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be necessary to make fresh experiments, in order to determine the length of time that the ivory pipes ought to remain in water, and how long they must afterwards be exposed to the vicissitudes of the air (28), or in general to what preparation they must be submitted in order to acquire a lasting degree of consistence before they are made use of. For this purpose it will also be expedient to compare hygrometers recently made, with older ones, both to ascertain whether they have undergone any alterations, and in what degree I likewise am of opinion, that when we wish to fix the point of extremed humidity, we must be very careful not to make use of any ice but what is very clean, as well internally as externally ; left any dust should stick to the ivory pipes, which might hinder the water from penetrating into the pores : this is what I thought of myself too late I do not know whether for the same reason it would not be right to wash these tubes with spirits of wine before we put them into the water, to remove any greasy scurf they may have gathered by handling ; and afterwards to repeat this at times, in order to carry off any little deposit of various kinds, which may in course of time have been left upon them by the air. Moreover it will be right to inquire whether there is not a difference between the effects of the heat upon the irory of the hygrometers and upon the glass of the thermometer, sensible enough to be attended to, in correcting the effects of this cause upon the hygrometer.
74. Having already discovered so many causes, more or less probable, of the differences I observed
