with the 
oil, a moment which is very definitely seen. Any change in 
the vertical diameter of the fruit is of course perceptible in the 
micrometer readings. With practice the micrometer is reliable 
to o-oi mm., and a third decimal place can be estimated. For 
some reasons it would be better to use mercury for the 
reflecting surface, but contact with oil is far more easily seen, 
and the error arising from evaporation or clogging of oil is 
spread over many days. 
Other sources of error are more serious. The fruit-bearing 
branch must be securely tied in several places to heavy retort- 
stands, or to stakes stuck in the soil ; if this is not done the 
fruit will not be steady. I can only say that I was fully alive to 
this possible source of error from the first. The fruit rested 
on a glass plate supported on an iron tripod ; the ebonite pot , 
was steadied on the curved surface of the fruit by a bedding of 
putty. As the fruit grew, the oil- pot gradually moved laterally 
with regard to the needle, so that at intervals of about twenty- 
four hours it was necessary to recentre it. 
Another source of error is the expansion by heat of the 
iron rod which supports the micrometer. When the rod 
expands the readings of the micrometer will show an apparent 
shrinking of the fruit, and vice versa . 
I have not given the correction for temperature ; the follow- 
ing examples show the extent of error. 
July 12, 1893. The vertical height of the fruit (which gives 
the length of iron rod whose expansion has to be allowed for) 
was 113 mm. In round numbers the expansion of iron for 
i°C. is o-oooo 1 of a unit of length. 
The following readings give a fair range of temperature. 
Microm. Temp. 
5 p.m. 4.615 24.6 
11 » 7-430 17.5 
The rate of growth without correction for temperature is 
7*8 per minute, the corrected rate is 7-7 9. As I only give 
I i 2 
