"...To invest and reinvest the principal and any unexpended income and to expend such portion of the income as may seem desirable to my said trustee for the benefit of the Town of Wrentham for some public improvement, such as care or improvement of the Town Common, the care of its share trees or for any other public purposes of a kind that would ordinarily not be likely to be provided for by town appropriation, it being my intent and purpose that the income from this fund shall be expended for the benefit of said Town, to be used for purposes which might be considered luxuries, the expenditure for which would not ordinarily be considered advisable to be raised by the ordinary tax levy."